12 Workmanship, installation, control and maintenance
12.1 General
This section gives recommendations for the preparation, fabrication, storage, handling,
installation, control and maintenance of materials and components. These recommendations are
necessary conditions for the applicability of the design rules given in the Manual.
Workmanship in the preparation, fabrication and installation of materials should conform
in all respects to accepted good practice. Where necessary reference may be made to BS 8000,
in particular part 5121.
The specified materials should be installed in such a way as to perform adequately the
functions for which they are designed.
For the structure to conform to the principles and practical considerations of the design
there should be adequate supervision throughout its preparation, construction and maintenance.
12.2 Members
12.2.1 Condition of timber members
Members which are damaged, crushed or split beyond the limits permitted for similar defects
in the grading should be rejected or repaired to the satisfaction of the designers and approving
authority.
The deviation from straightness measured midway between the supports should, for
columns and beams where lateral instability can occur, or members in frames, be limited to 1/500
times the length of glued laminated timber or LVL members and to 1/300 times the length of
solid timber.
12.2.2 Moisture content
Before being used in construction, solid timber and other structural timber products should be
dried as near as practicable to the moisture content appropriate to their climatic condition in the
completed structure (see Table 12.1).
As a guide, at the time of erection the moisture content of solid timber members designed
for use in service class 1 or 2 should not exceed 18%, and the moisture content of wood-based
panels for use in service class 1 should not exceed 12% (see Section 12.5).
The determination of moisture content by a properly calibrated moisture meter used in
accordance with the manufacturer’s instructions will normally be considered sufficiently accurate
if insulated probes inserted 20mm or one quarter of the timber thickness (whichever is less) are
used. Where a more accurate average determination of moisture content has to be made, and for
meter calibration, the oven dry method detailed in EC5 NA 4.1 should be used.
12.2.3 Dimensions
The size, shape and finish of all members and materials should conform to the detailed drawings
and specifications. Dimensions and spacings should not be scaled from drawings.
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12.2.4 Modifications
The cutting, notching, drilling or other modification of members, other than that allowed by
the drawings and specification or by Sections 5.2.1.4, 5.2.1.5, 7.3.4 or 10.8.5.2, should not
be permitted. Care should be taken to ensure that notches and holes are not so positioned in
a member that the remainder of the cross-section contains a knot or other defect which will
significantly affect its strength.
Where service holes are drilled in wall studs, the services should be protected if necessary
to avoid penetration by plasterboard nails.
12.2.5 Treatment of cut surfaces
The cutting of timber after preservative treatment should be avoided. When it is unavoidable,
exposed untreated timber should be given a liberal application of a permitted preservative in
accordance with Manual on Preservative Treatment122.
12.3 Connections
12.3.1 General
Wane, fissures, knots or other defects which have not been allowed for in the design, and which
may affect significantly the load-carrying capacity, should not be permitted at a connection.
The spacing and predrilling of holes for nails, screws, bolts and dowels should conform
to the design drawings.
It is essential that the fastener types and corrosion protection specified in the design are
used, particularly square twisted nails, ringed shank nails, screws with a specified minimum root
diameter, or high strength steel bolts.
12.3.2 Nailed connections
If predrilling is specified in the design, holes with a diameter not exceeding 0.8 times the nail
diameter must be drilled before inserting the nail.
12.3.3 Screwed connections
Screws should be turned, not hammered.
Screws should be tightened so that the members fit closely together.
Coach screws with a timber headside member should be fitted with a special steel washer
in accordance with Table 3.29.
Predrilling is not required:
•
for special screws if their manufacturer can provide characteristic load capacities from
tests on complete connections carried out in accordance with BS EN 2689188 and
matching the connection configuration used in the design
•
for wood screws with a smooth shank diameter G 6mm in softwood, unless the Engineer
has specified predrilling on the drawings
In other cases predrilling is required, with the following requirements:
•
the lead hole for the smooth shank should have the same diameter as the shank and the
same depth as the length of the shank
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•
for tk G 500kg/m3 the hole for the threaded portion should have a diameter of
approximately 70% of the shank diameter; while for tk > 500kg/m3 the predrilling
diameter should be determined by tests where tk = the characteristic timber density.
12.3.4 Bolted connections
The maximum diameter of bolt holes should be:
d + 1mm
in timber
d + 2mm
max . )
1.1d mm in steel plates
Where d = diameter of bolt (mm)
Special steel washers in accordance with Table 3.29 must be fitted between timber and the bolt
head or nut.
Bolts should be tightened so that the members fit closely together.
12.3.5 Dowelled connections
The minimum dowel diameter should be 6mm. The tolerances on the dowel diameter should be
-0/+0.1mm.
The diameter of pre-bored holes in the timber members should not exceed that of the
dowels.
12.3.6 Connections made with timber connectors
Toothed plate, split ring and shear plate should be fitted in accordance with the design drawings.
Members connected with toothed plates should be drawn tightly together by means of a
high tensile steel bolt. With other types of connector recesses should be accurately cut concentric
with the bolt hole, and all chips and shavings removed.
The diameters of bolts used with timber connectors should conform to Table 6.32.
12.3.7 Adhesively bonded joints
Joints in structural components made from separate pieces of timber, plywood or other wood
based board that are fastened together with adhesive (e.g. box beams, single web beams,
stressed skin panels, glued gussets) should be manufactured in accordance with BS 644695. The
manufacturing process should be subject to quality control.
Large (full depth) finger joints should be manufactured in accordance with
BS EN 38797.
The adhesive manufacturer’s recommendations with respect to mixing, environmental
conditions for application and curing, moisture content of members and all factors relevant to the
proper use of the adhesive should be followed.
For adhesives which require a conditioning period after initial set to attain their full
strength, the application of load to the joint should be delayed for the necessary time.
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12.4Trussed rafters
Requirements for the fabrication of trusses are given in BS EN 14250123.
12.5 Storage and handling
12.5.1 Protection of materials and components
Precautions should be taken during storage prior to delivery, and on site, to minimise changes in
moisture content due to the weather. Rain, damp and direct sunlight are all potentially harmful to
timber and wood-based components.
Materials and components should be stored clear of the ground on dry bases, and stacks
should be evenly supported on bearers with spacer sticks at regular intervals. Stacks should be
protected from the weather either by a roof, or by tarpaulins or other impervious materials so
arranged to give full cover but at the same time to permit free passage of air around and through
the stack.
Solid timber delivered packaged, i.e. strapped, should not be stored in packaged form
for lengthy periods. Where early use is not possible, packages should be opened and the timber
should be open-piled and protected from the weather as above. Care should be taken not to deform
stacked material.
For wood-based panel products any shrink-wrapping should be left untouched until
24 to 48 hours before installation (see Table 12.1).
Where it is essential that materials and components have low moisture contents, it may
not be possible to maintain suitable conditions on site other than for short periods, and deliveries
should be arranged accordingly.
12.5.2 Handling
Undue distortion of components during transportation and handling should be avoided. Similarly,
damage from chafing or slings should also be avoided. Where design assumptions for long,
flexible or heavy components dictate certain methods of handling, lifting points should be marked
on the components and methods of lifting should be shown on the design drawings.
12.6Assembly, erection and installation
12.6.1 General
The method of assembly and erection should be such as to ensure that the geometry of assembled
components, as specified by the designer, is achieved correctly within the specified tolerances.
During assembly and erection, no forces should be applied to a component that could
cause the permissible stresses to be exceeded in that or any other component. Special care is
necessary when handling framed arches and shaped beams. Any instructions about methods and
sequences of erection included in the health and safety plan or shown on the design drawings
should be rigorously followed.
12.6.2 Trussed rafters
Trusses should be checked for straightness and vertical alignment prior to the fixing of any
bracing.
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At the time of fabrication, the truss members should be free from distortion within the
limits given in BS EN 14250123. However, if members become distorted during the period
between fabrication and erection but can be straightened without damage to the timber or the
joints and can be maintained straight, then the truss may be considered satisfactory for use.
The maximum bow in any truss member after erection should be limited. Provided that
it is adequately secured in the completed roof to prevent the bow from increasing, the permitted
value of the maximum bow should be taken as 10mm.
The maximum deviation adev,perm of a truss from true vertical alignment after erection
should be limited to:
adev,perm
= min. )
r
10 + 5 (H - 1) mm
25mm
WhereH = height of truss (m)
If a girder truss has to be assembled on site, the first truss should be laid out on level supports
to ensure that it is flat. The ceiling tie members must be fastened together as specified by the
designer. Rafter and web members should be bolted or nailed together in accordance with the
designer’s directions or as recommended in BS PD 66932. Gaps greater than 3mm between chord
members should be packed with suitable material before fastening.
12.6.3 Floors
Requirements for the installation of floor decking are given in Table 12.1.
Requirements for the anchoring of timber floors to masonry walls are given in
Figures 12.1 to 12.3.
When installing engineered timber joists the manufacturer’s instructions should be
followed. See Section 8.3.6.2 for safety notes relating to joist supports.
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Table 12.1 Requirements for the installation of wood-based decking for floors and roofs
Plywood decking
Particleboard and OSB
decking
Item
Solid timber boards
Storage
All decking materials must be fully protected from the weather
before installation
Time of installation Decking should not be installed until the building is weathertight
unless it is factory-faced with a protective film
No special
Moisture contenta m.c. G 18% at
requirement but
installation.
Floor boarding intended must be dry
as a decorative feature
must dry out to 12% ± 2%
m.c. before being laid,
and should not be laid
until the building has
dried out
Boards should be
conditioned by laying
them in place at
least 24 hours before
fixing down. They
should not be fixed
to joists or noggings
which have a m.c.
> 20%
Dust and debris
Before fixing decking, all dust and debris should be brushed or
blown from the joist surfaces
Fastener type
and size
Decking should be attached to joists with flat head annular
ringed shank nails, or countersunk head traditional wood screws in
predrilled holes, 3.0mm to 3.35mm in diameterb; or with countersunk
head self-drilling wood screws, No.8 screw gauge or 4.00mm in
diameter. Nails should have a minimum length of 2.5t and screws a
minimum length of 2.0t where t is the decking thickness. Nail heads
should be punched and screw heads driven 2-3mm below the
surface. Where necessary a countersinking hole should be drilled for
screws
Access provision
Screws should be used where access to underfloor services is
required. With panel products a purpose-designed screwed access
panel should be provided
1 nail at each joist
Recommended
crossing,
maximum
fastener spacingc or 3 for boards wider
than 175mm
Position of
fasteners
150mm around the edges of a board,
300mm wherever boards cross a joist (EC5 10.8.1)
Nails should be 15 to
Fasteners should be positioned at least
20mm from the edges of 3d from the edges of boards, joists and
the board. (If there is a
noggings
third nail it should be in
the centre)
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Table 12.1 Continued
Solid timber boards
Gluing
In addition to nailing or screwing as above, it is recommended
that all decking should be glued to clean, dry, frost-free joists using
a PVAc adhesive of durability class D3 to BS EN 204124. (This is to
improve the floor performance and reduce the risk of creaking. The
site gluing of floor decking should not be treated as a substitute for
mechanical fixings)
Orientation of
boards
Boards should be laid
across the joists
Continuity over
joists
Boards should span over at least 3 joists
Support on joists
Floorboard ends should
meet in the centre of a
joist
Edges supported on joists should meet on
the centre of joists at least 38mm thick
End joints
End joints supported on
the same joist should be
at least 2 board widths
apart
The joints between the short ends of
adjacent boards should be staggered.
(This improves the strength of the floor as
a horizontal diaphragm. See Figure 5.4)
Support of edges
Before fixing, squareedged boards should
be butted tightly.
Square edged solid
timber boards should
not be used in timber
frame buildings or in
any buildings where
floor diaphragm action
is required (EC5 10.8.1).
T & G boards should
be glued with a PVAc
adhesive of durability
class D3 to BS EN 204 or
an equivalent adhesive,
and be cramped tightly
together
All edges should be tongued and
grooved to adjacent boards, or
supported on joists or noggings, or
(at the walls supporting the joists) be
supported on header joists or perimeter
noggings. T & G joints should be glued
with a generous bead of PVAc adhesive
of durability class D3 to BS EN 204 or an
equivalent adhesive, applied to both
tongue and groove before fixing. All
boards should be tightly pulled together
before fixing with nails or screws.
Softwood noggings minimum 38mm wide
x 38mm deep should be skew nailed at
each end to the joists with two 3mm z
nails. Panel edges should be fastened
and glued to noggings as to joists
242
Plywood decking
Particleboard and OSB
decking
Item
Board should be
laid with the face
grain at rightangles to the joists
(see Table 3.17
note a)
Square-edge
chipboard is normally
laid with the long
edges along the
joists. In all other
cases the long edge
should be at rightangles to the joists
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Table 12.1 Continued
Item
Solid timber boards
Expansion gap
Plywood decking
Particleboard and OSB
decking
Timber floors in masonry buildings –
An expansion gap of at least 10mm
should be allowed between the decking
and the perimeter wall in each room
area to prevent possible buckling as it
absorbs moisture.
On floors longer than 10m the gap
should be 2mm per metre run of deck,
divided between each end of the floor,
but manufacturers may recommend
intermediate expansion gaps instead. The
gap may be masked by skirting board or
loose cover strip, or filled with an easily
compressible material. It must be kept
clear of plaster and other building debris
Timber floors in timber frame buildings –
The decking must be connected to the
outer edge of the perimeter joists to
provide adequate diaphragm action. It is
therefore imperative that the decking be
conditioned to the in-service temperature
and humidity before it is fastened down
Notes
a Moisture content for roof decking will be dependent on the form of construction
and building control requirements.
b 3.00mm diameter is preferable for 38mm thick joists to reduce the possibility of
splitting.
c For service class 2 buildings the maximum fastener spacing around the perimeter of
a floor may be determined by tying requirements (see Appendix A).
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Wherever possible the tongue
of the hanger to be located away from
perpend joint below
Strap skew-nailed to joist
Underside of
joist notched
to provide flat
soffit for ceiling
Strap to turn down
a minimum of 100mm
and be tight against
the face of the
walling inner leaf
Fig 12.1 Timber joists supported by joist hangers on blockwork
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Strap to turn down a minimum of 100mm
and be tight against the face of
the walling inner leaf
Note
No straps are necessary in dwellings up to 3 storeys if the joist spacing is 1200mm or less
and the joists bear at least 90mm into the wall or are supported on specially designed joist
hangers. Otherwise strap as shown – either on top of the joist with the strap turned up, or
on one side of the joist with the strap turned sideways – 2m apart up to 3 storeys or 1.25m
apart above 3 storeys. Straps to be galvanized mild steel 5mm x 30mm, nailed as indicated
in Table 5.9 to resist the forces specified in Appendix A.
Fig 12.2 Timber joists supported on blockwork
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Strap to turn down
100mm and be tight
against the facing of the
walling inner leaf
Strap to be rebated into top
of joists to allow the
floorboards to lay flat
Nogging should
extend at least half the
depth of the joist
and be at least
38mm thick
Solid nogging to
be fixed between
joists under the
straps to take the fixings
Gap between the wall
and the first joist to be
filled with a timber packing
at strap positions
Alternative strap location
using full depth noggings
Note
The strap should be carried over at least three joists and be secured with four fixings of
which at least one should be in the third joist or in a nogging beyond the third joist. Strap
specification, spacing and nailing as for Figure 12.2.
Fig 12.3 Timber joists spanning parallel to a blockwork wall
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12.7 Treatments
12.7.1 Preservative and flame retardant treatments
Preservative and flame retardant treatments should be applied in accordance with the manufacturer’s
instructions. Flame retardants should be used under such conditions and in such a manner that
they will not adversely affect other materials or processes. Leach resistant retardants should be
used for materials exposed to the weather or to regular wetting, and they may need to be renewed
after a period recommended by the manufacturer.
12.7.2 Anti-corrosive treatments
The anti-corrosive treatment of metal fasteners and fittings should be sufficient to ensure their
satisfactory performance and structural integrity throughout the intended life of the structure. The
degree of protection required will depend on the dimensions of the fittings and the environment
to which they are to be exposed (see Table 3.25).
12.7.3 Decorative treatments
Where structural timber is painted, varnished or otherwise decorated, the work should be in accordance
with BS 6150125. Care should be taken to ensure that paints, preservatives, flame retardants and
adhesives are compatible. Advice can be obtained from the Wood Protection Association.
12.8 Production and site control
12.8.1 Control plan
A control plan should be drawn up to ensure that the construction is built in accordance with the
design specification. As a minimum the control plan should cover:
•
any necessary preliminary tests, e.g. tests for suitability of materials and production
methods
•
checking of delivered materials and their identification, e.g.
–
for wood and wood-based materials: species, grade, marking, treatments and
moisture content
–
for glued constructions: adhesive type, production process, glue-line quality
–
for fasteners: type, corrosion protection
–
for all materials, checking of correct dimensions and quantities
•
transport, handling of materials and site storage
•
checking of assembly and erection
•
checking of structural details, e.g.
–
splitting
–
number of nails, bolts etc.
–
sizes of holes, correct predrilling
–
spacings and distances to end and edge of members
•
final checking of the result of the production process, e.g. by visual inspection or proof
loading.
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12.8.2 Inspection
Reasonable facilities and access for inspection should be provided during and at completion of
fabrication and erection of a structure. These facilities and access conditions should be agreed
between the parties concerned.
12.9 Maintenance
12.9.1 Responsibility
The responsibility for maintenance should be agreed between the parties concerned. A maintenance
schedule should be drawn up to ensure that the building continues to function properly.
12.9.2 Tightening of bolts
It is advisable to check the tightness of bolts some six weeks to eight weeks after completion of
the structure, when the timber may have reached equilibrium moisture content. Where tightening
is to be carried out access for this purpose should be provided. A second inspection of large solid
timber members about 12 months after completion is recommended.
12.9.3 Ancillary components
It is imperative that features of the construction which are essential to the structural performance of
timber and timber-based components, e.g. vapour barriers, ventilators, seals, etc., are maintained
in an effective condition throughout the intended life of the structural timberwork.
12.9.4 Structural metal-work
Any corroded fittings should be treated with an anti-corrosive or, if necessary, replaced.
12.9.5 Other matters
The maintenance schedule should include inspection (and renewal when necessary) of wood
stains or other protective treatments on exterior woodwork, the removal of any debris or
vegetation which could retain moisture, and normal building maintenance on drains, gutters, flatroofing, etc., to ensure that the structural timberwork remains well ventilated and either remains
dry or does not remain wet for prolonged periods.
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