An expansion joint is a deliberately placed gap in a structure that allows for the expansion and contraction of materials. Concrete, timber and metal expand in heat and contract in cold. Without expansion joints, a structure can crack, deform or be damaged by this movement.
Why are expansion joints needed?
All building materials move:
- Concrete shrinks during curing and expands in heat
- Timber moves (shrinks and swells) due to moisture absorption
- Metal expands considerably with rising temperature
A 10-metre concrete floor can expand by 3-5 mm with temperature changes. Without a joint, this force leads to 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 joints in DIY projects
Building a deck
Expansion is essential in a timber deck:
- Leave 5-8 mm spacing between deck boards — timber swells in rain
- Leave 10-15 mm spacing between the deck and a wall — the entire deck surface can expand
- Do not use boards longer than 4-5 metres without a joint
Foundations and concrete floors
When pouring a concrete slab (for example for a garage):
- Place an expansion joint every 4-5 metres
- For an L-shaped slab, always include a joint at the intersection
- At a junction with an existing wall or foundation: always include an expansion joint, as old and new concrete move differently
Building a garage
For a brick-built garage:
- Between garage and house, always include an expansion joint — the structures settle differently
- In the concrete floor, joints at a maximum of 4-5 metre intervals
- For a long garage wall (over 6 metres): consider a joint in the brickwork
Types of expansion joints
| Type | Application | Material |
|---|---|---|
| Saw cut | Concrete floors | Sawn groove, filled with sealant |
| Expansion profile | Tiled floors, facades | Plastic or aluminium profile |
| Compriband | Frames, junctions | Self-expanding foam tape |
| Sealant joint | Facades, concrete, natural stone | Elastic sealant |
| Open joint | Decks, fences | No filler, just space |
How wide should an expansion joint be?
- Concrete floor: 5-10 mm
- Facades (brickwork): 10-15 mm
- Timber deck: 5-8 mm between boards
- Frame in wall: 5-10 mm (fitting clearance = expansion)
- Tiled floor: 5-8 mm along the edges
Common mistakes
- No joint in a large concrete floor — The concrete cracks in random places instead of at a controlled location
- Filling the expansion joint with hard mortar — The joint can no longer move and becomes useless
- Deck boards tight against each other — At the first rain, the boards swell and buckle upward
Related terms
- Concrete slab (floor slab)
- Frost line
- Deck board
- Slope (fall)