6.2 External timber framed walls

6.2.8 Differential movement

Timber structures shall account for differential movement between the timber frame wall and other building elements.

As the timber frame dries out, it will shrink and the overall height will reduce. The extent of the differential movement increases with the number of storeys, and will typically occur between the timber frame and other parts of the construction, including:

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

Where lightweight cladding is fixed to the timber frame, movement across floor zones should be allowed for in the cladding system and its supporting structure.

Movement joints should be provided to accommodate the expected movement. Joints should be detailed to:

  • accommodate the expected amount of shrinkage or expansion safely
  • provide an additional allowance for the residual thickness of any compressible filler materials after movement has occurred
  • provide a weather resistant and durable joint
  • be protected by a cover strip where the movement gap/joint is expected to be more than 35mm.

Where compressible filler materials are specified, they should be specified based on the anticipated residual gap size after movement has occurred.

In the absence of project-specific calculations, gaps in accordance with Table 1 should be provided.

Table 1: Gap sizes with masonry cladding to accommodate differential movement

Notes

1. Ground floor or lowest level of timber frame.
2. Calculations, where required, are to be based on BS EN 1995-1-1.

Table 1 is based on the following:

  • the table allows for a 2mm thickness of compressible material in closing gaps. Check the manufacturer’s product details
  • timber components are not saturated and have normal moisture contents at the time of construction, eg less than 20% and tight-jointed construction
  • the ground floor is concrete. For ground floors of timber joists, add 15mm for solid timber and 10mm for engineered timber joists
  • timber joist and ring beam/header joist have a maximum depth of 240mm
  • timber frame floor cross-section is as shown below, with maximum 45mm deep timber plates/binders
  • single head binder at the eaves. Maximum double sole plates
  • outer leaf brickwork with expansion rates no greater than 2.5mm per storey
  • brickwork up to five storeys, with lightweight cladding above five storeys
  • lightweight cladding – floor level joints must be 15mm for solid timber joists and 10mm for engineered timber joists.

Differential movement should be accommodated by the services where they:

  • are within the timber frame construction/envelope and pass across floor zones
  • pass through the external envelope.

Common details

The following sketches consider downward movement of the
timber frame and upward brick expansion, taken as 2.5mm
per storey of clay masonry. Cavity trays, cavity barriers, open
perpend vents etc are omitted for clarity.

Eaves and verges

Where a movement gap provided between the top of an exterior cladding of masonry and the eaves or verge soffit exceeds 10mm and is not protected by a purposely designed overhang, the gap should be protected with a compressible filler material or mesh to prevent the entry of birds, etc. The filler material or mesh should be flexible so as not to inhibit the differential movement between the exterior cladding of masonry and timber frame. Where a flexible filler is used, it should be sized based on the anticipated residual gap after movement has occurred.

Services

Drive through

Lightweight cladding and masonry plinth

Last updated: 2nd January 2024

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