Which foundation cracks are structural (and which are just ugly)

Nearly every poured basement wall in America has cracks. A small minority of them matter. Telling the two apart comes down to direction, displacement and whether it is still moving.

Basement walls crack. Concrete shrinks as it cures, and a long wall restrained at both ends has nowhere to put that shrinkage except into a crack. Walk through any twenty-year-old subdivision and nearly every poured foundation will have at least one. Very few of them are a structural problem.

The difficulty is that the ones that matter look, to an untrained eye and in a photograph, much like the ones that do not. Here is how to read them in the order that actually discriminates.

1. Direction, which matters most

Vertical: usually shrinkage

A vertical or near-vertical crack in a poured wall, roughly uniform in width, is the signature of curing shrinkage. It is the commonest crack there is and it is structurally boring. If it leaks, injection at $400–$950 solves that, provided it is not moving.

Diagonal from an opening: stress concentration

Corners of windows and doors concentrate stress, so cracks start there. Often benign. Also, however, the way settlement first shows itself — so this is the category where measurement over time earns its keep.

Horizontal: take seriously

A basement wall is a retaining wall. Soil pushes on it laterally, and that load is greatest at mid-height. A horizontal crack, especially in the middle third of a block wall, is that wall failing in bending. Width is nearly irrelevant to the diagnosis. This one gets an engineer.

Stair-step through mortar joints: differential movement

Cracking that steps along mortar joints is following the weak path, and it means one part of the foundation has moved relative to another. Settlement, or on expansive soil, heave — and on expansive soil it is heave more often than homeowners expect. Why that happens.

2. Displacement, which is the strongest single signal

Run a finger across the crack. If you can feel a step — one face standing proud of the other — the two sides are moving independently, out of plane. That is not shrinkage. Shrinkage opens a crack; it does not offset it.

Same test vertically on a stair-step crack: if one side is higher than the other, part of the wall has dropped or risen relative to the rest.

3. Progression, which settles it

A crack that has been the same width since 1998 is a scar. The same crack growing a millimetre a year is an active failure. Nobody can distinguish those from a single photograph — including the person quoting you, however confident they sound.

So measure it:

1. Mark two pencil marks across the crack at one chosen point and write the date on the wall.
2. Measure the width at that marked point with callipers or a crack comparator card, not a tape measure. You are looking for tenths of a millimetre.
3. Repeat monthly for at least three months; a full year is better, because concrete and soil both move seasonally.
4. Photograph it with the ruler in frame each time. Dated photographs with a scale are the single most useful thing you can hand an engineer.

The tracker does the arithmetic and draws the trend, and stores everything in your own browser rather than on our server.

The signals that mean stop reading and call an engineer

• Any horizontal crack in a block wall.
• Stair-step cracking through mortar joints.
• Visible bowing or inward lean of a wall. Past roughly 50 mm of deflection the options narrow sharply.
• Out-of-plane displacement — a step you can feel across the crack.
• Measured growth over months.
• Doors and windows that have gone out of square, or floors that have started to slope.
• Anything wider than about 6 mm.

Rate your crack against all of these at once.

Who to ask

An independent structural engineer, meaning one who does not sell repairs. They charge roughly $400–$1,200 for an inspection and a written letter with a recommended scope. That letter does two things a free inspection cannot: it gives you an opinion from someone with no product to sell, and it defines a single scope that three contractors can bid on like for like.

Without it, three quotes for a foundation problem are three different scopes at three different prices, and comparing them is meaningless. With it, they are comparable, and the spread is often startling. How to run that process without paying twice.

What a free inspection is and is not

It costs no money. It is a sales visit by a person whose company sells one family of products, and the diagnosis tends to correlate with the catalogue. That is not fraud; it is incentive, and it operates whether or not anyone involved intends it. Use free inspections to gather information and prices. Do not use one as your structural assessment, and do not sign during one.

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Page data updated 2026-08-12