When to Worry About Cracks in Basement Floor: The Warning Signs

When should you worry about cracks in basement floor?

Worry when the crack shows movement, not just width. ASHI inspector Tom Feiza says to be concerned if the cracks are parallel to the wall and footings or if there is vertical movement (opens in a new tab). Thin, random cracks with flush edges are normal shrinkage.

These are the warning signs, roughly in order of how much they matter:

• One side of the crack is higher or lower than the other.
• The floor is humped, tipped or sloping toward the crack.
• The crack runs parallel to a wall, a short way in, near the footing.
• The floor crack lines up with a crack in the wall.
• The crack is wider than 1/4 inch.
• The crack keeps getting longer or wider over months.

If none of those apply, the crack is almost certainly cosmetic. The basement floor cracks guide explains why most floors crack and what each pattern means.

How wide is too wide for a basement floor crack?

Over 1/4 inch is the line inspectors use. InterNACHI’s guidance says slab cracks less than 1/4” and which exhibit no significant vertical or horizontal displacement (opens in a new tab) are not regarded as structurally threatening. Wider cracks, or narrower ones with displacement, deserve a closer look.

The same article admits there is no absolute standard for evaluating slab cracks. Basement floor crack width is a guide, not a verdict. A 1/8 inch crack with one edge raised tells you more than a flush 1/4 inch crack.

Measure at the widest point with a crack width card or a set of feeler gauges. A tape measure is too hard to read on a thin crack.

Sources: InterNACHI (opens in a new tab), ASHI Reporter (opens in a new tab), NRMCA CIP 6 (opens in a new tab).

What does an uneven basement floor crack mean?

It means the ground under one side of the slab has moved. Either the soil pushed one side up (heave) or it dropped away under the other (settlement). An uneven basement floor crack is the clearest sign that the problem is below the concrete, not in it.

To check, lay a level or any straight board across the crack. If it rocks, or you can slide a coin under one side, the edges aren’t flush. Run your hand across it too: you can feel a step you can’t see in a dim basement.

Feiza is direct about it: floors that are heaved and cracked are a problem (opens in a new tab), and floors that are tipped and cracked need to be evaluated. If the floor is lifting, read the heaving cracks guide next.

Is a crack running along the basement wall a bad sign?

It can be. A crack that runs parallel to the wall, a short way in from it, often sits right over the edge of the footing. Feiza notes that there often will be a crack in the floor where the footing has started to drop (opens in a new tab).

Look at the wall above that crack. If the wall has a crack that meets the floor crack, Feiza says floor cracks that align with wall cracks can indicate a problem. The foundation crack guide covers which wall cracks matter.

Don’t confuse this with the gap where the floor meets the wall. Feiza notes floors often have gaps between the floor and the wall (opens in a new tab) because the slab shrinks away as it cures. That gap is a water path, not usually a structural sign.

How do you check if your basement floor is sloping?

Use a long level or a laser level and measure how far the floor drops over a set distance. InterNACHI reports that many structural engineers agree one inch of slope in twenty feet is tolerable (opens in a new tab), and that people rarely notice a difference until it’s more than that.

1. Set a 4 foot level, or a straight board with a level on it, across the floor at the high side.
2. Raise the low end until the bubble centers, and measure the gap under it.
3. Move along the floor and repeat, adding up the drop over 20 feet.
4. Note where the floor is lowest or highest. A hump or a dip next to the crack matters more than a gentle slope across the room.

A floor that slopes evenly toward a floor drain may have been poured that way. A floor that dips at a crack, or humps up in the middle, has moved.

What other signs go with a serious floor crack?

Movement under a slab rarely shows up in just one place. Check these at the same time as the crack:

• Doors and windows. The Colorado Geological Survey says doors and windows that don’t open and close properly, or have gaps at the frame, may be due to heaving from swelling soils (opens in a new tab).
• Drywall. The same guide notes that diagonal cracks around door and window frames are probably the result of swelling soils, settling or other foundation movement. Straight cracks are more often taping or framing shrinkage.
• Pipes. Water, sewer and gas lines should be checked to see if they are bowing or pulling apart, according to the same guide. Feiza adds that crushed lead plumbing pipes can show up where they enter the concrete floor.
• Wall cracks. New cracks in the foundation walls, especially near the floor crack.
• The sump. A sump pump that runs constantly, or never runs during wet weather, points to water under the slab.

One of these on its own may mean nothing. Two or three together, plus an uneven floor crack, is a pattern worth an engineer’s time.

Which basement floor cracks are not a concern?

Random, spiderweb and inside-corner cracks with flush edges are normal. Feiza says not to be concerned with random spiderweb-like cracks or cracks that occur from an inside corner (opens in a new tab). They come from the concrete shrinking as it dried.

The NRMCA says early random cracks, although unsightly, rarely affect the structural integrity (opens in a new tab) of the concrete. A crack along a control joint is the joint doing its job.

These are worth sealing only if water, soil gas or dirt comes through, or before you put down flooring. The basement floor crack repair guide shows how, and the before new flooring guide covers what to do under tile or plank floors.

What should you do if your floor crack shows a warning sign?

Record it, watch it and have an engineer look before anyone fills it. Filling the crack hides the evidence, and a contractor who sells repairs has a reason to find a problem.

1. Photograph it

   Take a close photo with a ruler or crack card across the widest point, and a wide photo that shows where the crack sits in the room.

2. Mark and measure

   Pencil a line across each end and two ticks at the widest point. Write the width and the date beside them.

3. Check the edges and slope

   Lay a straightedge across the crack and note any step. Check the floor for a hump or a dip with a level.

4. Look for the other signs

   Doors, drywall, pipes, wall cracks and the sump. Write down what you find.

5. Keep measuring

   The Colorado Geological Survey suggests measuring cracks every month (opens in a new tab) and noting whether they stay the same, steadily increase, or open and close with the seasons. The crack monitoring guide shows how.

6. Call an engineer

   A structural engineer can tell you whether it’s heave, settlement or footing movement and what fixes it. See engineer vs contractor for what to ask.

If water is coming up through the crack at the same time, look at drainage first. The survey also says control of surface and subsurface drainage will reduce the risk (opens in a new tab) of damage from swelling soils, and Feiza ties heaved floors to drain tile and sump problems.