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Institution of Gas Engineers and ManagersStructural Timber Association Differential Movement in Platform Timber Frame6.2.8 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 juliette 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
Note(s)
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.
Figure 11: Movement allowance at window sill with masonry cladding

Figure 12: Movement allowance at window head with masonry cladding

Figure 13: Movement allowance at window head and lintel with masonry cladding

Figure 16: Movement allowance at roof to vertical abutment – before movement

Figure 17: Movement allowance at roof to vertical abutment – after movement

Figure 18: Timber frame interface with concrete or masonry communal areas – before movement

Figure 19: Timber frame interface with concrete or masonry communal areas – after movement

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
Figure 23: Drive through section details

Figure 24: Drive through Section A-A

Figure 25: Drive through Section B-B

Lightweight cladding and masonry plinth
Figure 28: Balcony abutment – lightweight cladding

Figure 29: Balcony abutment – lightweight cladding

Figure 30: Balcony abutment – masonry cladding

Figure 31: Balcony abutment – masonry cladding

Figure 32: Slotted holes in balcony support brackets

Figure 33: Sliding movement joint in balcony support bracket

Figure 34: Juliette balcony support fixed to masonry cladding

Last updated: 2nd January 2025