How far apart should concrete control joints be?

Why control joints exist

Concrete shrinks as it cures. It keeps shrinking, slowly, for months afterwards. The slab is held down by friction against the ground, so as it tries to pull inward something has to give — and concrete is weak in tension. It cracks.

That is not a defect. It is what concrete does. A control joint does not stop the crack; it decides where the crack happens. Cutting a groove a quarter of the way through the slab creates a deliberately weak plane, and the shrinkage crack forms there, below the surface, in a straight line you chose.

So the honest way to say it: a slab without joints is not a slab that will not crack. It is a slab that will crack somewhere you did not pick.

The spacing rule, and why you will see two versions of it

Search this question and you will find two rules that look like they disagree:

• “Space joints 24 to 36 times the slab thickness”
• “Space joints 2 to 3 feet for every inch of slab thickness”

They are the same rule. The first is how ACI 302.1R writes it, in consistent units — thickness in inches, spacing in inches. The second is the trade shorthand, which has already divided by 12 to give you feet.

Same number, two ways of writing it. If you have been going back and forth between sources trying to work out which one to trust, that is why they never reconciled.

The part that does matter: the cap

ACI 302.1R puts a ceiling on it. Joint spacings of 24 to 36 times the thickness have produced acceptable results up to a maximum of 18 feet. Apply the 2–3× shorthand without that cap and a 7-inch slab gives you 21 feet, which is outside the guidance it came from.

There is a second figure in circulation. NRMCA CIP 6, the ready-mix industry's bulletin on slab joints, recommends keeping spacing to a maximum of 15 feet. Several websites attribute that 15-foot number to ACI; we could not find it in the ACI text. It comes from NRMCA, and plenty of contractors use it as the conservative working limit.

Spacing by slab thickness

The ACI range, and the same range held to the 15-foot working cap:

Slab thickness	ACI range (24–36×)	With a 15 ft working cap
4 in	8 – 12 ft	8 – 12 ft
5 in	10 – 15 ft	10 – 15 ft
6 in	12 – 18 ft	12 – 15 ft
7 in	14 – 18 ft *	14 – 15 ft
8 in	16 – 18 ft *	15 ft

* Capped. At 36×, a 7-inch slab would give 21 feet and an 8-inch slab 24 feet — both above the 18-foot ACI maximum, so the table stops there.

One thing worth saying plainly: this rule is written for unreinforced slabs. Rebar and fibre change the picture, and not in the direction most people assume. See the common mistakes below.

Panel shape matters as much as spacing

You can get the spacing right and still crack the slab if the panels come out long and thin. Keep each panel as close to square as the slab allows, and never longer than 1.5 times its width.

A 6 ft × 20 ft panel is inside the spacing rule in one direction and badly outside it in the other. It will crack across the middle of the long span, because that is where the shrinkage stress concentrates. Two joints turning it into three 6 × 6.7 ft panels fixes it.

How deep to cut

One quarter of the slab thickness.

Slab thickness	Cut depth
4 in	1 in
5 in	1¼ in
6 in	1½ in
8 in	2 in

A shallower cut does not weaken the section enough to attract the crack, and the slab cracks somewhere else instead — usually a few inches away from your tidy line, which looks worse than no joint at all.

When to cut

The window is narrower than people expect, and it moves with the weather.

• Hot weather: 6 to 12 hours after finishing
• Cool weather: up to about 24 hours
• Early-entry dry-cut saws: sooner, almost as soon as the slab carries your weight

Cut too early with a conventional saw and the edges ravel — the blade tears the green concrete instead of cutting it. Cut too late and the slab has already cracked where it wanted to, and your joint is decoration.

If you are hand-tooling joints with a groover rather than sawing, that happens during finishing, while the concrete is still plastic. Either method works; what matters is the depth.

Worked example: a 20 × 24 ft garage slab at 5 inches

Start with the rule and work down to a layout:

1. Spacing range: 24–36 × 5 in = 120–180 in = 10 to 15 feet
2. Pick a figure: 10 ft, to keep the panels square and stay well under the cap
3. Across the 20 ft side: 20 ÷ 2 = two 10 ft bays
4. Across the 24 ft side: 24 ÷ 2 = two 12 ft bays
5. Result: a 2 × 2 grid of 10 × 12 ft panels
6. Check the shape: 12 ÷ 10 = 1.2 : 1, inside the 1.5 : 1 limit ✓
7. Cut depth: 5 ÷ 4 = 1¼ inches

Three saw cuts in total: one down the middle each way. Add a joint at any inside corner — around a step or a door opening — because cracks start there regardless of your grid.

Where joints go besides the grid

• Inside (re-entrant) corners. Around posts, steps, door openings, any place the slab turns a corner into itself. Shrinkage stress concentrates at those corners and a crack will start there first. A short joint running diagonally out from the corner gives it somewhere to go.
• Anywhere the slab changes width. An L-shaped slab should be jointed into rectangles.
• Against walls, columns and footings — but that is a different joint. An isolation joint is a full-depth gap with a compressible filler, so the slab can move independently of whatever it meets. A control joint is a partial-depth cut. Most slabs need both, doing different jobs.

Common mistakes

• No joints at all, because the slab is “small”. If any dimension runs past about 10 feet, it wants a joint.
• Panels long and thin. The 1.5 : 1 rule gets forgotten more often than the spacing rule.
• Cuts too shallow. Half an inch on a 4-inch slab is not a control joint, it is a scratch.
• Cutting a day late. In hot weather the slab can crack within 12 hours. The joint has to be there before the concrete needs it.
• Assuming reinforcement replaces joints. It does not. Rebar and fibre control how wide a crack opens, not whether the slab shrinks. The 24–36× rule is written for unreinforced slabs, and ACI notes that reinforcement can actually increase random cracking at wider joint spacings. Reinforcement and joints solve different problems.

Frequently asked questions

Does a small shed slab need control joints?

Yes, if any dimension runs past about 10 feet. A 10 × 12 ft shed slab at 4 inches is already at the edge of the 8 to 12 ft range, so one joint across the long direction is cheap insurance. Below roughly 10 feet square you can usually skip them.

What is the difference between a control joint and an expansion joint?

A control joint is a cut partway into the slab that tells a shrinkage crack where to form. An expansion or isolation joint is a full-depth gap filled with a compressible material, used where the slab meets something it must not bond to — a wall, a column, a footing. You usually need both.

How deep should a control joint be cut?

One quarter of the slab thickness. That is 1 inch on a 4-inch slab and 1½ inches on a 6-inch slab. A shallower cut does not weaken the section enough, and the crack forms wherever it likes instead.

How long do I have to cut control joints?

Six to twelve hours after finishing in hot weather, and up to about 24 hours in cool weather. Early-entry dry-cut saws can go in sooner, almost as soon as the slab carries weight. Cut too early and the edges ravel; cut too late and the slab has already cracked where it wanted to.

Do rebar or fibres mean I can skip control joints?

No. Reinforcement controls how wide a crack opens, not whether the slab shrinks. The 24 to 36 times rule is written for unreinforced slabs, and ACI notes that reinforcement can actually increase random cracking at wider joint spacings.

What happens if control joints are spaced too far apart?

The slab cracks between them, on its own line. That crack is wider and more visible than a tooled joint, it collects water and dirt, and on an exterior slab it will open further with freeze and thaw.

Helpful resources

These are the primary sources for the figures above. Where two of them differ — the 18-foot and 15-foot caps — both are cited so you can see where each number comes from.

• Guide for Concrete Floor and Slab Construction, ACI 302.1R, Chapter 3 (PDF) Source: American Concrete Institute. The 24 to 36 times rule and the 18-foot maximum spacing, in the original wording.
• CIP 6 — Joints in Concrete Slabs on Grade (PDF) Source: National Ready Mixed Concrete Association. The 15-foot working recommendation, cut depth, and sawing time windows.
• Contraction (Control) Joints in Concrete Flatwork Source: Portland Cement Association. What the joints do, where to place them, and re-entrant corners.

Conclusion

Space joints at 24 to 36 times the slab thickness — the same as 2 to 3 feet per inch — and hold the result under 15 feet. Keep panels within 1.5 : 1. Cut a quarter of the depth, and cut within 6 to 12 hours in warm weather.

Get the thickness right first, because every number on this page depends on it — our guide to how thick a concrete slab should be covers that, and the gravel base underneath matters just as much. When you are ready to order, the concrete calculator turns your slab size into cubic yards and bag counts.