What is thermal expansion (dilatation)?
Thermal expansion in construction refers to the deliberate provision of a joint (gap) in a structure that allows for the expansion and contraction of materials. Concrete, timber and metal expand when heated and shrink when cooled. Without expansion joints, a structure can crack, deform or be damaged by this movement.
Why is expansion provision necessary?
All building materials move:
- Concrete shrinks as it cures and expands when heated
- Timber moves (shrinks and swells) as it absorbs or loses moisture
- Metal expands significantly with rising temperature
In a 10-metre concrete slab, thermal expansion can reach 3–5 mm across a temperature range. Without a joint, this force causes uncontrolled cracking.
An expansion joint absorbs this movement by creating a flexible break in the structure. The joint is filled with a flexible material (sealant, foam or a special profile) that moves with the structure.
Expansion provision in DIY projects
Building a deck
With a timber deck, expansion is essential:
- Leave 5–8 mm gaps between the deck boards — timber swells when wet
- Leave 10–15 mm between the deck and any wall — the entire deck surface can expand
- Do not use boards longer than 4–5 metres without an intermediate joint
Foundation and concrete slab
When pouring a concrete slab (for example for a garage):
- Include an expansion joint every 4–5 metres
- At an L-shaped slab, always provide a joint at the corner
- At a junction with an existing wall or foundation: always provide an expansion joint, as old and new concrete move differently
Building a garage
With a masonry garage:
- Always provide an expansion joint between the garage and the house — the two structures expand differently
- In the concrete floor, joints at maximum 4–5 metre intervals
- With a long garage wall (more than 6 metres): consider a joint in the masonry
Types of expansion joint
| Type | Application | Material |
|---|---|---|
| Saw cut | Concrete slabs | Sawn groove, filled with sealant |
| Expansion profile | Tiled floors, facades | Plastic or aluminium profile |
| Compriband | Frames, junctions | Self-expanding foam strip |
| Sealant joint | Facades, concrete, natural stone | Flexible sealant |
| Open joint | Decks, fences | No filling, just a gap |
How wide should an expansion joint be?
- Concrete slab: 5–10 mm
- Facades (masonry): 10–15 mm
- Timber deck: 5–8 mm between boards
- Frame in a wall: 5–10 mm (setting clearance = expansion)
- Tiled floor: 5–8 mm along the edges
Common mistakes
- No joint in a large concrete slab — the concrete cracks randomly rather than at a controlled location
- Filling an expansion joint with hard mortar — the joint can no longer move and is pointless
- Fitting deck boards tight together — at the first rainfall the boards swell and bow upward
Related terms
- Concrete slab (floor slab)
- Frost line
- Decking board
- Fall / slope
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