Concrete Control Joints: Spacing, Depth, Timing and Layout

Concrete control joints are straight grooves tooled or sawed into a slab so that it cracks along a line you picked instead of wherever it wants. Space them no more than 24 to 36 times the slab thickness (8 to 12 feet on a 4-inch slab), cut them at least a quarter of the way through, and get them in within hours of finishing, before the slab cracks on its own.

Why concrete needs control joints

Concrete shrinks as it dries and cools, and it’s weak in tension. When the ground holds it back from shrinking, it pulls itself apart and cracks. You can’t stop that, but you can decide where it happens.

That’s all a control joint is. NRMCA’s guide to joints in slabs on grade calls joints “pre-planned cracks.” The groove makes the slab thinner along a line, so when the stress builds, the crack forms under the groove, mostly out of sight.

NRMCA notes that random cracks are ugly but generally don’t affect the integrity of the concrete. Steel doesn’t stop them either: NRMCA says wire mesh won’t prevent cracking, it only keeps cracks from opening up. Our rebar vs wire mesh guide covers what steel actually does.

You’ll also hear these called contraction joints. They are not the same as isolation (expansion) joints, which separate a slab from the house, steps, a garage slab, or a sidewalk or curb where a driveway meets it.

Control joint spacing

The working rule: the maximum spacing in feet is 2 to 3 times the slab thickness in inches. NRMCA writes it as 24 to 36 times the thickness, which is the same thing, gives about 10 feet for a 4-inch slab, and caps every slab at 15 feet.

Engineer Kim Basham, writing in his rules for designing contraction joints, says the 2 to 3 times rule works if some panel cracking is acceptable. For better crack control, he keeps it to 2 to 2.5 times. Closer joints also keep the cracks under them narrower, so the two sides lock together better.

Slab thickness	Max spacing (2 to 3 ×)	Tighter spacing (2 to 2.5 ×)	Joint depth (1/4, min 1 in.)
4 in.	8 to 12 ft	8 to 10 ft	1 in.
5 in.	10 to 15 ft	10 to 12.5 ft	1 1/4 in.
6 in.	12 to 15 ft (15 ft cap)	12 to 15 ft	1 1/2 in.

Not sure how thick your slab is, or should be? Our guides to concrete patio thickness and concrete driveway thickness cover that first.

How to lay out control joints

Spacing is only half of it. Panel shape matters just as much. These rules from NRMCA and Basham apply to every slab.

• Keep panels close to square. The long side should never be more than 1.5 times the short side. Basham aims for 1.25 times for better crack control.
• No L-shaped or T-shaped panels. Avoid long, narrow panels too.
• Run joints all the way through. A joint that stops partway or jogs over sends its crack straight on into the next panel.
• Put a joint at every inside corner. Cracks start at re-entrant corners, the inside corner of an L-shaped slab, a notch around a step, or a cutout. Run joints out from those corners, or, as Basham allows, set short rebars diagonally in front of them.

Patios

Take a 12 by 16 foot patio, 4 inches thick. One joint across the middle of the 16-foot side leaves two 12 by 8 foot panels. That’s right at the 1.5 limit and at the top of the spacing range. Add a second joint down the middle of the 12-foot side and you get four 6 by 8 foot panels, which is a much safer layout.

Walks

For sidewalks and driveways, Basham spaces the cross joints at about the width of the slab. A 4-foot walk gets a joint every 4 feet, which gives square panels.

Driveways

Basham adds a joint down the center of any 4-inch driveway or walk wider than about 10 feet. A 20-foot-wide driveway gets a center joint, which makes each lane 10 feet wide, then cross joints every 10 feet, so every panel is 10 by 10.

Where the driveway meets the garage slab, a walk or a curb, NRMCA calls for an isolation joint, not a control joint.

Related: how long before you can drive on a new concrete driveway

On a project with plans, the engineer or architect lays out the joints. Without plans, Basham notes, the concrete contractor becomes the designer, so ask to see the layout before the pour. Sketching the slab to scale first, on graph paper or in our free hardscape designer, makes bad panel shapes easy to spot.

When to cut control joints

There are four ways to make the joint, and the timing depends on which one you use. NRMCA lists them this way:

Method	When	Depth
Plastic or hardboard strip	Pushed in before finishing	To the required depth
Hand groover (tooled)	Early in finishing, then run again later	1/4 of slab depth
Early-entry dry-cut saw	1 to 4 hours after finishing	At least 1 in.
Conventional saw	Within 4 to 12 hours after finishing	1/4 of depth, at least 1 in.

The right moment to saw is as soon as the blade stops tearing stones out of the joint edges, which is called raveling. If the edges ravel, NRMCA says to wait. If you wait too long, the concrete is hard to saw and random cracks may already be forming. Basham accepts a little edge raveling to get the joints in before the shrinkage stress gets too high.

Weather moves the window. NRMCA’s cold weather guide says a 20°F drop in concrete temperature roughly doubles the setting time, so a cold slab is ready to saw later. Warm concrete sets faster, so on a hot day the window opens and closes sooner. Our guide to pouring concrete in cold weather covers the rest of a cold pour.

Once the joints are in, the slab still has to cure. Our guide to how long concrete takes to cure covers keeping it moist for at least 7 days and why strength is judged at 28 days.

How deep to cut control joints

The groove has to be deep enough to be the weakest line in the slab. NRMCA’s rule is at least a quarter of the slab thickness, and never less than 1 inch. That’s 1 inch on a 4-inch slab, 1 1/4 inches on a 5-inch slab and 1 1/2 inches on a 6-inch slab.

Early-entry saw cuts are the exception. They go in sooner and shallower: NRMCA sets a 1-inch minimum, and Basham calls for 1 1/4 inches, give or take 1/4 inch, on slabs up to 9 inches thick. Some specs go deeper, to a third, but Basham notes deeper cuts leave less aggregate locking the sides together.

Two things change the depth. Fiber-reinforced slabs usually need deeper cuts, because the fibers carry tension under the groove; Basham says to get the fiber maker’s recommended depth. If the slab has wire mesh, NRMCA says to cut out every other wire, or better, stop the mesh at each joint.

Common control joint mistakes

These are the mistakes that put cracks where nobody planned them:

• Joints too far apart. A 4-inch patio jointed every 15 feet is past the 12-foot limit for its thickness.
• Long, narrow panels. A 4-foot walk jointed every 10 feet has panels 2.5 times longer than wide.
• Grooves that are too shallow. A light decorative line isn’t a control joint.
• Sawing the next morning. By then the slab may already have cracked where it wanted.

DIY or hire a pro

A small walk or patio is a fair DIY job with a hand groover, as long as you have help and the joint lines are marked before the pour. Tooled joints go in early in the finishing and get run again later, while the rest of the slab still needs floating and edging, so plan on a second set of hands.

Driveways and big patios are different. They need a saw on site at the right hour, and the layout has to account for the garage, walks and any corners. Hire a crew that saws its own joints, and ask for the layout in writing.

Frequently Asked Questions

How far apart should control joints be in a concrete driveway?

Space them at about the width of the driveway, or of each half once it has a center joint, and never more than 24 to 36 times the thickness: 8 to 12 feet on a 4-inch driveway. Add a joint down the center of a 4-inch driveway wider than about 10 feet so the panels stay close to square.

How long can you wait to cut control joints?

NRMCA says conventional saw cuts should go in within 4 to 12 hours after finishing, and early-entry saw cuts 1 to 4 hours after. Cold slabs set slower and hot ones faster, so start once the blade no longer ravels the edges.

How deep should control joints be in a 4-inch slab?

At least 1 inch. NRMCA’s rule is a quarter of the slab thickness, with 1 inch as the minimum on any slab.

Are control joints and expansion joints the same thing?

No. Control joints are grooves that make the slab crack in a straight line. Isolation joints, often called expansion joints, separate the slab from the house, steps, a garage slab, a curb or another slab so each can move on its own.

Sources: NRMCA, CIP 6: Joints in Concrete Slabs on Grade; Kim Basham, PhD, PE, FACI, Rules for Designing Contraction Joints (For Construction Pros); NRMCA, CIP 27: Cold Weather Concreting

Written and reviewed by Thayne Muthler, owner of Muthler Landscaping & Construction (PA HIC #PA212584). How we check facts: editorial policy.