6.2 External timber framed walls

SITEWORK STANDARDS

6.2 - S1 Sitework standards
6.2 - S2 Construction
6.2 - S3 Breather membranes
6.2 - S4 Wall ties
6.2 - S5 Insulation
6.2 - S6 Vapour control layers
6.2 - S7 Cladding
6.2 - S8 Control of fire
6.2 - S9 Services

6.2 - S1
All sitework shall:
(a) meet the Technical Requirements
(b) take account of the design
(c) follow established good practice and workmanship

Sitework that complies with the design and guidance below will be acceptable for external walls of timber framed dwellings, substantially timber framed dwellings and wall panels.

All relevant information in a form suitable for the use of site operatives should be available on site before construction starts including:

  • full set of drawings
  • materials specification
  • fixing schedules
  • nailing details
  • manufacturers' recommendations relating to proprietary items.

CONSTRUCTION

6.2 - S2
Construction shall ensure that the building is structurally adequate

Items to be taken into account include:

(a) setting out

The substructure should be correctly set out to receive the timber frame which will be manufactured to close tolerances. The timber frame should be checked to ensure that it is erected accurately both on plan and vertically. The load from the frame should be supported as intended in the design. Where packing under sole plates is necessary it should be in accordance with Appendix 6.2-B.

12mm maximum overhang of frame over supporting structure (20mm for 140mm wide studs)

Ledges can form moisture traps. Where these occur protection should be provided.

ledge protected by breather membrane

(b) anchoring the frame

The frame should be anchored to resist both lateral movement and uplift. Care should be taken to avoid splitting timber plates or damaging the substructure.

When shotfiring, care should be taken not to spall edges of masonry or slabs. When shotfiring into masonry, solid concrete blocks with a minimum crushing strength of 7.3N/mm2 should be used, positioned to receive fixings.

concrete blocks with a crushing strength of 7.3N/mm2

fixing to studs

(c) fixing panels

All fixings, including nailed joints and sheathing, should be as scheduled in the design.

Wall panels should be securely fixed together and to floor and roof framing.

Sole plates and head binders should be provided to bind the panels together. Joints in sole plates and head binders should not coincide with those between panels. Joints in head binders should occur over a stud.

If head binders are not provided, joists and roof trusses, including girder trusses and other similar loads, should bear directly over studs. Individual studs should be not less than 37mm wide, at maximum 600mm centres, unless other adequate support is provided for wall boards and fixings.

A lintel and cripple studs should be provided to any opening in loadbearing panels except when the opening does not affect the stud spacing or where the supported loads are carried by a rim beam or perimeter joist.

Unless otherwise clearly specified by the designer, multiple studs should be included to support multiple joists.

(d) supporting claddings

Wall ties should be in accordance with the design and fixed to studs.

Battens supporting lightweight cladding should be fixed to studs.

(e) fixing plasterboard

Plasterboard should be fixed in accordance with Chapter 8.2 'Wall and ceiling finishes' (Sitework) unless the design specifies closer fixing centres

Particular care should be taken at the junction between walls and roofs. Reference should be made to Chapter 8.2 'Wall and ceiling finishes' (Sitework).

(f) movement between timber frame wall and other elements

The timber frame will reduce in overall height as it dries out. The anticipated amount of shrinkage of the timber frame and where it will occur between other parts of the structure is given in Appendix 6.2-C.

Movement will occur at:

  • door and window openings
  • eaves and verges
  • balconies (including Juliet balconies)
  • openings for drive-throughs
  • staircases and lift enclosures (where they are not timber framed)
  • service entries
  • the interface of the timber frame with any other construction.

The extent of the differential movement increases with the number of storeys. Movement gaps should be filled with suitable materials to take up the expected movement. The manufacturer of the material should be consulted on the suitability for the extent of the movement expected.

Where the movement gap is expected to be more than 35mm it should be protected by a cover strip.

All claddings that are fixed directly to the timber frame, should have a horizontal movement joint at each floor level. See Appendix 6.2-C.

Masonry claddings should not be supported by the timber frame.

(g) prefabricated chimneys

Prefabricated chimneys should either be supported by:

  • the masonry cladding, or
  • the timber frame, including any roof construction supported by the timber frame.

(h) cavities

A cavity should be provided to reduce the risk of rain penetrating to the frame. The following minimum cavity widths, measured between the cladding and sheathing, should be provided:

Cladding Minimum cavity width
Masonry 50mm nominal
Render on backed lathing 25mm nominal
Vertical tile hanging without underlay No vertical cavity required when a breather membrane is fitted to the sheathing
Other cladding* 15mm

*See Chapter 6.9 'Curtain walling and cladding'.

A clear cavity for at least 150mm below dpc should be maintained. Weep holes (eg open perpends) should be provided where necessary to prevent water build up in the cavity.

BREATHER MEMBRANES

6.2 - S3
Breather membranes shall where required protect the sheathing from dampness

Special attention should be given to the following details:

  • laps, which should be at least 100mm on horizontal joints and 150mm on vertical joints as shown in the following diagram
  • direction of laps - upper layers should be fixed over lower layers to ensure rain runs away from the sheathing. Vertical joints should be staggered wherever possible
  • fixing should be at regular intervals to prevent damage by wind and should be not less than 500mm centres and in accordance with the manufacturer's recommendations
  • marking the stud positions for wall tie fixing
  • shedding water away from the lowest timber
  • use of self extinguishing grade membrane
  • use of high performance breather membrane in areas of Very Severe exposure to wind driven rain (as defined in Appendix 6.1-A) (unless the alternatives given in Design Clause D4(c) are adopted)
  • use of fixings described in Chapter 6.2 (Materials).

Breather membranes shall where required protect the sheathing from dampness

Damaged membranes should be repaired or replaced before proceeding with the cladding.

When bitumen impregnated fibre building board is used and a breather membrane is not specified the joints of the boards should be closely butted and horizontal joints sealed to prevent water ingress.

When a breather membrane is not required the bottom frame members should be protected from water in the cavity.

When a breather membrane is not required the bottom frame members should be protected from water in the cavity

WALL TIES

6.2 - S4
Wall ties shall be correctly installed

Wall ties should be:

  • of the type specified in the design
  • fixed to the studs not the sheathing
  • be sufficiently flexible or of a type that fully permits differential movement
  • be kept clean and free from mortar droppings
  • spaced at a maximum of 600mm horizontally and 450mm vertically
  • spaced at jambs of openings and at movement joints in masonry a maximum of 300mm vertically within 225mm of the masonry reveal or movement joint
  • spaced within 225mm of the top of a masonry wall
  • inclined away from the sheathing so that the slope is maintained following differential movement.

INSULATION

6.2 - S5
Insulation shall be correctly installed

Insulation should cover the whole wall area between studs. No gaps should be left:

Insulation should cover the whole wall area between studs. No gaps should be left

  • against studs or rails
  • at corners
  • against noggings
  • at junctions with partitions
  • behind service panels.

VAPOUR CONTROL LAYERS

6.2 - S6
Vapour control layers shall be correctly installed

Before fixing a vapour control layer the framing timbers should have a moisture content of 20% or less.

The vapour control layer should be fixed on the warm side of the insulation and frame.

The vapour control layer should be the material specified in the design. 500 gauge (120 microns) polyethylene sheet or vapour control plasterboard should be used. Where vapour control plasterboard is used joints between sheets should be positioned on studs or noggings and the joints should be filled, taped and finished.

Where polyethylene is used all joints in the vapour control layer should have at least 100mm laps and be located on studs or noggings.

Vapour control layers should be fixed at 250mm centres to the top and bottom of the frame, at laps and around openings.

When cutting vapour control plasterboard care should be taken not to displace the vapour control material.

Any holes made in a vapour control layer should be made good.

The vapour control layer should cover the external framed wall area including rails, studs, reveals, lintels and sills.

The vapour control layer should lap with the dpc.

CLADDING

6.2 - S7
Cladding and its fixings shall not obstruct the cavity and shall allow for differential movement

Allowance for differential movement between various components should be made as detailed in Clause S2 of this Chapter.

Horizontal battens, which obstruct the drained and vented cavity, should not be used to support cladding except tile hanging.

The cavity should be vented to allow some limited, but not necessarily through, movement of air. Where wall areas are divided by horizontal cavity barriers and openings should be provided to each section (see Design clause 6.2 - D7).

The ventilation openings should be:

  • equivalent to open brick perpends every 1.2m.
  • placed to prevent the ingress of rain or should be below the lowest timber.

Proprietary perpend ventilators are available.

These openings can also provide drainage.

The cavity should be kept clean, free of obstructions and be capable of draining freely.

All proprietary cladding should be fixed in accordance with the manufacturer's recommendations.

Masonry cladding should be constructed in accordance with Chapter 6.1 'External masonry walls'.

CONTROL OF FIRE

6.2 - S8
Fire spread shall be controlled as detailed in the design

Guidance on the prevention of fire during construction is given in 'Fire prevention on construction sites' jointly published by the Construction Confederation and the Fire Protection Association.

CAVITY BARRIERS

Cavity barriers should be installed in positions detailed by the design and relevant Building Regulations.

Cavity barriers should be formed of materials specified in the design. If no specification is available the advice of the designer should be obtained.

Horizontal cavity barriers (except under eaves) should be protected with a dpc tray. The tray should have a minimum upstand of 100mm. Alternatively polyethylene encased cavity barriers providing a minimum upstand of 100mm should be used.

Breather membrane lapped over dpc tray

Vertical timber cavity barriers should be protected from moisture by a dpc.

Vertical timber cavity barriers should be protected from moisture by a dpc

FIRE-STOPPING

Fire-stops should be installed in positions detailed in the design and relevant Building Regulations.

Only those materials specified in the design should be used for fire-stopping.

If details of fire-stop design, location and materials are not available they should be verified with the designer before construction commences.

Fire-stops should be installed in positions detailed in the design and relevant Building Regulations

SERVICES

6.2 - S9
Services shall not reduce the fire resistance or stability of the dwelling

Services should not pass through separating wall cavities.

Service outlets should not impair the fire resistance of floors and walls.

Only those services shown in the design should be installed in separating walls.

Notching or drilling of structural timber members should be carried out as detailed in the design. If these details are not available the designer should be consulted before such operations are started.

See Appendix 6.2 - C.