A concrete control joint is a straight groove of even width, tooled or saw cut into the slab on purpose, usually spaced at regular intervals. A crack is jagged, wanders and varies in width. A thin crack running along the bottom of a joint is the joint doing its job, while a crack beside it or across a panel means the joints didn't control it.
Is this a crack or a control joint?
Look at the line’s shape. A control joint is straight, the same width end to end, and has clean edges, because someone made it with a groover or a saw. A crack is jagged, changes width, and goes wherever the concrete was weakest.
Spacing is the next clue. Control joints repeat across the slab at regular intervals, usually in a grid, and meet the edges at right angles. A crack shows up once, at an angle, often starting from a corner or a joint.
| Sign | Control joint | Crack |
|---|---|---|
| Shape | Straight line | Jagged or wandering |
| Width | Even from end to end | Varies along its length |
| Edges | Clean, square or rounded | Rough and irregular |
| Pattern | Repeats at regular spacing, often a grid | One-off, often at an angle |
| Depth | A groove about 1/4 of the slab thickness | Can run through the full slab |
| Where it starts | Edge to edge across the slab | Often at a corner, post or joint |
If you’re still not sure, run a fingertip or a coin across it. A joint has a groove you can feel with square walls. A crack is a split in a flat surface.
What is a concrete control joint?
A concrete control joint is a groove cut or tooled into a slab so it cracks there instead of somewhere random. The NRMCA puts it plainly: joints are simply pre-planned cracks (opens in a new tab). The groove makes the slab thinner along a line, so that’s where it splits as it shrinks.
InterNACHI’s concrete inspection guide explains that control joints weaken concrete by making it thinner (opens in a new tab), and that shrinkage cracks appear where the concrete is weakest. They’re also called contraction joints.
Slabs have three kinds of joints, and they look different:
- Control (contraction) joints are grooves in the surface, tooled while the concrete is soft or saw cut within hours of finishing.
- Isolation (expansion) joints are full-depth gaps filled with a fiber strip, foam or old wood, where a slab meets a wall, step, post or another slab.
- Construction joints are where one day’s pour meets the next. They’re usually straight and run full depth, with no groove.
Is a crack inside a control joint a problem?
No. A thin crack at the bottom of the groove means the joint worked. The slab shrank, cracked where it was meant to, and the crack is hidden in the joint.
InterNACHI’s guide shows that pattern and describes it as cracking taking place in the control joints, as it should (opens in a new tab). You don’t need to repair it.
Outdoors, you may still want to seal it so water and weeds stay out, the same way you’d seal the joint itself. Indoors, a basement floor crack in a joint only matters if water comes through it.
Why is there a crack next to the control joint?
Because the crack formed before the joint could control it. The usual reasons are a joint that was cut too late, cut too shallow, or spaced too far from the next one.
- Cut too late. The NRMCA says conventional saw cuts should be run within 4 to 12 hours (opens in a new tab) after finishing, and that if sawing is delayed too long, uncontrolled cracking may occur. The slab cracked on its own before the saw got there.
- Too shallow. The groove should be at least 1/4 of the slab thickness and not less than 1 inch deep, per the same sheet. A shallow groove doesn’t weaken the slab enough to pull the crack in.
- Too far apart. InterNACHI notes that if shrinkage cracking and control joint spacing both seem excessive, the cracking is probably related to spacing (opens in a new tab).
A shrinkage crack beside a joint is the same as any other shrinkage crack. Thin and flush, it’s cosmetic, and the hairline cracks guide covers how to fill and seal it.
If the crack has a raised or sunken edge, the ground under the slab has moved and the joint is not the issue. The why concrete cracks guide covers settlement and other causes.
How far apart should control joints be?
The NRMCA’s rule is 24 to 36 times the slab thickness, with 15 feet as the maximum. Its example: joint spacing for a 4 inch thick slab should be about 10 feet (opens in a new tab). InterNACHI uses a similar rule of no more than 30 times the thickness.
| Slab thickness | Joint spacing | Minimum groove depth |
|---|---|---|
| 4 inches | 8 to 12 feet | 1 inch |
| 5 inches | 10 to 15 feet | 1 1/4 inches |
| 6 inches | 12 to 15 feet | 1 1/2 inches |
Spacing is 24 to 36 times the thickness, capped at 15 feet. Depth is 1/4 of the thickness, at least 1 inch. Source: NRMCA CIP 6 (opens in a new tab).
Panel shape matters too. Both the NRMCA and InterNACHI say panels should be square or close to it, with the long side no more than 1.5 times the short side. Long, narrow panels and L-shaped panels tend to crack across the middle or at the inside corner.
If your slab has joints much farther apart than this, random cracks between them are expected. They’re not a sign of a bigger problem by themselves.
What’s the difference between a control joint and an expansion joint?
A control joint is a partial-depth groove that makes the slab crack in a straight line. An expansion joint, which the NRMCA calls an isolation joint, is a full-depth gap that separates the slab from something else so each can move on its own.
The NRMCA lists where isolation joints go: between slabs and walls, footings, or columns (opens in a new tab), and between driveways and patios and sidewalks, garage slabs, stairs and light poles. You’ll usually see a fiber strip, foam or wood in the gap.
The concrete joints guide covers both types, and the expansion joint repair guide shows how to clean out and refill the gaps.
What should you do about it?
That depends on which of the four you have:
- A control joint with no crack. Nothing, unless it’s outdoors and you want to keep water and weeds out. Then seal it with a flexible joint sealant over a backer rod.
- A crack inside a joint. The same. It’s a sealed joint either way.
- A thin crack beside a joint or across a panel. Fill it with a flexible filler. The product chooser matches the filler to the crack width and location.
- A crack with uneven edges, or one that keeps widening. Don’t fill it yet. Check the basement floor warning signs or the foundation crack guide, and have an engineer look if it’s moving.
Use flexible material in joints, not mortar or a rigid patch. Joints exist because the slab moves, and a rigid filler cracks and pulls away the first time it does.
Questions
Is a slab without control joints a defect?
Inspectors flag it. InterNACHI's concrete inspection guide says the main concerns with control joints are over-spaced joints, and especially a lack of joints (opens in a new tab). The slab will still be usable, but expect random cracks.
Does my basement floor have control joints?
Look for straight saw cuts across the floor and a gap or fiber strip around the edge where the floor meets the wall. The edge gap is an isolation joint, which lets the floor move without cracking the wall.
My old driveway has wood strips between the sections. What are they?
Those are expansion joints, also called isolation joints. Wood strips rot over time and grow weeds. The expansion joint repair guide covers replacing them.
Should I fill control joints?
Outdoors, sealing them keeps water and weeds out. Use a flexible joint sealant over a backer rod, never a rigid filler, because the joint has to keep moving.