Concrete guide
How Deep Should Concrete Control Joints Be?
A common rule is to cut control joints to at least one quarter of the slab thickness — 1 inch on a 4 inch slab, 1¼ inches on a 5 inch slab, and 1½ inches on a 6 inch slab. Cuts that are too shallow may not create a weak enough plane, and the slab can crack beside the joint instead of in it.
The one-quarter thickness rule
A control joint works by making the slab thinner along a line, so shrinkage stress cracks it there first. Removing about a quarter of the thickness is the long-standing rule of thumb for conventional saw cuts and tooled joints. It is the same depth guidance shown by the MudHub joint layout calculator.
| Slab thickness | Minimum cut depth (¼ t) |
|---|---|
| 4 in | 1 in |
| 5 in | 1¼ in |
| 6 in | 1½ in |
| 8 in | 2 in |
Why shallow cuts fail
If the cut is too shallow, the remaining section is still strong enough that the crack follows the path of least resistance somewhere else — often a few inches to the side of the joint, or across a panel. A crack running alongside a control joint is one of the most common signs of a cut that was too shallow, too late, or both.
Early-entry saws
Early-entry (soft-cut) saws cut sooner and usually shallower than conventional saws. Depth guidance for that method comes from the saw manufacturer and the project specifications, not the one-quarter rule. Follow those instructions when using early-entry equipment.
Depth is only half of it
The right depth cut too late still may not control cracking — see when to saw cut concrete. Plan spacing, depth, and timing together.
These are general planning rules of thumb. Project specifications, the engineer of record, and local conditions take priority.
Frequently asked questions
- How deep should I cut a 4 inch concrete slab?
- At least 1 inch, which is one quarter of the 4 inch thickness, when using a conventional saw.
- Why did my concrete crack next to the control joint?
- Common causes are a cut that was too shallow, a cut made after shrinkage cracking had already started, or panels that were too large or too long and narrow.