External timber framed walls

6.2

6.2.4Load-bearing walls

Load-bearing timber framed walls shall be constructed to support and transfer loads to foundations safely and without undue movement. Issues to be taken into account include:

  1. timber elements
  2. joints between panels and other elements
  3. positioning of sole plates
  4. packing under sole plates
  5. fixing panels
  6. support of prefabricated chimneys.

Timber elements

Load-bearing timber framed walls should be in accordance with BS EN 1995-1-1, and take into account:

  • wind loads
  • roof loads
  • floor loads.

Timber in external framed walls should be:

  • a suitable grade in accordance with BS EN 338 and BS EN 14081-1
  • dry graded and marked in accordance with BS 4978
  • I-studs assessed in accordance with Technical Requirement R3.

Individual timber studs should have:

  • a minimum width of 37mm
  • a maximum spacing of 600mm, unless other adequate support is provided for wall boards and fixings.

Narrow or inaccessible gaps between studs which are difficult to insulate should be avoided.

Lintels and cripple studs should be provided at openings in load-bearing panels except where:

  • the opening does not affect the stud spacing, or
  • supported loads are carried by a rim beam or perimeter joist.

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

Where masonry cladding is used, additional studs may be required at openings to fix wall ties.

Multiple studs should be included to support multiple joists and other point loads, unless otherwise specified by the designer.
Where head binders are not provided, joists and roof trusses, including girder trusses and other similar loads, should bear directly over studs.

Joints between panels and other elements

Wall panels should be:

  • securely fixed together, and securely fixed to the floor and roof framing
  • constructed to prevent buckling.

At joints between wall panels, sole plates and head binders should be provided to bind panels together. Joints in sole plates and head binders should:

  • occur over a stud
  • not coincide with joints between panels.

Positioning of sole plates

When setting out:

  • the substructure should be correctly set out to receive the timber frame
  • the timber frame should be checked to ensure that it is erected accurately, both horizontally and vertically
  • the load from the frame should be supported as intended in the design
  • protection should be provided where ledges form moisture traps.

Packing under sole plates

Where packing is required to ensure the timber frame or sole plate is level:

  • permanent packing should be used for gaps less than 5mm
  • grout and mortar should not be used for gaps less than 5mm
  • hollow plastic packing with reduced bearing surfaces should not be used
  • temporary spacers can remain in place provided they are durable and non-degradable.

Permanent packing should be:

  • designed and approved by the timber frame designer to suit the horizontal and vertical loads on the sole plate
  • at least the same plan area as the load points, e.g. studs or posts.

Packing exceeding 20mm should be agreed between the timber frame manufacturer’s engineer and NHBC. The following methods are generally acceptable to NHBC for packing up to 20mm.

Permanent structural packing under sole plate

The sole plate should be levelled on temporary spacers.

When the first lift construction (including wall panels, first floor structure, or roof structure in a single storey building) has been erected, permanent packing should be placed under the sole plate, which can be:

  • free-flowing non-shrinkable grout for the full length and width of the sole plate, or
  • individual packers placed under each load point, e.g. stud or post.

Bedding of the sole plate

The sole plate should be laid and levelled on a continuous bed of mortar prior to the erection of the wall panels.

The bedding should extend the full width of the sole plate.
Care is needed to ensure that the bedding is not disturbed during the fixing of the sole plate.

Double sole plate ‘sandwich’

The lower sole plate should be fixed to follow the contours of the supporting structure.

The upper sole plate should then be fixed on top and levelled with temporary spacers inserted between the sole plates.

When the first lift construction has been erected, permanent packing should be inserted under each load point, e.g. stud or post. As this method uses an additional sole plate, the extra shrinkage should be taken into account.

Fixing panels

The wall panels should be adequately fixed to the sole plate so that the frame can resist both lateral and vertical forces.

When fixing panels:

  • fixings, including nailed joints and sheathing, should be as scheduled in the design
  • they should be securely fixed together, to the floor and to the roof framing
  • sole plates and head binders should be provided to bind the panels together.

Support of prefabricated chimneys

Prefabricated chimneys should be supported by the:

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