Control Joints in Concrete: Spacing, Depth & Timing
By Marko VisicBSc Physics, University of Ljubljana
A concrete slab control joint does not stop the slab from cracking. It gives the crack a place to form, as long as the joint is in before the slab cracks on its own.
Space control joints 24 to 36 times the slab thickness, about 10 ft for a 4-inch slab and never more than 15 ft, cut at least a quarter of the depth (at least 1 in), and saw within 4 to 12 hours of finishing (NRMCA CIP 6).
10ft
between control joints on a four-inch slab
How far apart should control joints be?
The NRMCA rule is to space contraction joints 24 to 36 times the slab thickness, which works out to about 10 ft for a 4-inch slab, and to keep them to no more than 15 ft apart. The table gives that range and the cut depth for common slab thicknesses.
How deep to cut a control joint
Cut the groove at least a quarter of the slab thickness, and never less than 1 inch. That is deep enough to force the crack to start at the bottom of the groove and run straight down, hidden, instead of wandering across the surface. A 4-inch slab needs a one-inch cut; a 6-inch slab, an inch and a half.
When to saw cut joints
For conventional concrete saws, run the saw-cut joints within 4 to 12 hours after finishing. Early-entry dry-cut saws go sooner, 1 to 4 hours after finishing. If the edges of the cut ravel, wait a little; if you wait too long, uncontrolled cracking may occur before you cut.
The ACI FAQ gives the same 4 to 12 hours window but counts it from placing, not from finishing, so check which starting point a figure uses.
Other ways to form a joint
A joint can also be tooled in with a groover while the concrete is still plastic, run early in finishing and rerun later so the groove does not bond back together. It can also be formed with preformed plastic or hardboard strips pressed into the surface to the required depth before finishing. All three do the same job as a saw cut: a planned weak line that steers the crack.
Laying out the panels
Aim for square panels. Keep each panel's length to no more than 1.5 times its width, and avoid L-shaped panels, which crack at the inside corner.
Run joints through the openings left for columns, and cut or stop wire mesh at each joint so the steel does not keep the joint from opening. The concrete slab guide covers the thickness, base and reinforcement that go with the jointing.
Concrete patio control joints
A 12 by 12 ft patio at 4 inches wants a joint each way at the midpoint: that makes four 6 by 6 ft panels, every side well under the 4-inch slab's 10 ft guide and square, so none exceeds the 1.5-to-1 shape limit. Size the slab first in the patio thickness guide.
Driveway joints
A 12 by 40 ft driveway at 4 inches takes transverse joints every 10 ft, making four 12 by 10 ft panels (a 1.2-to-1 shape, within the limit). See the driveway thickness guide and crack repair for the rest of the driveway.
Control-joints FAQ
How far apart should control joints be in concrete?
Space them 24 to 36 times the slab thickness, which is about 10 ft for a 4-inch slab, and never more than 15 ft (NRMCA CIP 6).
Are control joints necessary?
Yes. Cracks in concrete cannot be prevented entirely, only controlled. Joints create planned weak lines so the slab cracks where you want it to, under the joint, instead of randomly across the surface.
How do you make a control joint?
Either saw a groove with a concrete saw, or tool one in while finishing, or set a preformed plastic strip before finishing. Cut it at least a quarter of the slab depth, and at least 1 inch.
Can you cut control joints a full day later?
It is risky. NRMCA says to saw within 4 to 12 hours of finishing; wait too long and the slab can crack on its own before you cut it. Early-entry dry-cut saws go sooner, 1 to 4 hours after finishing.
Sources & methodology
- National Ready Mixed Concrete Association (NRMCA). Concrete in Practice CIP 6: Joints in Concrete Slabs on Grade. 2014 revision
- American Concrete Institute (ACI). FAQ: When should saw cuts be made on a concrete slab, and what precautions should be taken to prevent damage?. page as retrieved Nov 2026 (undated)
- Storey County (NV) Community Development. Slab / Slab-Gravel Inspection Checklist (cites IRC R506.1). Checklist dated May 5, 2022 (IRC edition adopted by Storey County)
Spacing is the slab thickness times 24 to 36, capped at 15 ft; the cut depth is a quarter of the thickness, at least an inch. Full method on the methodology page.
- Concrete Slab GuideThickness, base, reinforcement, cost and joints for a residential slab, in one place.
- Concrete Driveway Crack RepairMatch the fix to the damage once a slab has already cracked.
- How Thick Should a Concrete Patio Be?Size the slab before you plan the joints that steer its cracks.
Marko Visic
I'm Marko Visic, a physics graduate who builds data-driven calculators. I run ConstructionCalc through my company, Moving Data Systems d.o.o.
The calculators help you plan a DIY job or check a contractor's quote before you sign. Each one cites its sources and shows its method, and cost figures are computed from public primary data. If a number looks wrong, email info@constructioncalc.org and I'll trace it to its source or fix it.