A control joint is a straight cut or groove placed in a concrete slab to control where it cracks. Concrete shrinks as it cures and moves with temperature changes, so it’s going to crack somewhere. The joint creates a planned weak line so cracks form neatly inside it. Control joints stay visible in most finishes and may get a flexible filler.
What Control Joints in Concrete Are For
Concrete shrinks as it dries and grows and shrinks again with the weather. When the slab is held in place by walls, posts, or even its own weight, that movement builds up stress that leads to cracking. You can’t stop it, but you can tell the crack where to go.
A control joint works like the perforated line on a sheet of paper towels. It makes a planned weak spot so the slab breaks along a straight line in the joint instead of zigzagging across the floor. The crack is still there, but it’s hidden at the bottom of the groove.
How Control Joints Are Made
Control joints are either tooled into wet concrete or saw-cut soon after it’s poured. Timing matters. Cut too late and the slab may already have cracked on its own.
The cut has to be deep enough to do its job, at least one-quarter of the slab’s thickness. Joints are laid out in a grid, and how far apart they go depends on how thick the slab is. That’s why garage floors, basements, and driveways usually show a pattern of squares.
Control Joints and Your Floor Finish
If you’re polishing, staining, or coating an existing slab, the control joints come along for the ride. They’re part of the slab’s structure, so a good contractor works with them, not over them. If a joint is simply covered over, the slab can keep moving underneath and the crack may show up in the finish instead.
Joints may be filled with a flexible filler to keep out dirt and give a smoother look. On jointed floors that get coated, your contractor should decide how to treat each joint based on how much it’s expected to move. Learn more in our concrete floor coatings guide.
Control Joint FAQ
What is a control joint?
A control joint is a groove cut or tooled into a concrete slab to create a weak line where the slab can crack neatly. Concrete always shrinks and moves a little, so cracking is expected. The joint hides that crack in a straight line instead of letting it wander.
Do control joints need to be filled?
Not always, but filling is common indoors. A flexible filler keeps dirt and debris out and makes the floor easier to clean. Your contractor should match the filler to how much the joint is expected to move.
Is it normal for control joints to crack?
Yes. A crack at the bottom of a control joint means the joint did its job. Cracks that wander outside the joints, or edges that crumble or shift, are worth having a contractor look at.
What is the difference between a control joint and an expansion joint?
A control joint is a partial-depth cut that guides cracking inside one slab. An expansion joint is a gap that separates the slab from another part of the building, such as a wall or a neighboring slab, so each can move on its own.