Building Control
Charnwood Borough Council
Southfield Road
Loughborough
LE11 2TN
BUILDING CONTROL GUIDANCE SHEET
USE OF BREATHABLE ROOFING FELTS.
Last updated 16/11/2010
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PURPOSE:
This Guidance Note is for the benefit of local architects, builders and the general public.
Its purpose is to provide information, promote good practice and encourage consistency
of interpretation for all. It is purely advisory in nature and does not cover every aspect
of the topic concerned, but tries to cover the main, commonly encountered points. If
more details are required then the relevant Approved Documents, British Standards or
manufacturers’ instructions should be consulted. The Guidance Note is not intended to
outline the only way of carrying out the type of work referred to. If in doubt, please
contact your local Building Control Authority.
This document should not be submitted as part of any Building Control application.
The Council offers a complete Building Control service, from pre application advice on
technical aspects and how to make an application, through to plan checking and an
inspection regime to ensure a safe and compliant building. We also offer an Energy
Performance Assessment service which uses SAP methodology to produce Energy
Performance Certificates (EPC’s) in respect of new and existing buildings. Please
contact Building Control for any further information:
Building Control Contact Details:
General enquiries:
Tel:
01509 634757
01509 634924
01509 634749 (24 hour answer machine)
Fax:
01509 260536
Email: building.control@charnwood.gov.uk
Web:
www.charnwood.gov.uk/pages/buildingcontrol1
Further information can be obtained from The Building Regulations 2010 or from the
Council’s Building Control Service on request. The views expressed in this document
are those of Charnwood Borough Council and do not necessarily represent a
guaranteed methodology for compliance with the requirements of the Building
Regulations 2010. Charnwood Borough Council accepts no liability for any claim that
may arise in relation to reliance on the information contained in this document.
1.0 INTRODUCTION
1.1 This document applies to dwellings with a cold or warm roof of traditional construction.
This does not apply to where a breathable membrane is used with cladding.
1.2 Please read all of the questions below.
1.3 Breathable roofing membrane has been used in building for many years now and, more
recently, in cold pitch roof construction without traditional eaves ventilation. The benefit of
reduced heat loss and not having to incorporate ventilators has seen their use grow but at
the same time has attracted noticeable comment both from supporters of the application
and from detractors who favour traditional ventilation solutions.
2.0 TYPES OF BREATHABLE MEMBRANES ARE DEFINED AS FOLLOWS,
2.1 Breathable membranes have a water vapour resistance of less than or equal to 0.6MN.s/g
a) High water vapour resistance membrane (type HR) has a water vapour resistance
greater than to 0.25MN.s/g
b) Low water vapour resistance membrane (type LR) has a water vapour resistance less
than 0.25MN.s/g
c) This document applies to type LR only - in light of this the following document has been
prepared to try to answer some of the common questions raised regarding using this
type of construction.
3.0 WHAT IS A ROOFING MEMBRANE AND HOW DOES IT WORK?
3.1 The particular membrane in question is typically a micro porous membrane which has a
structure sufficiently fine to prevent liquid water penetration in service conditions, but not
too fine to prevent water vapour (molecules of water in air) penetration.
3.2 All materials, including vapour control layers, have a finite permeability to water vapour
transfer.
3.3 However, in the context of this article, a breathable roofing membrane is a material
which, in service conditions, is sufficiently permeable to water vapour transfer to adequately
limit the risk of condensation in roofs.
3.4 The function of these membranes is to provide all of the normal demands made on a
roof underlay and at the same time allow water vapour egress without making specific
provision for ventilation.
4.0 WHEN DO I NEED TO PROVIDE VENTILATION TO A ROOF FITTED
WITH A BREATHABLE MEMBRANE?
4.1 It is often wrongly assumed that the breathable membrane is a straightforward
replacement for the sarking or more traditional roofing felt normally used to the roof
structure and once fitted there is no need to ventilate the roof space.
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4.2 As a general rule of thumb however it will be necessary to provide ventilation to a roof
void fitted with breathable felt under the following circumstances,
a) The breathable membrane is fitted tight across the rafters and no counter battens are
fitted.
b) The new roof fitted with the breathable membrane is linked to an existing roof fitted
with traditional felt or the extension results in a combination of different breathable
membranes being used.
c) The new roof fitted with the breathable membrane is linked to an existing roof which is
already vented.
d) The building does not fully comply with the requirements of Approved Document F,
ventilation or BS5250, Control of condensation in buildings.
e) Tight fitting slates and tiles are used.
f) Where it is recommended by the manufacturers or where the BBA certificates requires
ventilation. Some breathable felt manufacturers recommend ventilation at ridge level.
g) BBA or BRE certificates not complied with fully
5.0 CAN THE ROOFING MEMBRANE BE USED ‘SIMPLY’ INSTEAD OF
CONVENTIONAL EAVES VENTILATION?
5.1 Only if a number of measures are taken to limit the ingress of water vapour into the
unvented space. In this regard, the roof system must be considered in its entirety, from
ceiling to roof tiles, including the following design points:a) All penetrations into the roof space must be properly sealed
b) Loft hatches must incorporate effective compressible draught seals
c) The rooms below the ceiling must include provision for the dispersal and rapid dilution
of water vapour in accordance with the Building Regulations, including extractor fans in
rooms that may experience high humidity
d) All water tanks n the loft space must be covered
e) Any vent pipes should be arranged so that they do not discharge water vapour into the
loft space.
f) Cavities closed at plate level to stop vapour transfer to roof space.
g) Where it is proposed to install spotlights/down lighters always refer to the relevant
BBA/BRE certificate for guidance.
h) Always check the BBA for the felt to be used to check suitability and how to install in
your roof design.
6.0 WHAT ARE THE ADVANTAGES OF USING THE ROOFING MEMBRANE
UNVENTED SYSTEM?
6.1 One of the main advantages is reducing the amount of heat lost by air leakage through
the ceiling and from the loft space to the outside. Up to 25% of the heat lost through a
conventional roof system is by this mechanism.
6.2 The unvented roof system will reduce this mechanism of heat loss, more so if the
underlay laps are sealed. In addition, not having to provide eaves and ridge ventilation can
help to save time and cost on site and can give more desirable appearance to the finished
roof?
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7.0 WHAT ARE THE DISADVANTAGES OF USING THE ROOFING
MEMBRANE SYSTEM?
7.1 Extra time and care is needed to ensure that penetrations into the ceiling are properly
sealed and that the loft space is protected from sources of water vapour, in accordance with
the requirements of the certificate.
7.2 If the ceiling is constructed without due attention to sealing joints and around
penetrations, the opportunity for moisture ingress into the loft space and consequent risk of
condensation occurring is increased, this is also true of conventionally ventilated
constructions where the effect of wind movement over the roof can reduce the air pressure
in the loft space and ‘suck’ moisture laden warm air from living space into the loft space.
7.3 However, it is recognised that the rate of moisture removal via the ventilated and
unventilated approach is not the same. It is therefore important that the ceiling is sealed to
an appropriate level.
7.4 Where the membrane is draped between rafters there have been reports of flappingnoise in roofs incorporating some types of permeable roofing membranes in certain wind
conditions. In a worst case scenario this could result in tiles lifting off the roof surface.
8.0 HOW DURABLE ARE ROOFING MEMBRANE SYSTEMS?
8.1 In normal conditions found in roofs, the membranes are durable and can be expected to
have a life comparable to that of traditional roof tile underlay. Specifiers should however
follow the requirements of the relevant Certificate relating to the period for which the
underlay can be exposed to the elements before slating/tiling and on the use of eaves guards
for open eaves construction.
8.2 With regard to water vapour permeability, the BBA assessment included measurements
of the effect of heat ageing; effect of UV light exposure and of accelerated dust/debris
accumulation and found no discernible reduction in performance.
9.0 DO I NEED A VAPOUR CHECK CEILING?
9.1 Not always but it can help. Most manufacturers recommend a vapour control layer be
fitted over bathrooms, utility rooms and kitchens. It is important to remember that vapour
will pass through the path of least resistance in a ceiling. Gaps and cracks will provide an
effective ‘short cut’ however high the vapour resistance of the rest of the ceiling is.
9.2 We would always recommend that a vapour barrier be installed. This is of particular
importance where there are sloping ceilings and/or engineered joists within the roof
construction.
10.0 WHAT DO I DO ABOUT SOURCES OF MOISTURE IN THE LOFT?
10.1 Water tanks should have lids on and vent pipes arranged to avoid venting moisture into
the loft space.
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11.0 DO I NEED TO COUNTER BATTEN?
11.1 In general, counter battens are not required if the underlay is draped between rafters,
i.e. a nominal 10mm. If however it is not draped but pulled taut, counter battens are
required. Please refer to specific Certificates for the holders detailed installation instructions.
The drape or counter battens are there to allow water vapour to disperse or drain away
12.0 DOES THE ROOFING MEMBRANE HAVE TO RUN PARALLEL TO
EAVES OR PERPENDICULAR (eaves to ridge)?
12.1 Traditionally, roof tile underlays are laid in runs parallel to the eaves with subsequent
runs ‘weather lapped’ by 150mm. Some Certificate holders have had an alternative approach
also assessed, where the underlay is drawn from the eaves, over the ridge down to the eaves
on the opposite side of the building. This entails the use of a device to hold the roll of
underlay horizontally and allows it to be unwound.
12.2Due to safety issues the eaves/ridge/eaves method is rarely used. Counter battens are
needed to secure laps between adjacent runs of underlay. Users should refer to the relevant
Agreement Certificate for full details of this alternative method of installation.
13.0 WHAT TYPE OF ROOFING BREATHABLE FELT COVERING CAN BE
USED?
13.1 The issued Certificates cover conventional slates and tiles installed to the relevant
clauses of BS 5534-1: 2003, Code of practice for slating and tiling (including shingles). Some
tightly fitting tiles and slates need special consideration and designers should check with the
tile or slate manufacturer in case they have particular design requirements, for example a
ventilation space beneath them.
14.0 WHAT HAPPENS TO CONSTRUCTION MOISTURE? (Water added
during the construction process)
14.1 The amount of construction moisture in a new building depends on the materials used
and type of construction. The risk of condensation is however greatest when there is a
significant drying out taking place and cold temperatures.
14.2 This may in some circumstances lead to condensation during the first heating season,
but should not persist into the second heating season in a properly designed and constructed
roof.
14.3 BS5250, Code of practice for the control of condensation in buildings, states you can
use a 5mm ridge vent to help improve moisture removal.
15.0 WHAT IS THE DIFFERENCE BETWEEN A ‘NON-VENTILATED’ SYSTEM
AND A ‘SEALED SYSTEM’?
15.1 Both systems are non-ventilated but the sealed system includes adhesive tape at all laps
in the membrane, penetrations and edge details. Not all membranes are required to be
sealed. Seals are normally specified where maximum thermal efficiency is required.
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16.0 CAN BREATHABLE MEMBRANES BE DRESSED INTO THE GUTTER?
16.1 No, It is generally accepted as good practice for all roofing underlays to be stopped
short of the gutter, overlapping a ‘proprietary eaves protective device’ (BS5534) this is
normally a UV (sunlight) resistant eaves sheet such as an eaves carrier or dpc felt that
replaces the underlay as a drip into the gutter.
16.2 The roofing underlay should not be left permanently exposed to direct sunlight.
17.0 CAN BREATHABLE MEMBRANES BE LEFT EXPOSED TEMPORARILY
BEFORE SLATES/TILES ARE FITTED?
17.1 Yes. The ultra violet stability of the membrane allows them to be left exposed for a
period before being covered. This can be up to four months but check with manufacturer to
confirm exact limit for the membrane specified.
18.0 CAN I USE THE BREATHABLE ROOFING MEMBRANE ON VERTICAL
CLADDING?
18.1 No, there is a difference in the requirement and the membrane manufacturer should be
consulted for further advice.
NOTE: Before any final decision is made with roof design and breathable membrane
specification always contact the manufacturer’s technical service department for clarification.
DOCUMENTS REFERRED TO
BS 5534-1 2003, Code of practice for slating and tiling (including shingle)
BS5250, Code of practice for the control co condensation in buildings
Building Regulations Approved Document F
USEFUL LINKS
http://www.bbacerts.co.uk
http://construction.tyvek.co.uk
http://www.proctorgroup.com/Products/CondensationControl.aspx
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