NHBC Standards 2006

6.2 External timber framed walls


This Chapter gives guidance on meeting the Technical  Requirements and recommendations for external walls of timber framed dwellings, substantially timber framed dwellings and wall panels (including relevant  certification procedures). This Chapter applies to timber framed walls up to three storeys high. Some modification may be required for walls more than three storeys high.


6.2 - D1 Design standards
6.2 - D2 Statutory requirements
6.2 - D3 Loadbearing walls
6.2 - D4 Moisture control and insulation
6.2 - D5 Preservative treatment
6.2 - D6 Exterior cladding
6.2 - D7 Control of fire
6.2 - D8 Other elements of timber frame buildings
6.2 - D9-D10 Provision of information

6.2 - D1
Design shall meet the Technical Requirements

Dwellings with a timber frame superstructure require certification indicating that the design has been checked by an NHBC timber frame certifier. This procedure may also apply (at NHBC's discretion) to other combinations of timber framed wall panels (see Appendices 6.2-A, B and C).

Attention is drawn to Clause D9 and Sitework clause S1 which require that certain information about the erection of the timber frame should be available to appropriate personnel.

Design that follows the guidance below will be acceptable for external walls of timber framed dwellings, substantially timber framed dwellings and wall panels. This Chapter applies to timber framed walls up to three storeys high. A closer analysis of differential movement may be required for walls more than three storeys high.


6.2 - D2
Design shall comply with all relevant statutory requirements

Design should be in accordance with relevant Building Regulations and other statutory requirements..


6.2 - D3
Loadbearing timber framed walls shall be designed to support and transfer loads to foundations safely and without undue movement

Structural design of loadbearing timber framed walls should be in accordance with BS 5268 : Parts 2 and 6. The design should take into account:

  • wind loads
  • roof loads
  • floor loads.

Items to be taken into account include:

structural elements

All structural timber should be:

  • of a suitable grade in accordance with BS 5268 and
  • dry graded and marked in accordance with BS 4978.

Sheathing and its associated fixings should be structurally adequate to resist racking due to wind and other forces.

Individual studs should be not less than 37mm wide, at maximum 600mm centres, unless other adequate support is provided for wall boards and fixings and agreed by NHBC.

A lintel and cripple studs should be provided to any opening in load-bearing panels except when the opening does not affect the stud spacing or where the supported loads are redistributed by a perimeter joist.

Additional studs should be considered at openings for fixing wall ties where masonry cladding is used.

Multiple studs should be included to support multiple joists unless otherwise specified by the designer.

Load bearing lintel supported by cripple studs

junction with substructure

The wall panels should be adequately anchored to the substructure to resist lateral movement of the frame. Typical details are shown in Clause S2.

Where frames are fixed to masonry or beam and block floors by shotfiring the blocks should be 7.3N/mm2 concrete blocks to BS EN 771.

joints between panels and other elements

Wall panels should be securely fixed together and fixed to floor and roof framing where appropriate. Appropriate measures should be taken to prevent buckling.

If head plates are not provided joists and roof trusses should bear directly over studs.

In Scotland, at joints between wall panels, sole plates and head binder plates should be provided to bind panels together. Joints in sole plates and head binder plates should not coincide with those of the panels.


6.2 - D4
The design shall ensure that the structure is adequately protected from the effects of moisture

Items to be taken into account include:

(a) provision of dpcs and dpms

Dpcs should be installed below all ground floor walls, including internal partitions, to protect timber from rising damp and residual construction moisture.

Dpcs, waterbars and trays should be fitted at openings where needed to prevent rain penetration.

In Scotland, Northern Ireland, the Isle of Man and other places where the exposure to driving rain is Severe or Very Severe, masonry should form a rebate at the reveals of openings to avoid a straight through joint where the frame abuts the masonry.

Jamb - sheltered site. Sill - sheltered site

Jamb - exposed site. Sill - exposed site

Head of opening

(b) membranes

Breather membranes and other barriers should be lapped so that each joint is protected and moisture drains outwards as shown in Clause S3.

In areas of Very Severe exposure a high performance breather membrane should be used (unless the alternatives given in Clause D4(c) below are adopted). Exposures are defined in Chapter 6.1, 'External masonry walls' Appendix 6.1-A .

(c) cavities in external walls

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

CladdingMinimum cavity width
Masonry50mm nominal
Render on unbacked lathing50mm nominal
Render on backed lathing25mm nominal
Vertical tile hanging without underlay No vertical cavity required when a breather membrane is fitted to the sheathing
Other cladding10mm

In areas of Very Severe exposure (as defined in Appendix 6.1-A ) the wall construction should include a 50mm cavity between the sheathing and the cladding and:

  • a high performance breather membrane, or
  • a masonry cladding which is rendered or clad with an impervious material, or
  • plywood sheathing treated by 10 minute immersion in organic solvent preservative complying with type F/N of BS 5707.

The cavity should be extended at least 150mm below dpc, to form drainage to the cavity and should be kept clear. Weep holes should be provided where necessary to prevent water build up in the cavity.

Weep holes should be provided

The cavity should be ventilated although it is not necessary to provide through ventilation. Where wall areas are divided by horizontal cavity barriers (see Clause D7) ventilation openings should be provided to each section, preferably at the top, although it may be aesthetically preferable to place the lowest ventilation slots below the lowest timber.

The ventilation openings should be equivalent to open brick perpends every 1.5m.

Ventilation slots should be placed to prevent the ingress of rain or should be below the lowest timber. Weather protected proprietary perpend ventilators are available.

(d) spillage or floor washing

Moisture or water should be prevented from passing from one dwelling to another beneath the separating wall, or adversely affecting timber.

Reinforced concrete raft foundation with channel

Where individual concrete floors are used, drainage can be via the cavity of the separating walls.

Drainage via cavity in separating wall

(e) insulation

The BRE Report 'Thermal insulation: avoiding risks' discusses aspects of insulation relevant to external timber framed walls. In England and Wales account should be taken of Robust Details.

Insulation should normally be placed between the sheathing and the vapour control layer. Insulation between the sheathing and the cladding is not permitted unless designed and approved in accordance with Technical Requirement R3 as an integral part of a wall system.

Combined sheathing and insulation should be assessed in accordance with Technical Requirement R3 and installed in accordance with the assessment.

Wall insulation should be of a type which breathes eg, mineral wool (rock or glass).

Insulation with an integral backing should have the backing on the warm side. The backing should not form the vapour check.

Service pipes, conduits, etc within walls should be on the warm side of the insulation.

(f) vapour control layers for walls

A vapour control layer, where provided, should be fixed on the warm side of wall insulation.

The vapour control layer should cover the external wall including bottom rails, head rails, studs, lintels and window reveals.

Vapour control layers should be of 500g polyethylene or vapour control plasterboard. Foil backed plasterboard or the integral backing of insulation should not be used as a vapour control layer.

The moisture content of the frame should be 20% or less before fixing the vapour control layer.

All joints in the vapour control layer should have at least 100mm lap and should 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.

Joints in vapour control plasterboard should be located on studs or noggings. They should be filled, taped and finished.

Vapour control layer preventing moisture getting into timber frame walls


6.2 - D5
Timber and timber products shall either be naturally durable or where necessary be treated with preservative to give adequate resistance against decay and insect attack

Items to be taken into account include:

(a) timber framing

Timber framing should be treated in accordance with the guidance in Chapter 2.3 'Timber preservation (natural solid timber)', to which reference should be made.

(b) timber cladding

Timber cladding should be treated in accordance with the guidance in Chapter 2.3 'Timber preservation (natural solid timber)', to which reference should be made.

(c) joinery

Joinery should be treated in accordance with the guidance in Chapter 2.3 'Timber preservation (natural solid timber)', to which reference should be made.


6.2 - D6
Exterior cladding shall be compatible with the timber frame

Items to be taken into account include:


A cavity between exterior cladding and the sheathing should be specified as detailed in Clause D4. This cavity should not contain electricity cables other than meter tails.

wall ties

Wall ties should be:

  • of a type which fully permits differential movement between the timber frame and the cladding - see Clause D6(c) below
  • fixed to studs, not sheathing
  • spaced at a maximum of 600mm horizontally and 450mm vertically
  • spaced at jambs of openings a maximum of 300mm vertically within 300mm of the masonry reveal. In this case additional studs may be needed
  • inclined away from the sheathing so that the slope is maintained following differential movement.
allowance for differential movement


In balloon construction, dwelling height studs are used, to which the floor is attached. This form of construction is not commonly used in the United Kingdom. The timber frame designer's recommendations for differential movement allowances should be followed.


This is the use of storey height panels separated by floor joists.

When using this form of construction either the timber frame designer's recommendations should be followed or the allowances shown in the table below:

LocationSuspended timber ground floor when panels are supported on ground floor joists or perimeter joistsOther ground floor construction
Ground floor openings5mm3mm
First floor openings12mm9mm
Second floor openings18mm15mm
Eaves and verges
Add 3mm to the allowance for openings on that floor

Where the ground floor construction is totally masonry and the upper storeys are timber framed the allowances relate only to the timber frame construction. Walls which have cladding fully supported by the timber frame do not require allowances at openings, junctions with roofs or above the wall.

For timber framed buildings more than three storeys high allowances should be extrapolated from the table by 6mm for each additional storey.

The designer should detail how the differential movement is allowed for at openings.

Timber shrinkage should not affect the performance of, or transfer loads to, any flue. Typical details are shown in clause S2.

At eaves and verges a gap should be left above the masonry which can close as the timber shrinks. Sealing the gap is not required if it is protected by the eaves overhang.

In buildings of four storeys or more the designer should ensure that the wall ties are capable of allowing for the likely differential movement.

At sills of openings there should be a gap left, sealed with a compressible sealant. Purpose made sealants are made to suit wider movement gaps on higher buildings (over three storeys).

At heads of openings allowance should be made for a gap to open. This should be filled with an expandable sealant or well weathered. Lintels supporting outer leaf masonry should not be fixed rigidly to the timber frame. A space should be left between the lintel and its clips to allow for differential movement.

At jambs of openings allowance should be made for shear movement between masonry and joinery.

A gap should be left at horizontal junctions between lightweight cladding and masonry claddings. It should be protected against rain penetration.

Allowance should be made for shear movement at vertical junctions between masonry and lightweight cladding.


6.2 - D7
Walls and panels shall resist the spread of fire

All elements should have adequate fire resistance.

Service mains should not pass through separating wall cavities.

In Scotland services are not permitted within a timber framed separating wall.

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

Items to be taken into account include:

(a) cavity barriers

The design should detail the position and materials for cavity barriers in accordance with relevant Building Regulations.

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.

Dpcs should be used to cover horizontal and vertical cavity barriers and to shed moisture away from sheathings. Typical details are shown in Clause S8.

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

(b) fire-stops

The design should detail the position and type of fire-stops in accordance with relevant Building Regulations.

A typical fire-stop detail is shown in Clause S8.


6.2 - D8
The relevant Parts and Chapters of the Standards shall be applied when designing elements of dwellings not covered by this Chapter

The design of timber framed dwellings may incorporate elements of design obtained from each Part and other Chapters of these Standards.

The list of Chapters should be consulted in order to ensure that advantage is taken of all the details of Technical Requirements and Guidance available.


6.2 - D9
All relevant information shall be distributed to appropriate personnel

Ensure that design and specification information is issued to site supervisors and relevant specialist subcontractors and/or suppliers.

Where proprietary products are to be used, manufacturers usually have specific requirements for fixing and/or assembly of their products. This information should also be made available for reference on site so that work can be carried out satisfactorily in accordance with the design and specification.

The fixing schedule should allow for every structural connection made on site including fixing details for framing, wall ties, breather membrane, sheathing and vapour control layers, and must show as appropriate:

  • number and spacing of nails and staples
  • size and type of nail including material and corrosion protection
  • method of nailing (eg skew, end, etc.).

Copies of the fixing schedule should be given to the person doing the job.

6.2 - D10
Design of the superstructure shall be checked by an NHBC approved timber frame certifier

The complete design package should be placed with an NHBC approved timber frame certifier so that a certificate can be issued in accordance with Appendices 6.2-A, B and C.