October 2014 | Issue 15
Focus on cavity trays and DPCs
Movement joints in external masonry walls
Slating and tiling for pitched roofs – BS 5534
Type Approvals: Certificate changes
Building Regulations – Regulation 38
Publication of Category 4 Screening Levels for land affected by contamination
Copings and cappings to brick chimneys
Suspended beam and block concrete floors
Installation of GRP dormer windows
Audible cracking noises in intermediate floors
Information and support
Technical news
Technical Extra | Issue 07 | July 2012 | Page 1 page 3 page 7 page 9 page 11 page 12 page 14 page 18 page 20 page 24 page 25 page 26 page 27

Our lead article in this edition focuses on the importance of correctly installing cavity trays and DPCs, particularly around openings and at abutments, expanding on an area we touched on in the annual claims review published in
.
The importance of these critical areas can’t be stressed enough, and this article highlights how apparently small deficiencies in detailing can lead to both costly repairs and considerable disruption to homeowners. If you’ve had to revisit your sites, do take the opportunity to learn from the repairs you’ve had to undertake and ensure current work practices address any inherent deficiencies of the past.
Within Regulation and compliance, we discuss the transfer of Fire safety information to the responsible person – Regulation 38 of the Building Regulations.
If you are uncertain of your requirements in this regard, please contact your
NHBC Building Surveyor.
Other articles highlight changes to the code of practice relating to slating and tiling for pitched roofs and vertical cladding, and the publication of Category 4 screening levels for land affected by contamination.
Among the articles in Guidance and good practice, we review GRP dormer windows and copings and cappings to brick chimneys, two additions to the roof line where shortcomings in detailing and construction can greatly affect long-term performance. We also draw attention to suspended beam and block floors and, in particular, the need to ensure that adequate structural performance of floors is not impaired by the method of achieving sufficient levels of thermal insulation.
Finally, whilst technical content has been regularly updated, the current format of the Standards has remained largely unchanged for over 20 years. NHBC is therefore pleased to announce that we have embarked on a full review of the
Standards format. Work is advancing well and we intend to publish the newly formatted Standards during 2015; for now, the current edition remains in place until the new Standards are launched. In the meantime, we’ll be publishing a new
Chapter that will set the benchmark for waterproofing of basements and below ground structures. The new Chapter will be released shortly and accompanied by a special edition of Technical Extra.
Head of House-Building Standards

NHBC STANDARDS
Who should read this: Technical and construction directors and managers, architects, designers and site managers.
INTRODUCTION
As we now enter autumn, it seems wise to reflect on the somewhat unprecedented weather conditions in terms of wind and rain that were experienced during the winter of 2013/14.
BACKGROUND
During a three-month period in the winter of 2013/14, claims made to NHBC increased by over 40% compared with normal levels for the same period in previous years. Around half of the claims reported during this period were in relation to external wall issues, with an increase of over 150% above ‘normal’ levels experienced. NHBC received circa 1,200 claims related to inadequate cavity trays and DPCs in the three-month period. This compares with 1,700 in total over the previous three full years.
Analysis of the findings of investigations into these defects has revealed very few, if any, cases of construction that had been built in accordance with
NHBC Standards having failed; in most cases, the cause was found to be directly related to workmanship. This indicates that the Standards, including classification of exposure areas, are suitable for purpose, but that ensuring construction is in accordance with the Standards is the major issue.
The issues surrounding cavity trays include trays being installed at the incorrect height, not linking with flashings or being omitted completely.
Further analysis in relation to external walls and
NHBC's ongoing campaign to raise the standard of pitched roofs also shows a significant increase in flashing and upstand failures at abutments. This highlights that a clear understanding between trades as to who will do what is necessary to ensure correct installation and satisfactory construction quality.
A clear understanding and co-ordination of individual trade's responsibilities is vital to ensure correct installation and satisfactory construction quality.
underlay turned up behind flashing lead flashing wedged into joint below wall dpc at least 75mm at least 150mm clip free edge of flashing - method depends on exposure
Internal damage due to horizontal tray not being lapped with flashing, causing water ingress within roof void and damp to ceiling
NHBC Standards 7.2 - S12(f)
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 |

REQUIREMENTS
In addition to the above, NHBC sees simple workmanship issues with horizontal trays – from the flexible material being too narrow, weep holes being omitted or trays being bedded down onto brickwork, making cutting out for flashings necessary and damage inevitable. We also see issues with horizontal trays above meter boxes and flues or ducting, including examples where flues and ducts have been installed through the tray.
Extract duct core drilled through cavity tray, which is also installed too high
Flexible material too narrow for cavity width
n Weep holes should be provided and spaced at a maximum of 450mm intervals; each opening should have at least two weep holes.
n Cavity trays or combined lintels should have stop ends. Where cavity trays are used, they should extend at least 25mm beyond the outer face of the cavity closer and cover the ends of the lintel.
n At abutments, the cavity tray should be linked to a flashing to prevent water penetrating into the enclosed area.
n Mortar should be raked out whilst still green to allow the flashing to be tucked under the cavity tray by a minimum of 25mm.
Good practice stop end combined lintel projects at least
25mm beyond the outer face of cavity closure at least 2 weep holes per opening, not more than 450mm apart at least
140mm total rise in cavity tray at least
100mm rise in cavity tray from front of cavity dpc oversails lintel to protect timber door and window heads groove in window head prevents rain penetration
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
| October 2014 | Issue 15 | Technical Extra
AIR BRICK cavity tray
METER BOX cavity tray

REQUIREMENTS (CONTINUED)
To install a stepped cavity tray correctly, careful consideration should be given to the setting out. For this to be successful, trade collaboration is paramount. It is good practice to use a template, e.g. an appropriate roof truss or frame (with guide lines), to assist with setting out the stepped trays. As a rule of thumb, the back of the tray should be 170-200mm above this truss (NHBC Standards require stepped flashings to have a minimum upstand of
85mm above the top of the finished roof line). Utilising an appropriate truss or frame, as per the example, should enable bricklayers to install stepped trays in the correct position and alignment.
Stepped tray not linked with flashing Stepped tray incorrectly installed below roof line
n To comply with the guidance within NHBC Standards, stepped cavity trays should be preformed.
If any other method is used, this will need to be agreed with your Inspector together with the method by which you will ensure correct installation.
n The lowest tray should project beyond the eaves line of the roof; there should be a starter tray
(stop end at both ends) and a weep hole should be included.
n Each intermediate tray should overlap the previous with the stop end closest to the roof line.
n A ridge tray, with open ends, should be provided at the ridge.
n Mortar should be raked out whilst still green to allow the flashing to be tucked under the cavity tray by a minimum of 25mm.
n Tile selection will dictate if a secret gutter, proprietary soakers or soakers, or a cover flashing is required at the abutment.
preformed stepped cavity trays at least
85mm
Example of a truss for setting out NHBC Standards 6.1 – D6(c) and 6.1 – S4(f)
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 |

REQUIREMENTS (CONTINUED)
Experience during the ‘storm period’ has reinforced previous analysis regarding the number of claims related to superstructure; with some 33% of all section 3 claims (years 3-10 of the Buildmark Policy) dealt with by NHBC in 2013 being in relation to external walls. This article has focused on cavity tray and DPC issues as the dominate cause; however, our review has highlighted further areas requiring attention. A brief summary of these issues follows; more substantial guidance and support will be made available in 2015 (see Next steps).
Over the last three years, the top three failures of render in the UK have consistently related to deboning from substrate, shrinkage (render or substrate) and number of coats or thickness. A render finish is commonly adopted in Scotland and a prominent area of failure, with an average of eight homes per 1,000 finalled having experienced render issues.
70% of mortar claims are due to workmanship issues.
These include poor pointing, unfilled joints and irregular or inadequate bonding. A further 25% can be attributed to the mix specification.
Issues with parapets and copings is almost exclusively caused by inadequate fixings. Northern Ireland has very few (if any) failures. England and Wales and
Scotland see failures in one in 2,000 homes built.
Proportionately, it is a much bigger issue in Scotland.
NHBC has already taken some action and released additional guidance in Technical Extra 14 for the fixing of copings to gable walls. This will be included within the NHBC Standards 2015 .
In order to assist the industry to reduce the number of defects in superstructures, NHBC is in the process of developing appropriate and sustainable actions to raise awareness and improve construction quality in these areas.
This is due to be launched in early 2015 and is likely to include: n Updated Standards and guidance.
n Free nationwide training seminars for site managers and trades.
n On-site trade talks.
n Technical articles within trade press.
YOU NEED TO…
■ Review your construction details and make sure that all trades involved understand the importance of getting the detail right.
■ Make sure that installation is right first time – physically check the detail as constructed, paying particular attention to the first unit constructed for each house type or by different sub-contractors.
■ Ensure that there is a clear understanding and co-ordination of individual trade's responsibilities; this is vital in achieving correct installation and satisfactory construction quality.
■ Look out for more information on NHBC initiatives, including site manager training seminars, to be held in the New Year.
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
| October 2014 | Issue 15 | Technical Extra

NHBC STANDARDS
Who should read this: Technical and construction directors and managers, architects, designers and site managers.
INTRODUCTION
As previously reported, the use of concrete facing bricks has significantly increased. Providing that they are installed correctly, they can provide a durable and aesthetically pleasing part of the building envelope. But, as with clay bricks, it is extremely important to ensure that the detailing is correct, particularly in relation to the accommodation of movement.
REQUIREMENTS
During late 2013 and early 2014, NHBC inspectors reported an increase in the use of concrete facing bricks. This type of product has been used successfully for many years, but the more widespread use has highlighted a need to review what should be considered for the accommodation of movement, which is different to that required for walls constructed with clay bricks.
In March 2014, NHBC published Technical Alert 01, which provided general guidance for the spacing of vertical movement joints in walls constructed from concrete facing bricks. The alert suggested that movement joints should be spaced at no more than
6m intervals unless specific evidence is provided to justify wider spacing. Since then, we have received some enquiries asking in what circumstances it may be acceptable to increase the spacing.
Guidance for avoiding cracking can be found in British
Standards as well as NHBC’s Technical Standards.
However, the guidance does not clearly distinguish between what should be considered specifically for high or low density concrete masonry. Complying with the guidance and adopting the minimum distance of
6m for the spacing of vertical movement joints should provide a robust solution, but it can be overly onerous in situations where higher density concrete masonry is used.
Movement of concrete masonry can be attributed to thermal and moisture movement, the effects of which can be more significant when low density materials, which are more prone to movement than higher density materials, are used. As with walls constructed with clay bricks, the ratio of width to height of the wall panels will also have an effect which needs to be considered in the design, along with the localised use of bed joint reinforcement where required.
Lightweight concrete masonry units are generally made of aggregates that have a gross density not exceeding 1,500kg/m 3 . Dense concrete masonry units are generally made of aggregates that have a gross density exceeding 1,500kg/m 3 . Typically, concrete facing bricks have a higher density in excess of this figure.
The next edition of NHBC’s Standards will be updated to include the revised table overleaf. This provides guidance for the spacing of vertical movement joints that distinguishes between dense and lightweight concrete blocks and bricks.
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 |

REQUIREMENTS (CONTINUED)
Material
Clay brick
Joint width (mm)
16
Normal spacing
12m (15m maximum)
Calcium silicate brick 10 7.5 to 9m
Lightweight concrete block and brick
(autoclaved or using lightweight aggregates)
Dense concrete block and brick (using dense aggregates)
10
10
6m
7.5 to 9m
Any masonry in a parapet wall 10
Half the above spacing and 1.5m from corners (double the frequency)
In addition to the revised table and the guidance published in British Standards, such as BS 5628-3 and PD 6697, the manufacturer’s advice should also be considered in the design of walls, particularly the accommodation of movement.
YOU NEED TO…
■ Ensure that walls are designed to accommodate movement safely without cracking by following the guidance in NHBC Standards, British Standards and the manufacturer’s recommendations.
■ Revised guidance will be published in the next edition of NHBC Standards, but the above guidance can be adopted now.
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
| October 2014 | Issue 15 | Technical Extra

NHBC STANDARDS
Who should read this: Technical and construction directors and managers, architects, designers and site managers.
INTRODUCTION
This article explains the main changes made in the revised BS 5534 ‘ Slating and tiling for pitched roofs and vertical cladding – Code of practice ’, which was published at the end of August 2014.
REQUIREMENTS
The revised BS 5534 has been re-structured and includes some significant changes that will affect the way slated and tiled roofs are constructed going forward.
One of the main changes relates to the specification and use of roof underlays. An unsupported roof underlay provides a secondary barrier to water ingress through the roof cover, but it also provides the majority of the resistance to wind uplift acting across a roof.
The type and number of fixings for the roof coverings are calculated taking account of the wind loads taken by the underlay. It is therefore important that the underlay does not balloon under wind conditions and make contact with the roof tiles, as this would transfer more wind loads to the tiles, which could cause them to lift or even dislodge and come off the roof.
Resistance to wind uplift figures, currently quoted by underlay manufacturers, are difficult to understand and apply. To address this, the British Standard now includes a new wind uplift resistance test and classification system for underlays. The classification system uses the wind uplift resistance test figures and relates them to the batten gauge being used and where in the country the site is located in relation to design wind pressures.
This information will appear on a ‘zonal-classification’ label (Table 1) provided by the underlay manufacturer and, subject to a few parameters, will enable the designer or installer to identify easily which underlays are suitable for a particular roof. The classification system will suit most roof designs. For others, a calculation to determine the required wind uplift resistance should be carried out and used to identify appropriate underlays that can resist those forces.
The underlay manufacturer will be able assist in these situations.
The underlay classification system will enable users to identify the weaker underlays that are only suitable for sheltered areas of the UK or roofs where the batten gauge is small, or where the underlay is supported on solid sarking or insulation boards, which themselves provide the wind uplift resistance.
The new wind uplift resistance tests include the provision of a lap in the underlay. The new tests have highlighted the importance of securing the laps to prevent them opening up and touching the roof tiles.
The British Standard describes how laps should be secured by covering with a separate batten, or extending the underlay to the nearest slating or tiling batten or sealing the laps with a proprietary sealant. If sealants are used, they must be durable and perform to the same standards as the underlay and should therefore be provided by the underlay, manufacturer and installed strictly in accordance with the manufacturer’s recommendations.
Changes have been made to take account of wind loadings in accordance with Eurocodes. As a result, calculated wind loads have increased, which in turn requires additional fixing of the roof tiles. It is likely that all roof tiles will now require fixing either by nailing or clipping, or a combination of the two. The current ‘Zonal Method’, which could be used to establish roof tile fixings without requesting a calculation and fixing specification from the roof tile manufacturer, has not been updated to take account of these new wind load requirements and should no longer be used. Therefore, all fixing specifications should now be provided by the tile manufacturer in line with the revised British Standard.
A third major change is that mortar bedding can no longer be relied upon as a method of fixing. In other words, any tensile strength provided by mortar
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 |

REQUIREMENTS (CONTINUED) bedding should be ignored, and all bedded ridge and hip tiles now require mechanical fixings. This change has brought the British Standard in line with NHBC
Standards, which have called for mechanical fixings of bedded ridge and hip tiles since January 2012. The ratio of soft and sharp sands in mortar mixes has also been revised in line with NHBC Standards.
Other changes include a requirement for a minimum of two mechanical fixings for perimeter tiles (subject to meeting the wind load requirements). One of the fixings can be a tile clip or dry fix unit if it can be shown by the clip or unit manufacturer to provide the necessary wind uplift resistance. There is also advice on the avoidance of creating small cut tiles at perimeters by the use of double, tile-and-a-half or half
Product
Top underlay
Batten gauge
Identification
LR
Declared wind uplift resistance, P
D
(N/m 2 )
Manufacturer
Underlaymakers Ltd.
Zone suitability tiles where available from the manufacturer. It is also recognised that, where small cut single lap tiles (less than half tile width) cannot be avoided, they should be mechanically fixed or bonded with adhesive to an adjoining full tile. This is in line with discussions NHBC has been having with the roof tile industry and it is important that any adhesive fixings should only be made with adhesive recommended by the roof tile manufacturer and used strictly in accordance with that tile manufacturer’s instructions.
To allow time to implement the changes, the previous edition of BS 5534 (2003+A1:2010) will not be withdrawn until 28 February 2015, but NHBC strongly recommends that the changes be adopted as soon as is practicable.
Website www.underlaymakers.com
Wind zone map
< 345mm 1 200 1 to 3
< 250mm 2 200 1 to 5
< 100mm > 2 200 1 to 5
Note 1 In this example, light grey indicates that the zone is suitable and dark grey indicates that it is not suitable. In practice, suitability might be indicated by the use of colour, e.g. green for suitable and red for not suitable.
Note 2 Zone suitability applies only for underlays in applications where a well-sealed ceiling is present, ridge height is not greater than 15m, roof pitch is between 12.5
o and 75 o , site altitude is not greater than 100m and no significant site topography is present. Other applications might require underlays with greater wind uplift resistance, and it is advisable to seek professional advice.
Note 3 Zones 3 and 4 apply to Northern Ireland.
Table 1 – Illustration of a zonal classification label for an underlay
Permission to reproduce extracts from British Standards is granted by BSI Standards Limited (BSI). No other use of this material is permitted.
YOU NEED TO…
■ Make yourself familiar with the revised BS 5534 and start to incorporate the new requirements in your roof designs and construction as soon as is practicable.
■ Make sure roof underlays are suitable for the intended location – refer to the zonal classification label.
■ Make sure that laps in underlays are adequately secured.
■ Ensure that you have a tile-fixing schedule in all cases – note that the zonal method of fixing should no longer be used.
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
| October 2014 | Issue 15 | Technical Extra

REGULATION AND COMPLIANCE
Who should read this: Technical and construction directors, architects, design teams and site managers.
INTRODUCTION
Type Approvals can simplify and speed up the design and approval processes, and give you earlier certainty about Building Regulations compliance. NHBC Building Control offers Type Approvals free to our building control customers. They are helpful for house, flat and garage types, and can also be used for standard details and specifications that will be repeated on different sites.
GUIDANCE
Building regulations in Wales have now devolved to the Welsh Government. As a result, Type Approval
Certificates for Regulations issued since the introduction of Part L 2014 will now include the following information: n Region: this will state whether the approval applies to England or Wales, or both England and Wales n Approved Documents (ADs): the applicable ADs will be referred to and fully listed in Annex A.
These changes will ensure that customers and NHBC surveyors and inspectors can quickly recognise the regulations and jurisdiction that the Type Approval has been designed to meet.
YOU NEED TO…
■ This article is for information. If you require any further advice please contact your
NHBC building surveyor.
For Building Regulations advice and support , call 0844 633 1000 and ask for ‘Building Control’ or visit www.nhbc.co.uk/bc
Technical Extra | Issue 15 | October 2014 |

REGULATION AND COMPLIANCE
Who should read this: Technical and construction directors, architects, design teams, site managers, building owners and managing agents.
INTRODUCTION
This article provides guidance on complying with Regulation 38 of the Building Regulations in respect of the transfer of fire safety information to the responsible person to help them operate and maintain the building with reasonable safety. Regulation 38 applies in both English and Welsh Building Regulations.
GUIDANCE
Whenever a building to which the Regulatory Reform
(Fire Safety) Order 2005 applies is built, formed by a change of use, altered or extended, compliance with
Regulation 38 of the Building Regulations (English and Welsh versions) is required.
Regulation 38 places a duty on the person who carries out the work (usually the principal contractor) to provide the responsible person (the employer, owner or person who has control of the premises as defined by Section 3 of the Order) with fire safety information to help them operate and maintain the building with reasonable safety. It is an expectation of the Order that this information will also be used by the responsible person as the basis for their fire safety risk assessment and ongoing fire safety management of the building.
The Building Regulations also place a duty on the building control body overseeing the work to take reasonable steps to ascertain that the fire safety information (as required by Regulation 38) has been provided to the responsible person before issuing a completion certificate or final certificate.
For less complex buildings, all that might be required is a set of ‘as-built’ plans of the building, detailing the following fire protection measures as stated within
Approved Document B, Appendix G: n Escape routes.
n Compartmentation and separation.
n Location of fire doors.
n Location of fire detectors.
n Any sprinkler system(s).
n Any smoke control system(s).
n Any high-risk areas.
n Specifications of any fire safety equipment .
n Any design assumption made regarding the ongoing management of the building.
n Any provisions incorporated for the evacuation of disabled persons.
For more complex buildings, or buildings incorporating fire-engineered solutions, more detailed records are likely to be required. These records should include all the items detailed above and, where appropriate, may include the following: n The fire safety strategy.
n Any assumed fire loading in the design of the fire safety systems.
n Any risk assessments undertaken.
n Any risk analysis undertaken.
n All assumptions in the design of the fire safety systems regarding ongoing management.
n Escape routes, and escape strategy, including muster points.
n All passive fire safety measures.
For Building Regulations advice and support , call 0844 633 1000 and ask for ‘Building Control’ or visit www.nhbc.co.uk/bc
| October 2014 | Issue 15 | Technical Extra

Building Regulations – Regulation 38
GUIDANCE (CONTINUED) n All active fire safety measures.
n Sprinklers systems, design and controls.
n Smoke control systems, design and controls.
n All high-risk areas and particular hazards.
n Any other details appropriate for the specific building.
Further detailed guidance is also available within
BS 9999:2008, Annex H (formerly BS 5588-12:2004,
Annex A).
The undertaking of a fire risk assessment is the cornerstone of the ongoing fire safety management of every building covered by the Fire Safety Order, and it is essential that all safety-critical information is made available to the building occupiers before occupation so that they may fulfil their duties under the Order. Fire brigades and Building Control bodies would encourage every builder to take actions to ensure that sufficiently detailed information is passed over to the responsible person, and would advise that it is far better to provide too much information about the design and construction of a building than not enough.
YOU NEED TO…
■ Be aware of your responsibilities in respect of Regulation 38. If you require any further advice, please contact your NHBC building surveyor.
For Building Regulations advice and support , call 0844 633 1000 and ask for ‘Building Control’ or visit www.nhbc.co.uk/bc
Technical Extra | Issue 15 | October 2014 |

REGULATION AND COMPLIANCE
Who should read this: Technical and construction directors and managers, architects, designers, consultants, specialist remediation companies, contaminated land professionals and land buyers.
INTRODUCTION
In 2012 revised Statutory Guidance for Part 2A of the Environmental Protection Act (1990) came into force for
England and Wales. This introduced a new four category approach for classifying land affected by contamination to assist decisions by regulators in cases of Significant Possibility of Significant Harm (SPOSH) to specified receptors, including humans, and significant pollution of controlled waters.
Following publication of the revised Statutory Guidance, DEFRA commissioned a research project to develop new
Category 4 Screening Levels (C4SLs) to provide a simplified test for regulators to aid decision-making on when land was suitable for use and definitely not contaminated land under the statutory regime.
The output from this research project was published by CL:AIRE in December 2013, with Policy Companion
Documents published in England by DEFRA in March 2014 and the Welsh Government in May 2014. The culmination of this work was the development of a framework and methodology for deriving C4SLs and the publication of final C4SLs for use as new screening values for six common contaminants. This article provides background information on the new C4SLs and their relationship to development and planning.
GUIDANCE
I n December 2013, the output of a DEFRA-funded research project (SP1010 – Development of Category 4
Screening Levels for the assessment of land affected by contamination) was published by CL:AIRE and included a draft methodology for determining Human
Health C4SLs.
In March 2014, DEFRA published a Policy Companion
Document for England to accompany the research project, which endorsed the draft methodology and framework for the derivation of C4SLs. DEFRA anticipated that regulators and risk assessors would use this in line with the Statutory Guidance. A separate Policy Companion Document was published by the Welsh Government in May 2014.
The four-category approach introduced by the revised
Statutory Guidance for Part 2A is indicated in Fig 1.
The C4SLs represent estimates of contaminant concentrations in soils that can, when used in accordance with defined risk management decisions in the methodology, be used to determine whether land is clearly not contaminated land for human receptors for specified land uses within the context of
Part 2A of the Environmental Protection Act (1990).
Risk
1
2
Point above which land is
‘contaminated land’ under
Part 2A
3
4
C4SL
SGV/GAC
Amount of land
Fig 1 – Land categories as defined by Statutory
Guidance and relationship of C4SLs and former SGVs
C4SLs therefore also have the potential to impact on the approach used by risk assessors and contaminated land professionals for assessing and remediating site(s) affected by contamination for the
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
| October 2014 | Issue 15 | Technical Extra

Publication of Category 4 Screening Levels for land affected by contamination
GUIDANCE (CONTINUED) purposes of demonstrating compliance with
NHBC Standards Chapter 4.1 ‘ managing ground conditions ’.
Land Quality –
(VI) and cadmium. In addition, the existing ‘idealised standard land uses’ of residential use (with and without) plant uptake, allotments, commercial and industrial have been extended to include two specified public open space uses.
Whilst considering the use of C4SLs, it is important to understand that the methodology used in their development represents a departure from the previous approach adopted by the Environment
Agency for the generation of the soil guideline values
(SGVs). SGVs were developed on the basis of ‘minimal’ risk levels, but in the context of Part 2A, these were generally considered to be ‘too precautionary’ to be a useful screening tool to aid in the determination of sites as contaminated land and they were withdrawn by the Environment Agency in 2011.
The methodology developed for determining C4SLs is described as being strongly precautionary, but has been based on a pragmatic approach to contaminated land risk assessment adopting an ‘acceptably low’, rather than a ‘minimal’, level of risk.
The derivation of C4SLs also introduced the concept of a low level of toxicological concern (LLTC), which represents the estimated concentration of a contaminant that would pose an ‘acceptably low’ risk to human health that, alongside modifications to exposure modelling, were used to develop the new screening criteria.
The full research project report, along with the Policy
Companion Document for England, is published by
DEFRA as Technical Report SP1010 and is available on the DEFRA website (http://randd.defra.gov.uk/Default.
aspx?Menu=Menu&Module=More&Location=None&Co mpleted=0&ProjectID=18341#RelatedDocuments). The report includes technical annexes for each of the six contaminants and details the selection of appropriate inputs, toxicological and exposure assessment parameters and statistical reviews. The report should be considered together with the Policy Companion
Document for England, which details the outputs (the final C4SLs) and the risk management (policy-based) decisions behind their derivation.
A separate Policy Companion Document was published by the Welsh Government in May 2014 and is available on the Welsh Government website
(http://wales.gov.uk/topics/environmentcountryside/ epq/contaminatedland/screening-levels-contaminatedland-assessment-/?lang=en).
One of DEFRA's requirements for the development of C4SLs was that these should allow a higher
(though still ‘acceptably low’) level of risk than the previously available SGVs, while maintaining a precautionary approach.
To test the framework and methodology developed for deriving C4SLs, the research project generated new screening values for six common contaminants: arsenic, lead, benzene, benzo(a)pyrene, chromium
Substance
Residential
(with home-grown produce)
Residential
(without home-grown produce)
The Policy Companion Documents also provide guidance to risk assessors on the consideration of normal background concentrations of contamination in England and Wales when using C4SLs. The full reports are considered essential reading for risk assessors and contaminated land practitioners, particularly where C4SLs are being considered for use on new residential development(s).
There is currently no change to the existing regimes for land affected by contamination in Northern Ireland and Scotland.
Allotments Commercial
Public open space (near residential)
Public open space (parks)
Former residential
(with plant uptake)
SGV/GAC (for comparison purposes)
Arsenic 37 40 49 640 79 168 32
Cadmium 26 149 4.9
410 220 880 10
Chromium (VI)
Lead
Benzo(a)pyrene
Benzene
21
200
5
0.87
21
310
5.3
3.3
170
80
5.7
0.18
49
2330
76
98
23
630
10
140
250
1300
21
230
4.3
450
0.83
0.33
Table 1: Category 4 Screening Levels (including previously published SGVs/GACs for residential land use for comparative purposes); quoted concentrations are presented as mg/kg.
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 |

Publication of Category 4 Screening Levels for land affected by contamination
GUIDANCE (CONTINUED)
The final C4SLs developed during the research project for the six common contaminants considered are shown in Table 2 on page 15. The previous SGVs for residential end use with plant uptake have also been included for comparison purposes.
It can be seen that the framework developed using the new C4SL methodology has provided screening values for England and Wales for the contaminants considered that are above the previously used, and withdrawn, SGV values. The exception to this is lead, as the report concluded that current evidence from worldwide toxicological studies and blood testing suggested that the withdrawn SGV level did not represent an ‘acceptably low’ risk to human health.
The Policy Companion Documents published for
England and Wales consider ‘wider issues’ in the content of the C4SLs, including their relationship with the planning regimes.
For England, the Policy Companion Document advises that the C4SLs were developed on the basis that they could be used under the planning regime as they would be in investigations under Part 2A of the
Environmental Protection Act 1990, but that planning policy falls within the remit of the Department for
Communities and Local Government (DCLG). In June
2014, the Planning Practice Guidance for England
(http://planningguidance.planningportal.gov.uk/ revisions/33/007/) was updated and placed on the
Planning Portal by DCLG. It makes reference to the
Policy Companion Document published by DEFRA and the use of C4SLs in providing a simple test for deciding when land is suitable for use and definitely not contaminated land. This position was affirmed in
DEFRA correspondence to local authorities on
3 September 2014, stated “revision of the Part 2A
Statutory Guidance was developed on the basis that
C4SLs could be used under the planning regime as well as within Part 2A” and that “exceeding a C4SL means that further investigation is required, not that the land is necessarily contaminated”.
For Wales, the Policy Companion Document states that C4SLs may provide a useful means of assisting local planning authorities in deciding whether land is suitable for its proposed use.
In Northern Ireland and Scotland, there is currently no change to the current requirements for consideration of land affected by contamination under the respective planning regimes.
The respective Policy Companion Documents for
England and Wales provide the background to the development of C4SLs and the final published screening values for the six selected contaminants.
The documents also provide guidance on the use of
C4SLs by risk assessors and regulators when assessing land affected by contamination. In relation to the use of C4SLs for residential developments,
NHBC considers that: n C4SLs may be used for schemes in England and Wales as generic screening levels for contaminants in soils, as long as they are justifiable and defensible in the conceptual site model for the site. Where representative contaminant concentrations exceed C4SLs, remediation or further detailed assessment will normally be required.
n Developers should, however, check that the use of C4SLs would be accepted by regulators under the relevant planning regime.
n In Scotland and Northern Ireland, care should be exercised, as a ‘minimal risk' level is used by regulators in these countries. Generic Assessment
Criteria (GAC) using a 'minimal risk' approach are therefore still likely to be the starting point, and developers would need to seek confirmation from regulators in these countries on their approach to lead.
n Where a land use scenario covered by a C4SL applies in England and Wales, that use of C4SLs will satisfy NHBC Standards – Chapter 4.1 requirements. For lead, the C4SL value should be adopted as the screening level, though normal background concentrations can be considered when appropriate.
n When the site or development deviates from the standard land uses or risk management assumptions covered by C4SLs, a site specific risk assessment should be undertaken to derive appropriate site-specific screening values for risk assessment and management purposes.
n Developers may still elect to take a more precautionary approach in line with their own risk management and liability policies (i.e. screening levels lower than C4SLs).
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
| October 2014 | Issue 15 | Technical Extra

Publication of Category 4 Screening Levels for land affected by contamination
GUIDANCE (CONTINUED) n Where C4SLs do not exist for a contaminant, there are still around 132 publically available GACs for use from recognised land contamination sector bodies. Additionally, the C4SL methodology could be used to develop screening values for other contaminants. n Though SGVs were withdrawn by the Environment
Agency in 2011 for purposes of determining land as contaminated, they may be appropriate as reference levels. These are generally precautionary and conservative assessment criteria as they are based on ‘minimal’ rather than ‘low’ risk.
YOU NEED TO…
■ Be aware of the changes to the Contaminated Land Statutory Guidance (England and Wales) and the supporting guidance that regulators will use when determining whether land is contaminated land under Part
2A of the Environmental Protection Act 1990.
■ Be aware that the framework and methodology used to generate C4SLs represents a departure from the previous approach adopted by the Environment Agency for the generation of SGVs – SGVs were based on
‘minimal’ risk levels, whereas C4SLs represent an ‘acceptably low’ risk.
■ Be aware that there are differences between regulatory approaches under planning for different countries in the UK, and check that C4SLs will be accepted by the regulators under the relevant planning regime.
■ Ensure that suitable consultants (conversant with the approach adopted for generating C4SLs) are appointed for site investigation, risk assessments, remediation and validation, to ensure that the land for new developments is appropriately assessed and remediated against appropriate standards.
■ Ensure that your developments are assessed on a site-by-site basis for contamination, including the development of a suitable conceptual site model basis using appropriate screening values for risk assessment and the development of remediation strategies.
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 |

GUIDANCE AND GOOD PRACTICE
Who should read this: Technical and construction directors and managers, architects, designers, consultants, manufacturers, specifiers and purchasers.
INTRODUCTION
Because of their exposed position on dwellings, brick chimneys are at high risk of rainwater penetration and frost damage. Frost can damage both the mortar and the bricks themselves if they are not sufficiently frost resistant.
The greatest risk is the brickwork at the top of a chimney and, once damage has occurred, it is often difficult and costly to rectify due to problems with gaining access to carry out repairs. In this article, we discuss these issues and how to address them.
GUIDANCE
Careful consideration at the design and material selection stages, together with correct installation on site, can significantly reduce the risk of water penetration and avoid saturation and subsequent frost damage. Guidance on the provision of dampproof trays and flashings, where a chimney abuts or passes through a roof, is provided in NHBC Standards
Chapter 6.8. Examples of chimney terminals and cappings are also given, and what follows is a more detailed explanation on the selection and construction of both copings and cappings to provide good weather protection.
NHBC Standards uses the term ‘capping’ for both copings and cappings, but strictly speaking, they are two different details and provide different levels of protection. BS 5642 ‘Sills, copings and cappings’ defines copings and cappings as:
Coping – construction that protects the top of a wall and sheds rainwater clear of the surfaces beneath.
Capping – construction that protects the top of a wall, but does not shed rainwater clear of the surfaces of the wall beneath.
A coping should have a projection beyond the brickwork together with a drip which stops water tracking back under the projection and on to the brickwork. A capping, comprising any projecting brickwork and a flaunching as described later, lacks a drip feature and rainwater runs over the face of the brickwork beneath.
Copings which throw rainwater away from the brickwork are the preferred option. Ideally, a coping should be in one unit to avoid having vertical joints through which rainwater will eventually penetrate.
If a jointed coping is unavoidable, it should be bedded on a DPC which should in turn be bedded onto the brickwork below. Likewise, if a natural stone coping is used, which could be slightly porous, it too should be bedded on a DPC.
Because water will penetrate brickwork laid horizontally, or at a low pitch, brick copings or cappings, without a flaunching as described below, should not be used to finish the top of a chimney stack, even if a DPC is incorporated beneath the brickwork. Such detailing is at a high risk of becoming saturated and defective through frost action.
Although a projecting coping can offer the best protection to the brickwork, cement flaunching has been a traditional means to cap chimneys. Although not the preferred option, if flaunching is selected, it needs to have good tensile strength and durability.
A 1:3 cement sharp sand mix is therefore recommended and should be trowelled to form a smooth hard surface. The junction between the flaunching and brickwork should be at least 25mm thick to resist edge spalling, and because flaunching can develop fine cracks over time, with the potential for rainwater penetration, it should be laid over a bedded DPC to protect the brickwork beneath.
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
| October 2014 | Issue 15 | Technical Extra

GUIDANCE (CONTINUED)
Flaunched chimneys often have projecting or corbelled feature brickwork at or near the top of the stack. Corbelling cannot be relied upon to act as a coping; because it lacks a drip feature, it can only be considered as a capping. Where brickwork corbels inwards, it forms an exposed horizontal ledge which can hold water and increase the risk of saturating the stack. Such ledges should be weathered by applying a
1:3 cement sharp sand fillet, as shown in Fig. 1.
To assist the keying in of the fillet, the mortar bedding to the brickwork behind should be racked back slightly.
Projecting brick surface weathered with fillet of mortar
1:3 (cement sharp sand) mix
Projecting brick surface weathered with fillet of mortar 1 : 3 (cement : sharp sand) mix
Unacceptable detail where projecting brickwork forms a ledge that holds water
Unacceptable detail where projecting brickwork forms a ledge that holds water.
n Pointing should be bucket handle or weather struck. Recessed and projecting joints which could hold water should not be used.
The diagrams (Fig. 2) illustrate the principal features that should be considered to ensure the tops of brick chimney stacks are constructed to avoid saturation of the brickwork and thereby reduce the risk of damage through frost action. The advice of the brick manufacturer on the selection of suitable bricks for chimney stack construction should be sought at an early stage.
Flue liner or chimney pot inserted 125mm min. or quarter of the length of the terminal into the stack, excluding the depth of the flauching
Mortar flaunching 1:3 (cement sharp sand) with smooth finish and falls to shed water
PROJECTING BRICKWORK DETAIL
Fig. 1 – Projecting brickwork detail
In addition to the above, the following features should be included: n Bricks used in a chimney stack should be frost resistant (F2). n Cappings should be laid in a durable mortar mix
M12 [designated mix (i)]. The same mortar mix should be considered for the whole stack. n M6 mortar [designated mix (ii)] may be suitable for brickwork protected by a coping.
n Capping mortar if smoke billowing engulfing the chimney stack is likely to occur.
n All mortar joints should be fully filled. n Where there is a risk of regular wetting of the brickwork, a low sulfate-resistant brick (S2) is advised to reduce efflorescence and sulfate attack.
25mm min.
High bond DPC bedded in mortar
Capping
Frost resistant brickwork
F2, sulfate S2, mortar
M12, i.e. [designation (i)] or as recommended by brick manufacturer
M12 mortar or cement grout joint between coping and flue/pot
Pre-cast coping, (preferably a single unit). Any joints in coping to be made with M12 mortar
Provide high bond DPC bedded in mortar where a jointed or porous coping is used
* Coping
30mm
50mm
* 50mm min. for concrete, cast stone or natural stone
Insulated concrete fill
Bucket handle or weather struck pointing (no recessed or projecting joints)
Fig. 2 – Section through chimney with flaunched capping or pre-cast coping (chimney stack construction may vary)
YOU NEED TO…
■ Apply the design principles described above and select the correct quality of bricks and mortar for use in the construction of brick chimney stacks.
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 |
30mm
50mm
*
*
Coping

GUIDANCE AND GOOD PRACTICE
Who should read this: Technical and construction directors and managers, architects, designers, manufacturers, specifiers and purchasers.
INTRODUCTION
This article draws attention to the use of beam and block suspended concrete floors in residential construction, and the need to ensure that adequate structural performance of floors is not impaired by the method of achieving adequate levels of thermal insulation.
GUIDANCE
Suspended beam and block floors comprising precast concrete beams and infill blocks with concrete topping is a popular type of residential floor construction in the UK. First introduced in the late 1970s, its use is currently estimated to account for approximately
40% of the market. Suspended concrete ground floors, when combined with an insulation material such as polystyrene infill blocks or as sheets laid over the top of blocks, can provide suitable thermal insulation, but adequate structural performance of the floor system should not be impaired by the chosen method of satisfying thermal insulation requirements.
Specifications for these floors are covered by the harmonised European Standard BS EN 15037 that is published in five parts. Part 1 (BS EN 15037-1) covers concrete beams, Parts 2 to 5 cover a range of block types, i.e. concrete blocks, clay blocks, expanded or extruded polystyrene blocks and lightweight blocks for simple formwork, respectively. However, this
Standard does not include any guidance for concrete topping.
The category of beam and the type of infill block specified for the floor will affect the specification of the concrete topping. This article is restricted to the use of self-bearing and non-self-bearing beams covered by BS EN 15037-1 and concrete or polystyrene blocks, covered by BS EN 15037 Parts
2 and 4, respectively.
Precast concrete beams may be either reinforced or pre-stressed normal weight concrete according to
BS EN 1992-1-1:2004.
BS EN 15037-1 defines the beams as ‘self-bearing’ or
‘non-self-bearing’. Performance requirements are described below. Guidance on appropriate structural floor systems using concrete or expanded polystyrene blocks is given in Tables 2 and 3 of this article.
Self-bearing Reinforced or pre-stressed concrete beams, which alone provide the final strength of the floor independent of any other constituent part of the system (i.e. blocks or structural screed).
Non self-bearing
Reinforced or pre-stressed concrete beams, which provide the final strength of the floor system in conjunction with cast in situ concrete screed and, possibly, with the top of the blocks.
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
| October 2014 | Issue 15 | Technical Extra

GUIDANCE (CONTINUED)
BS EN 15037-2 covers concrete blocks made of normal or lightweight aggregate concrete. Concrete blocks are defined as non-resisting (NR), semi-resisting (SR) and resisting (RR). Performance characteristics, described below. Guidance on appropriate structural floor systems incorporating concrete blocks is given in
Tables 2 and 3 of this article.
NR Perform no mechanical function in the final floor system, but act as formwork during construction of the floor.
SR
RR
Contribute to the mechanical function of the final floor system.
Contribute to the mechanical function of the final floor system.
BS EN 15037-4 covers EPS blocks defined as type R1
(R1a or R1b) or type R2. Performance characteristics are described below. Guidance on appropriate structural floor systems incorporating EPS blocks is given in Tables 2 and 3 of this article.
R1
R2
Perform no mechanical function in the final floor system, but may act as formwork during construction of the floor.
Contribute to the mechanical function of the floor system.
1.5
2.0
2.5
R1 (a & b)
R2
-
Table 1 – Approximate equivalent performance in terms of characteristic resistance between concrete and EPS blocks.
Four types of non-structural screed can be used: sand:cement screeds; enhanced sand:cement screeds; anhydrite screeds, and liquid cementitious screeds.
If insulation material is laid over the top of the blocks, the compressibility of the insulation material needs to be verified, and the minimum thickness of screed recommended is 65mm. If underfloor heating is used, the minimum cover above the pipes for the particular system needs to be met.
All of the above screeds can incorporate the use of polypropylene micro-fibres to control shrinkage cracking, provided the specification is substantiated by an independent assessment of use.
Table 2 gives the choices for concrete and EPS blocks when used with self-bearing beams and non-structural screed.
Structural screeds should be designed by suitably qualified persons in accordance with BS EN 206:2013 and its Complementary British Standard BS 8500-
2:2006. Reinforcement consisting of welded steel mesh should be in accordance with BS EN 15037-1.
Alternatively, an independent assessment of the beam and block flooring, including the structural concrete topping, may be acceptable (see Note 3 to
Tables 3 and 4).
In either case, structural screeds reinforced with microfibres will not be acceptable to NHBC, but polypropylene macrofibres or steel fibres could be acceptable for use as reinforcement.
Tables 3 and 4 give the choices for concrete and EPS blocks when used with non-self-bearing and selfbearing beams respectively, together with cast in situ structural screed.
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 |

GUIDANCE (CONTINUED)
Self-levelling or self-compacting concrete can also be used and should meet the minimum requirements of standard concrete, including the provision for reinforcement.
Since BS EN 15037 is a harmonised standard, the CE marking for both the beam and the block is acceptable.
The CE marking should be based on the harmonised standard with evidence on satisfying the required
Assessment and Verification of Constancy of
Performance (AVCP), formerly known as the attestation of conformity system level, as specified in
Table ZA.2 of the respective parts of BS EN 15037.
Where a floor is required to act as a diaphragm, the guidance given in BS EN 15037-1 Annex G should be followed.
For all types of infill blocks where the floor is to be used within a garage, the concrete shall be reinforced with a minimum A142 mesh to resist a point load of
10kN minimum.
Tables 2 and 3 give the choices for beams and blocks when used with non-structural and structural concrete toppings respectively, in accordance with BS EN 15037.
Table 4 provides an option for beams and blocks when used with structural concrete topping, but not specifically covered by BS EN 15037.
Blocks
Precast concrete beam type Block material
Performance type (min) (1)
Notes on the structural floor system
Self-bearing
Concrete
SR
RR (ungrouted)
R2
The precast beams alone provide the final strength of the floor system. The blocks participate in the transfer of floor loads to the beams. The finished surface will be non-structural
(e.g. screed, wood or floating screed).
EPS
Table 2 – Guidance on components for beam and block floor systems with self-bearing beams and non-structural screed as concrete topping.
Precast concrete beam type Block material
Blocks
Performance type (min) (1)
Notes on the structural floor system (3)
Concrete
EPS
NR
R1
The beams act in conjunction with the cast in situ structural concrete topping to provide the final strength of the floor system. The concrete topping forms the compression flange of the floor. The blocks perform no mechanical function in the final floor system, but will act as formwork during the construction of the floor.
Non self-bearing
Concrete RR
The beams act in conjunction with the cast in situ structural concrete topping to provide the final strength of the floor system. The concrete topping acts compositely with part of the top of the blocks to form the compression flange of the final floor system as they participate in the transfer of loads.
Table 3 – Guidance on components for beam and block floor systems with non-self-bearing beams and structural screed as concrete topping.
| October 2014 | Issue 15 | Technical Extra

GUIDANCE (CONTINUED)
Precast concrete beam type Block material
Blocks
Performance type (min) (1)
Notes on the structural floor system (3)
Self-bearing EPS R1 (2)
The precast beams alone provide the final strength of the floor system. The blocks perform no mechanical function in the final floor system, but will act as formwork during the construction of the floor. The concrete topping participates in the transfer of floor loads to the beams.
Table 4 – Guidance on components for beam and block floor systems with self-bearing beams and structural screed as concrete topping.
NOTES TO TABLES 2, 3 AND 4
1. Tables 2, 3 and 4 show the minimum strength of block required to perform the function indicated in the right hand column of the table.
A stronger block may be used, but no improvement in performance may be assumed in the design of the floor system.
2. The use of self-bearing precast concrete beams with EPS blocks type R1 is not covered by BS EN 15037. However, for residential purposes only,
NHBC will accept type R1 classification EPS blocks used in conjunction with self-bearing beams and cast in situ structural concrete topping for residential suspended ground floor construction, provided that the adequacy of the complete structural floor system (including beams, blocks and structural concrete screed) has been satisfactorily verified by calculation to BS EN 1992-1-1, together with full-scale testing (details to be agreed with NHBC prior to testing) by an appropriate independent technical approval authority and in accordance with NHBC Standards Technical
Requirement R3.
3. In all situations where the cast in situ concrete topping is required to perform the function of a structural screed, the reinforcement of the topping should consist of welded mesh in accordance with BS EN 15037-1. Alternatively, cast in situ structural concrete topping reinforced with polypropylene macrofibres or steel fibres may be acceptable to NHBC for residential suspended ground floor construction, providing that the adequacy of the complete structural floor system (including beams, blocks and structural concrete screed) has been satisfactorily verified by calculation to BS EN 1992-1-1, together with full-scale testing (details to be agreed with NHBC prior to testing) by an appropriate independent technical approval authority in accordance with NHBC Standards Technical Requirement R3.
YOU NEED TO…
■ Select the appropriate types of pre-cast concrete beam and block to suit the structural and thermal requirements of the suspended floor construction and ensure that the beam type is compatible with the block type specified, in accordance with BS EN 15037, Part 1 and Part 2 or 4 as appropriate.
■ Ensure that the design of the screed is structurally suitable for use with the beam and block system specified.
■ As an alternative to the first two bullet points above, and for residential purposes only, NHBC will accept structural suspended beam and block floor systems verified in accordance with the guidance given in Note 1 to
Tables 2 and 3.
■ Ensure that the design of the whole floor construction fulfils the structural requirements for its position in the building in accordance with BS EN 1992-1-1.
■ Ensure that all elements of the floor construction are installed in accordance with the manufacturers’ technical information.
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 |

Who should read this: Technical and construction directors and managers, architects, designers, consultants, manufacturers, specifiers and purchasers.
INTRODUCTION
Quick to install, reliable and robust, low maintenance and aesthetically pleasing, it’s not difficult to see why off-site fabricated GRP systems are a popular choice for building designers when considering dormer windows, but there are a few common pitfalls to avoid.
GUIDANCE
Standards Extra 44 (April 2009) raised concerns with the design, manufacture and installation of GRP products used to form building elements, including dormer roofs. Subsequently, the Construction Glassfibre
Manufacturers Association (CGMA) was formed, as reported in Technical
Extra 07 (July 2012). Most of the major manufacturers now have products that comply with the requirements of the CGMA scheme, or alternatively hold independent third-party assessments.
The good news is that there seems to be a marked improvement in the quality of products available and also in the way they are installed. However, feedback from NHBC’s Inspection Service suggests a number of instances where inspectors have found the detailing of the interface between the roof covering and the GRP dormer has been less than satisfactory. Without the intervention of our inspectors, these poor installations would have likely resulted in water ingress, and the possibility of costly repairs for the builder,
NHBC or perhaps, in the longer term, for the homeowner.
Common defects include the underlay not being finished correctly and being left to hang down into the rafter or roof void, and battens and tiles abutting the dormer incorrectly. These could have easily been avoided if the manufacturer’s installation instructions had been followed.
A reoccurring theme indicates that the installation instructions are not available at the time of installation, which creates unnecessary risk. The correct detailing of underlay, tiles and battens at interfaces is critical to the long-term performance of the roof. The detailing will vary between manufacturers, and therefore it is important that the product-specific instructions are followed. Leaving these details to ‘get over’ on site could significantly increase the risk of failure.
YOU NEED TO…
■ Ensure that GRP dormers have a third-party assessment or alternatively a CGMA certification.
■ Ensure fitting instructions are available at the time of installation and are followed for all phases of the construction, including the roof covering.
| October 2014 | Issue 15 | Technical Extra

GUIDANCE AND GOOD PRACTICE
Who should read this: Technical and construction directors and managers, architects, designers and site managers.
INTRODUCTION
In Technical Extra Issue 11 September 2013, we ran an article on audible cracking noises in intermediate floors that can occasionally occur in a small number of homes. This article recaps research work that NHBC, Gypsum
Products Development Association and UK Timber Frame Association jointly funded to establish the cause and possible solutions to the problem.
GUIDANCE
Research was carried out by Salford University
Acoustics Research Centre and involved both laboratory and on-site investigations using homes where the problem had occurred. The results showed that the source of each cracking sound emanated from locations where the drylining adhesive had extended upwards and made contact with built-in joist ends. The recommendation following the research was to ensure that the gypsum plasterboard adhesive did not spread up to set in contact with the floor joists, or where a cracking noise was apparent, to form a cut through the adhesive to debond the wall and ceiling drylining.
The various drylining installation instructions from plasterboard manufacturers show the adhesive dabs or ribbons at the top of the walls are kept down from ceiling. Drylining installation methods found on sites would suggest that not all dryliners have been achieving this gap, either because they have installed the adhesive tight to the ceiling or the adhesive has spread up too far when the plasterboard sheets have been installed.
To achieve a gap between the adhesive and the ceiling, it is suggested that the adhesive be kept
100mm down from the ceiling which, on application of the plasterboard, should still maintain the required isolation from the ceiling. Fig. 1 shows a suggested drylining detail at the wall/ceiling junction that has been adopted by some contractors with apparent success.
Other feedback suggests that, where remedial work has been carried out to ceilings with noise issues, plasterboard not being fixed tight up to the floor joists was a contributory factor. Fixing the plasterboard tight to the joists resolved that noise problem.
Clear gap between adhesive and ceiling board after installation of drylining
A continuous ribbon of adhesive approximately 100mm clear of the ceiling board prior to application of plasterboard
(the ribbon may coincide with the ribbon around window and door openings)
Fig. 1 – Section through a drylined wall/ceiling junction at an intermediate floor within a dwelling
YOU NEED TO…
■ Ensure the plasterboard adhesive is kept far enough down from the top of the wall to ensure the adhesive does not spread up to contact the ceiling board and joist ends. A distance of 100mm between the adhesive and the ceiling would be a reasonable distance to adopt. Where a ceiling already has a noise problem that it is suggested that checking the fixing of the existing plasterboard ceiling would be an appropriate first course of action.
For technical advice and support , call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 14 | May 2014 |

In response to the growing demand for the NHBC Home
User Guide (HUG), we’ve announced a further series of seminars to demonstrate this unique, online system and showcase what it can offer to both builders and homeowners.
The NHBC Home User Guide (HUG for short) is the ultimate online home management tool and is available free for every new plot registered with us for Buildmark cover. It’s co-branded, accessible from computer or tablet, and enables our registered builders to provide new home buyers with all the information they need to move in and run their new home.
Date Location
24 November 2014 Bristol
28 November 2014 Milton Keynes
1 December 2014 Cumbernauld, Scotland
4 December 2014
8 December 2014
11 December 2014
Nottingham
York
Croydon
For 23 years, Building for tomorrow (Bft) has been informing the industry on topics that directly impact current and future house building.
Ahead of further information and an agenda being published in the autumn, NHBC is pleased to announce the dates for next year’s Bft seminars.
Date
26 February 2015
5 March 2015
10 March 2015
12 March 2015
17 March 2015
24 March 2015
26 March 2015
14 April 2015
16 April 2015
Location
Shendish Manor, Hemel Hempstead
Thistle Haydock Hotel, Haydock
Leigh Court, Bristol
Sandown Park Racecourse, Esher
York Racecourse, York
Westerwood Hotel, Cumbernauld
Cambridge Belfry, Cambourne
National Motorcycle Museum,
Birmingham
Hilton Templepatrick, Belfast
For some time, NHBC has been developing a significant improvement to the way we work with builders to manage site and plot information. This was driven by feedback from our customers that we needed to make it much easier for you to work with us. We launched the NHBC
Portal in May this year, as an upgrade to the NHBC
Extranet. The response has been very positive; it saves you, our registered builder customers, considerable time and effort. As most builders have moved to the NHBC
Portal, we will be de-commissioning the Extranet in
November 2014. If you haven’t moved across, we therefore now need you to register for the
NHBC Portal as soon as possible – please go to www.NHBC.co.uk/PortalLogin.
If you are not familiar with the NHBC Portal, we wish to encourage you to share the success. The Portal enables you to check the status of your plots and sites online, making it much easier to manage them. You can now: n See any Reportable Items and other builder responsible items. By showing this online, it is much easier and quicker for any issues to be seen and resolved.
n Upload all the documentation about the site such as drawing, schedules and technical data, rather than waiting for the post and for them to be scanned.
n See and change plot details, such as selling price, fabrication, etc. , making sure this is up to date and track Plot Product Schedules (PPS).
forms, so we can provide you with instant quotes for Warranty and Building Control.
n Download management reports about any or all of your sites; the consolidated reports for all your sites
(or across all companies) make it easier to see site status at any time of your choice.
The NHBC Portal is provided to you as a registered builder. It is a free service and is available 24 hours a day.
The feedback has been that it is a comprehensive portal; it greatly improves our service to you, enabling you to manage your sites easily and securely, reducing your time and costs.
To see a video about the NHBC Portal and sign up, click www.nhbc.co.uk/portallogin
| October 2014 | Issue 15 | Technical Extra

As advised in the August edition of NHBC’s Clicks &
Mortar, the British Association of Reinforcement (BAR) has recently issued a compliance alert after independent tests identified some imported bars failed to comply with BS 4449.
The bars were found not to comply with the geometrical requirements of the British Standard. BAR has advised its members always to check rebar material for British
Standard compliance.
Action: visit the BAR website below for a copy of the alert in full at www.uk-bar.org/news/41.htm
Click below to view previous editions and subscribe to
NHBC’s monthly technical newsletter Clicks & Mortar www.nhbc.co.uk/NewsandComment/Joinournewsletter/
Pressure to increase the safety of automatic gates has intensified following a number of incidents involving children. Designers and builders should consider the following guidance when specifying powered residential driveway gates.
The Door & Hardware Federation (DHF) Powered Gate
Group represents the UK’s leading designers, manufacturers, installers and maintainers of powered gates. The specialist Powered Gate Group was formed with encouragement from the Health and Safety
Executive (HSE), which was anxious to see an industrywide trade body responsible for developing higher standards of safety for automated gates.
The first output was the publication of a comprehensive guide to powered gate safety to provide appropriate best practice technical guidance to the industry – the DHF
Guide to Gate Safety Legislation and Standards . The guide lists all of the current published standards for powered gate performance and the design requirements that should be followed to ensure every gate installation is safe. The guide advises manufacturers on the safe design and manufacture of safe powered gates and also makes recommendations on current safety solutions which go beyond the obligations set out in current standards.
The guide is available at: http://www.dhfonline.org.uk/ docs/1053-Gate-Guide-CURRENT.pdf
For maximum safety, not only must powered gates be designed and manufactured to the highest standards, they must also be correctly installed and maintained.
Member companies of the Powered Gate Group ensure that their key personnel are sent on an intensive two-day
DHF Powered Gate Safety Diploma training course. It is a condition of DHF membership that all members supplying powered gates send their relevant personnel through the training course.
Only when the training has taken place can member companies apply the DHF Safety Assured mark to their installations. This mark on a powered gate assures the specifier, owner and user of the gate that the company supplying it has: n Undergone the rigorous gate safety training programme n Achieved the Powered Gate Safety
Diploma qualification n Signed up to a rigorous and binding code of conduct n Committed to comply with or exceed all current safety standards.
For more information on the DHF Powered Gate Group, visit www.dhfpoweredgategroup.co.uk
Action: ensure automatic gates are correctly designed, manufactured and installed.
Building Information Modelling (BIM) is widely seen as the construction industry's answer to greater efficiency and improvement. Whilst some house builders are starting to realise the benefits of BIM, others are taking a more cautious approach.
The BIM4 Housing group, which is formed from a wide range of industry representatives to look at issues with the use of BIM specific to house building, is now holding a series of conferences across the UK. Speakers from a range of disciplines involved with BIM will be drawing on shared practical experience to dispel some of the myths and uncertainties around BIM to help builders make a more informed decision on its use.
A number of one-day conference dates remain: n Birmingham – 30 October n London – 12 November n Edinburgh – 8 December
Conference places can be booked here: http://constructingexcellencesw.org.uk/events
Technical Extra | Issue 15 | October 2014 |

Tel: 01908 747384
Email: technical@nhbc.co.uk
Web: www.nhbc.co.uk/builders/technicaladviceandsupport
Previous editions of Technical Extra are available on our website at www.nhbc.co.uk/Builders/ProductsandServices/
TechnicalExtra/
Buy online at: www.nhbc.co.uk/nhbcshop/technicalstandards or access the new digital format Standards Plus via the
NHBC Extranet at: www.nhbc.co.uk/builders/NHBCExtranet
For guidance on issues relating to Building Regulations, please visit NHBC’s TechZone at www.nhbc.co.uk/techzone
For Building Control queries, please call
0844 633 1000 and ask for ‘Building Control’ , or email buildingcontroladmin@nhbc.co.uk.
For Engineering queries, please call 0844 633 1000 and ask for ‘Engineering’ .
The NHBC Foundation facilitates research and shares relevant guidance and good practice with the house-building industry. www.nhbcfoundation.org
For information about training, please go to www.nhbc.co.uk/training, call 0844 633 1000 and ask for ‘ Training ’, or email training@nhbc.co.uk.
The UK Government has set out an ambitious plan for all new homes to be zero carbon from 2016. The Zero Carbon
Hub helps you understand the challenges, issues and opportunities involved in developing, building and marketing your low and zero carbon homes.
www.zerocarbonhub.org
NHBC regularly distributes information on a range of industry topics, including new products and services, the building industry market, house-building news and house-building statistics. To receive this industry information, please register at: www.nhbc.co.uk/newsandcomment/registerfore-news
For all other enquiries, including ordering products and services, please call 0844 633 1000, and ask for ‘Sales’ .
Copyright© NHBC 2014
NHBC is authorised by the Prudential
Regulation Authority and regulated by the Financial Conduct Authority and the Prudential Regulation Authority.
This leaflet has been printed on material which is produced from well-managed forests and is fully recyclable and biodegradable,
ECF (elemental chlorine free) and is made to ISO 14001
Environmental Certification.
NHBC, NHBC House,
Davy Avenue, Knowlhill,
Milton Keynes,
Bucks MK5 8FP
Tel: 0844 633 1000
Fax: 0844 633 0022 www.nhbc.co.uk
HB2801 10/14