Concrete Crack Diagnoser
Read a crack in concrete and pick a repair that holds — starting with why orientation matters more than width.
Orientation beats width
A hairline horizontal crack in a foundation wall is a more serious finding than a ¼″ vertical one — and most people rank them the other way round, because width is the thing you can see. A vertical crack usually means the concrete shrank, which it always does. A horizontal crack means the wall is being pushed inward by the soil behind it. That's a load the wall is losing an argument with, and unlike shrinkage it doesn't stop.
And a rigid repair of a moving crack just moves the crack. Epoxy cures stronger than the concrete around it, so if whatever opened the crack is still working, the next one forms alongside the repair instead of in it. Epoxy is for cracks that have stopped. Anything still moving needs a flexible sealant that can take the movement — and the choice comes from whether it moves, not from how wide it is.
Which makes measuring it twice the most useful thing you can do. Mark the ends with a pencil, date them, note the width, and look again in three months across a season change. A stable crack and a growing crack of identical width are different problems needing different repairs, and no single look can tell them apart.
- Crazing is the most worried-about and least serious thing concrete does. A fine network like crazed glaze is confined to the top millimetre and needs nothing at all.
- Stair-step cracking in blockwork is settlement, not shrinkage — the joints are the weak line and they've taken the movement.
- The ACI width tolerances are about corrosion, not strength. They're set to keep water and chlorides off the reinforcement, so a crack can be structurally unimportant and still wide enough to rust the steel behind it.
- One side proud of the other is always serious, whatever the width. That's displacement, not cracking.
- Filling a crack that's telling you something is the most expensive way to stop hearing it.
How to use
- Say where the crack is and what pattern it makes.
- Answer honestly about whether it is moving — that decides the repair.
- Mark and date the ends before repairing anything, if you have not already.
- Treat anything horizontal, displaced or growing as a reason to get someone on site.
Frequently asked questions
Is a horizontal crack in my foundation serious?
Yes, at any width. A horizontal crack in a foundation or retaining wall means the wall is being pushed inward by the soil behind it — that is a load the wall is losing an argument with, and unlike shrinkage it does not stop on its own. A hairline horizontal crack is a more serious finding than a quarter-inch vertical one, which is the opposite of how most people rank them.
Why does orientation matter more than width?
Because orientation tells you the cause and width only tells you how far it has got. A vertical crack in a wall is almost always drying shrinkage, which every concrete wall does. A horizontal one is a response to lateral pressure. The first is a durability question and the second is a structural one, and the widths overlap completely — so width alone cannot separate them.
Should I fill a crack with epoxy?
Only if it has stopped moving. Epoxy cures stronger than the concrete around it, so if whatever opened the crack is still working, the next crack forms alongside the repair instead of in it — you have moved the problem a few inches and spent money doing it. Epoxy restores structural continuity across a static crack, and that is the only job it is good at.
What do I use on a crack that keeps moving?
A flexible polyurethane sealant. It stretches with the crack instead of forcing the movement somewhere new, and it can be renewed when it eventually fails — which it will, because it is doing real work. The choice between rigid and flexible comes from whether the crack moves, not from how wide it is, and that is the single most common mistake in concrete repair.
How do I tell if a crack is still moving?
Mark both ends with a pencil, date the marks, note the width, and look again in three months across a season change. A crack monitor — a two-part plate that reads movement directly — costs very little and removes the guesswork. A stable crack and a growing crack of identical width are different problems with different repairs, and no single inspection can tell them apart.
What is crazing and should I worry?
A fine network of surface cracks like crazed glaze, caused by the surface drying faster than the concrete beneath it. It is confined to the top millimetre or so and does not extend into the slab. It is the most worried-about and least serious thing concrete does — filling it achieves nothing and usually looks worse than leaving it alone.
What does stair-step cracking in block mean?
Differential settlement. The crack follows the mortar joints because they are the weak line, and it steps because that is the shortest path through them — but the cause is that one part of the wall has moved relative to another. Repointing it without finding out why is treating the symptom, and the crack will come back in the same place.
How wide is too wide?
It depends on exposure, and the published tolerances are about corrosion rather than strength. Roughly 0.016 inches in dry air, 0.012 in soil or moist air, 0.007 where de-icing chemicals are used, and 0.004 for anything retaining water. Those figures exist to keep water and chlorides away from the reinforcement — a crack can be structurally unimportant and still wide enough to rust the steel behind it.
One side of the crack is higher than the other. Is that bad?
Yes, and it is a different problem from a crack. Displacement means the two sides have moved relative to each other rather than simply separating, which points at settlement, heave or a failure below. It is serious at any width and it is a reason to get somebody on site before doing anything else — a repair would hide the evidence without addressing the cause.
What about a crack that leaks?
Hydraulic cement sets against running water and is the practical answer for an active leak in a wall. Polyurethane injection foams into the crack and seals it from within, and stays flexible enough to survive some movement, which epoxy does not. Neither addresses why water is arriving there, and on a foundation that is usually a drainage question outside the wall rather than a sealing question inside it.
My slab cracked between the control joints. Why?
Because the joints were too far apart, cut too shallow, or cut too late. Concrete shrinks and something has to give — control joints decide where, and they only work if they are within the right spacing, at least a quarter of the slab depth, and cut before the slab has chosen its own cracks. A crack that is not on a joint is telling you which of those three went wrong.
Can I rely on this instead of an inspection?
No. This is a guide to reading a crack, and it deliberately errs toward getting somebody to look. Anything horizontal in a foundation wall, anything with one side proud of the other, anything growing, and anything in a wall holding back soil wants a person on site who can see the whole structure — the crack is a symptom and the cause is elsewhere by definition.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.