A control joint is a deliberately placed groove or gap in a block wall or concrete surface designed to control where shrinkage or thermal cracking happens, rather than eliminate cracking altogether.
Concrete and CMU block shrink slightly as they cure and shift with temperature over time. Without a planned weak point, that stress produces random cracking wherever the wall or slab happens to be weakest. Control joints are cut or tooled in at regular intervals so any cracking that does occur happens along a straight, planned line instead of scattering across the visible surface. They're standard in CMU block walls, poured concrete, and long runs of masonry.
A control joint isn't always required in brick veneer or stone, but it's expected in poured concrete work, block foundation walls, and long patio slabs, where spacing is generally set based on wall height or slab thickness so any cracking that happens stays predictable, straight, and low-visibility instead of random. Skipping them on a long run is a common shortcut that shows up as an ugly crack somewhere down the line.
More masonry vocabulary from the glossary.
The services where this term comes up.
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