Expansion joints separate panels, slabs, cladding and structures so thermal growth, shrinkage, wind sway and settlement close and open the gap instead of breaking material. The gap is sealed with an elastomeric sealant, a preformed seal or a manufactured cover system. Joint width is calculated from the expected movement and the sealant's movement capability, not chosen by habit.
Expansion joint at the parapet's 3/4 inch, but the design movement says we need a full inch even with a class 50.
Measure joints at install temperature: a joint sealed on a cold morning is already near its widest and will spend the summer in compression.
Also heard as
- movement joint
- EJ
Do not mix it up with
- Control joint: A control joint is a planned weak line so concrete or masonry cracks there; an expansion joint is a full separation that allows real movement.
Related terms
Control joint
A cut or formed groove that makes concrete or masonry crack in a straight, planned line instead of randomly.
Movement capability
How far a sealant can stretch and compress, as a percentage of joint width, while keeping its seal.
Backer rod
A round foam cord pushed into a joint to set sealant depth and keep the sealant from bonding to the back.
Width-to-depth ratio
The proportion between a sealant joint's width and the sealant's depth, usually targeted at about 2 to 1.