Foundation Cracks: Which Ones Leak and Which Ones Are Just Ugly

By the Accu-Dry team | Family-owned in Westmont since 1990

You moved a storage box and there it was. Now you have spent an hour online and come away with answers ranging from "every house has them" to "call an engineer today".

Both of those are true about some cracks. The direction, the width and the material tell you which one you are looking at, and you can read all three off a phone photo. Here is how.

Start With the Direction

Direction is the single most useful thing about a crack, and it takes one look.

• Vertical, in poured concrete. Most often shrinkage. Concrete gives up water as it cures and contracts slightly, and a thin vertical crack is the usual result, generally appearing in the first couple of years after the pour. This is the common one. It can still pass water.

• Diagonal, from a corner. Usually settlement. One part of the footing has moved down relative to another. Worth watching and worth measuring, especially if it is wider at one end than the other.

• Horizontal, running along the wall. Lateral pressure. Soil outside the wall is pushing in and the wall is bending, which is how bowing basement walls start. This is the shape that earns a call this week.

• Stair-step, following the mortar joints in block. The block equivalent of the horizontal crack. The wall is failing along its weakest line, which in block is the mortar. Same answer: get it looked at.

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Then the Width, With Something in the Frame for Scale

Width is where people guess, and guessing is why photographs with a coin in them are so much more useful than photographs without one.

Under about an eighth of an inch, in a vertical poured-concrete crack, is the common shrinkage territory. Around a quarter inch and up is worth having assessed whatever its direction. And width matters less than change: a crack that is the same width it was in a photo from two years ago is behaving very differently from one that has opened since spring, even if both measure the same today.

This is the argument for photographing it now with the date on the file, even if you do nothing else after reading this.

The Thing Width Does Not Tell You

A crack can be structurally uninteresting and still put water on your floor. Those are separate questions and homeowners routinely collapse them into one.

A hairline vertical shrinkage crack that goes all the way through the wall is a path. Through most of the year it stays dry, because the soil outside drains and the water table sits below it. Then fall saturates the ground, pressure builds against the outside of the wall, and the same crack that was fine for eleven years starts weeping. Nothing about the crack changed.

So a crack that has never leaked is not evidence that it will not.

What Each One Actually Needs

• Vertical shrinkage crack, dry or weeping. Injection. Epoxy or polyurethane, filling the crack through its full depth so it stops being a path. A genuine DIY job for a confident homeowner, and a short service call otherwise.

• Diagonal settlement crack. Assessment before anything else. The question is whether the footing has stopped moving. Filling it while it is still moving means filling it again.

• Horizontal or stair-step crack. An engineer or a foundation contractor, promptly. The repair depends on how far the wall has moved and what is pushing it, and options range from carbon fiber straps to wall anchors to excavating and relieving the pressure. Filling it is the one thing that helps nobody.

If water is arriving at the floor rather than through the wall face, the crack may be a red herring entirely, and the answer is a drain tile system rather than foundation crack repair. Worth ruling in or out before anyone injects anything. If it is running down from the top of the wall, check for sill plate seepage first.

A Word on Block Walls in This Area

Clay-rich soil across Westmont and the western suburbs swells when it takes on water and pushes hard against a foundation. Poured concrete resists that reasonably well. Cinderblock resists it along the mortar, which is why so much of what we see in older block basements around here is stair-step rather than vertical.

If your walls are block and the crack follows the joints, that is the local pattern and it is worth taking seriously rather than filling. The same mortar joints are where cinderblock foundation wall seepage usually starts.

How to Photograph It So Someone Can Help

Three photos and a date. One from a few feet back so the position on the wall is obvious. One close up with a coin or tape measure touching the crack. One along the wall at a shallow angle, which is the only way an offset between the two sides shows up.

Those three will get you a useful answer from anyone who knows what they are looking at, including over email.

Frequently Asked Questions

Q: When should I worry about foundation cracks?

Two shapes earn a call the week you see them: a horizontal crack running along the wall, and a stair-step crack following the mortar joints in block. Both suggest the wall is being loaded from outside rather than simply shrinking. Anything wider than about a quarter inch, anything offset so one side sits proud of the other, and anything that has visibly moved since you first noticed it belong in the same group.

Q: Are hairline vertical cracks normal?

In poured concrete, common. Concrete shrinks as it cures and a thin vertical crack is often the result, usually within the first year or two after the pour. Common does not mean it stays dry. A shrinkage crack that reaches through the wall will pass water once the soil outside is saturated enough, and that is a repair rather than a structural problem.

Q: What does a horizontal foundation crack mean?

Usually lateral pressure. Saturated clay soil expands against the outside of the wall, and a block wall in particular resists that badly. A horizontal crack is the wall telling you it is bending. Get it looked at rather than filled.

Q: Can I just fill a crack myself?

You can fill a hairline vertical crack in poured concrete with an epoxy or polyurethane injection kit and it may well hold. Filling a horizontal or stair-step crack hides the thing you needed to see. If the wall is moving, a cosmetic fill buys you a smooth surface and no information.

Q: How do I photograph a crack so someone can assess it?

Three photos. One from a few feet back showing where on the wall it sits, one close up with a coin or a tape measure in frame for scale, and one along the wall at a shallow angle so any offset between the two sides shows. Note the date. If you have an older photo of the same wall, that is the most useful thing you own.

Q: Does a crack always mean water will come in?

No. Plenty of cracks stay dry for decades because the soil outside them drains well and the water table never reaches them. The crack is a path; whether it carries water depends on what is happening outside the wall. That is why the fall and spring months are when dormant cracks start showing up wet.

Getting a Second Opinion

Accu-Dry has been repairing foundations out of Westmont since 1990 and offers a free inspection across the towns we cover. If you have a horizontal or stair-step crack, that is the one to book rather than sit on.