Wide cracks in concrete

A crack wide enough to accept a fingertip or a coin is doing more than relieving shrinkage. Something has moved, and the width is the gap that movement left behind.

Width is the thing homeowners notice and the thing search results obsess over, but it is not the most informative measurement available to you. Whether one side of the crack sits higher than the other, and whether the gap is still opening, both tell you more. This page is about reading a wide crack properly rather than reading its width.

What it is

A wide crack is a separation that has continued past the point where the two faces stayed in contact. In flatwork, that usually means one part of the slab is no longer supported the same way as the part next to it, or the slab has been pulled apart by movement it could not accommodate.

The two most useful distinctions are lateral versus vertical. A crack that has opened sideways but where both sides remain level is a different situation from one where one side has dropped — the second involves loss of support beneath, and repairing the crack without addressing that leaves the cause in place.

Wide cracks also tend to be self-reinforcing as water paths. Once the gap is wide enough to let surface water down to the base, that water can move fine material, which can remove support, which can widen the gap.

What seeing this cannot establish

• A wide crack does not establish that a slab has failed, and it does not establish that replacement is required. Plenty of wide cracks in flatwork are stable, repairable, and not going anywhere.
• Width does not indicate depth. A wide surface crack over sound concrete and a full-depth separation can present similarly from above.
• Nothing about a wide crack, on its own, distinguishes a settlement cause from a shrinkage crack that has been widened by years of water and freeze-thaw at its edges. Those call for different work.
• A crack in flatwork and a crack in a foundation wall or retaining wall are different subjects. Width in a structural element is a question for someone qualified to assess load, not for a website.

What this can be associated with

Candidate explanations, not findings. Nobody can establish which applies to your concrete without seeing it.

• Loss of support beneath one side of the slab, which can be associated with soil moisture change, poor drainage or a washout path
• A shrinkage crack that has been progressively opened by water entry and freeze-thaw cycling at its faces
• Movement at a joint that was not able to accommodate thermal expansion, transferring the movement into the slab instead
• Root growth beneath or beside the slab displacing a section
• Load beyond what the slab thickness and reinforcement were arranged for, such as repeated heavy vehicle use on a slab poured for cars

What you can safely check yourself

• Lay a straightedge or a long level across the crack and photograph it from the side. Any daylight under one end tells you the sides have moved vertically, which is the single most useful observation on this page.
• Measure the gap at three points along its length and note them. A crack that is wide in the middle and closes at both ends behaves differently from one that widens steadily towards one end.
• Check whether the gap has debris or vegetation growing in it. That indicates it has been open long enough to collect material, which speaks to age rather than severity.
• Pour a small amount of water into the crack from a bottle and watch. Water that disappears immediately suggests a path to somewhere; water that pools suggests the crack is not full depth or the base is saturated.
• Photograph both ends of the crack. Where a crack terminates is often more diagnostic than its middle.

When this is frequently cosmetic

• A wide but level, stable crack in a patio or walkway where nothing has dropped and the surface drains away from the house
• An old crack that has clearly been open for years, has collected debris, and shows no vertical offset

When professional evaluation matters more

• One side of the crack is higher than the other by enough to catch a toe or a mower wheel
• The gap has measurably widened between two dated photographs
• The crack is in a foundation wall, a retaining wall, or a slab that supports something
• Water disappears into the crack and reappears somewhere you did not expect, such as a basement wall
• The slab immediately beside the crack sounds hollow when tapped, or flexes underfoot

Evidence worth having before you ask for estimates

Every item here is something you can produce without tools or risk, and every one of them makes an estimate more accurate.

• A straightedge photograph showing whether there is vertical offset, and how much
• Gap measurements at three points, dated
• A photograph of each end of the crack
• A note of where roof water and surface water go in the area
• Whether anything heavy has been parked or driven on the slab that was not there before

Questions to ask whoever comes out

• Do you think the two sides of this crack have moved vertically, and how are you checking?
• If there is a support problem underneath, does your proposed repair address it or just the gap?
• How will you handle the fact that this crack will keep moving seasonally, if it does?
• What will the repair look like when it is finished — will it be visible, and will it match?
• If you filled this and it reopened in two years, what would that tell us?

Where estimates for this commonly differ in scope

These are the places two prices for apparently the same work stop being comparable.

• Filling a wide crack and stabilising what is under it are two different scopes. An estimate that quotes only the first may be perfectly reasonable — but you should know which one you are buying.
• Some estimates propose replacing the cracked panel and some propose repairing it. Those are not competing prices for the same work and should not be compared as if they were.
• "Flexible" and "rigid" fillers behave very differently in a crack that moves seasonally. An estimate that does not say which is leaving out the thing that determines whether it lasts.

Who tends to do this work

• A concrete repair specialist
• A concrete leveling specialist
• A residential flatwork concrete contractor

ConcreteOwner does not inspect, certify or approve contractors. This describes which trade usually holds this work, not a recommendation of anybody in particular.