A crack in a poured concrete foundation wall can be a harmless one left from the concrete curing or a sign the wall is moving. Which way it runs usually tells us which, and that decides whether it needs a seal or a structural repair.
Before any concrete work
Concrete holds up very well under the weight of your house pressing straight down on it.
A poured wall is much weaker when wet clay pushes on it from the outside, or when the soil under its footing settles and drops away. Both happen here.
The clay under homes in this area swells when it is wet and shrinks when it dries out. That is why Johnson County's residential foundation guideline warns that a foundation poured on dry, shrunken clay can crack and heave once that clay soaks up water again.
Not every crack in a poured wall means the soil is moving. Concrete shrinks a little as it cures in the weeks after it is poured, and that shrinking can leave a thin crack running straight up the wall.
A crack like that is usually not a structural problem, though it can still let water into the basement during a heavy rain.
What a crack can tell you
Grab a flashlight and check three things about the crack: which way it runs, whether it is wider at one end than the other, and whether the wall next to it leans in.
Look for a hairline crack running straight up and down a poured wall.
It usually formed as the concrete cured and shrank, years before you noticed it. It is rarely a structural problem, but it can still let water in during a heavy rain.
Some cracks run sideways along a poured wall instead of up and down, often partway up the wall.
That usually means the soil outside has been pushing on the wall from the yard side.
Hold a level or a long, straight board flat against the wall, running from the floor up toward the ceiling and across the crack. If the wall above or below the crack leans in, even slightly, you will see a gap behind the board.
A leaning wall is still moving, and it needs more than a seal.
Find a crack that runs up and down the wall, from the floor toward the ceiling. Check whether it is wide up near the ceiling and narrows to a hairline down by the floor, or the other way around.
That shape shows one part of the wall has dropped, which usually means the footing under that end of the wall has settled.
After a heavy rain, look for a damp, dark line along the crack or water dripping from it.
That crack is letting water through the concrete, even if the wall itself is not moving.
Look for a white, powdery crust along the crack, even if the crack is dry today. It is called efflorescence.
Efflorescence is the minerals water leaves behind as it dries, so that crack has leaked before and can leak again in the next storm.
A crack across the basement floor on its own is common, often from the concrete shrinking as it cured. What matters is whether it meets a crack in the wall beside it.
When the two line up, the ground under that part of the house has likely moved.
Slide your foot across the floor where it cracks. If one side of the crack sits higher than the other, the floor has lifted or dropped at that edge.
That means the ground under the slab has moved, not just the concrete on top of it.
Call us as soon as you notice the crack, even if it looks small. While you wait for your inspection, mark each end of the crack with a pencil and write the date beside it.
If the crack grows past those marks before we get there, fast or slow, it tells us the wall is still moving.
How we repair concrete
The right repair depends on what made the crack. A crack left from the concrete curing only needs sealing, while a crack from a wall or footing that moved needs the movement fixed first, or the seal will crack open again.
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Your concrete foundation inspection
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We start at the crack with you standing right there, because the direction it runs usually tells us what made it. A thin crack straight up the wall usually formed as the concrete cured, while a crack running sideways points to soil pushing from outside.
We point out what we see as we go, so you know which kind you are looking at and why it matters for the repair.
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Next we check the wall along the crack for any lean, from the floor up toward the ceiling, and whether one side of the crack has shifted out past the other. Either one means the wall is moving, not just cracked.
If it has moved, we show you where. A wall that is moving needs that fixed before any seal will hold, or the crack will open again.
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Then we check the footing, the wide concrete base the wall sits on, to see whether it is still level. We look at how the crack changes from the ceiling down to the floor, and whether the floor slopes toward that same corner.
A crack wider at one end than the other usually means part of the footing has dropped, and we will show you which end.
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Last, we look along the crack for signs of water: a dark, damp line after rain, a brown or rust-colored stain running down from it, or a white crust called efflorescence.
We also check the floor below the crack and the bottoms of any boxes or shelves against that wall. A crack can leak for years before anyone sees water on the floor, so these marks often show up first.
Where we work
We're based in Gardner, and we're a growing company. Right now we're expanding about 45 miles out in every direction, which covers most of the metro on both sides of the state line. Not sure if we make it out your way? Give us a call and ask.
Free estimate
In the message box, tell us which wall the crack is on, which way it runs and roughly how wide it gets at its widest point.
If you can take a photo with a coin held next to the crack, keep it handy for when we call you back.
Worth knowing
Most foundation cracks get caulked, painted over or ignored. Here is what to know before you do any of those.
Concrete shrinks a little as it cures in the weeks after it is poured. That shrinking often leaves a thin crack running straight up the wall, and many of those cracks were there years before anyone noticed them.
A curing crack is usually hairline thin, about the same width from the floor to the ceiling, and the wall on both sides of it sits flat. Mark its ends with a pencil, and if it never grows past those marks, the wall is not moving.
Those cracks do not need a foundation repair. You may still want one sealed, though, because even a hairline crack can let water into the basement during a heavy rain.
A crack that runs sideways, gets wider at one end or has the wall leaning in beside it is a different story. That points to a wall or footing that is moving, and it is worth having us look at.
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