External timber framed walls

6.2.8Differential 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
  • eaves and verges
  • balconies (including Juliet balconies)
  • service entries
  • openings for drive-throughs
  • staircases and lift shaft enclosures (where they are not timber framed)
  • the interface of the timber frame with any other construction at each floor level where cladding is fixed to the timber frame.

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 a weather resistant and durable joint
  • be protected by a cover strip where the movement gap/joint is expected to be more than 35mm.

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

Table 1: Gap sizes to accommodate differential movement


1 Ground storey 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, e.g. 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 I-joists
  • Timber joist and rim 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 and 10mm for engineered I-joists.

Differential movement should be accommodated by the timber frame and by the services affected, especially where they:

  • are within the timber frame construction/envelope
  • pass through the 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 are omitted for clarity.

Window head and sill with masonry cladding

Window head and sill with lightweight cladding

Roof to vertical abutment

Timber frame interface with concrete or masonry stairs and common areas

Eaves and verges


Drive through

Lightweight wall cladding – joint at each floor level (with and without insulation in cavity)

Lightweight cladding and masonry plinth

Balcony abutment – lightweight cladding

Balcony abutment – masonry cladding

Walls to flat roof abutment