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 jo Recommended joint widths per application, as given in the article.
int 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
