You’re replacing flooring. The carpet comes up. There’s a crack running across the slab. Your stomach drops.
Take a breath. Concrete cracks. Almost every slab has some. The question isn’t whether your slab cracked. It’s why, and whether it’s still moving.
This post gives you a simple way to sort the two big causes: shrinkage and movement.
Concrete shrinks. That’s normal.
The National Ready Mixed Concrete Association puts it plainly in its guide on slab joints. Concrete tends to shrink as it dries, and that shrinkage causes cracking early in the slab’s life. Those random cracks, the guide says, generally don’t affect the integrity of the concrete.
That’s why builders cut joints. A joint is a planned crack. It gives the slab a straight line to crack along instead of a random one. NRMCA describes three kinds: contraction joints, isolation joints and construction joints. It also says you can’t stop all cracking. You can only control where it goes. Later drying can widen early cracks a little.
So a thin crack in a slab poured decades ago is often just the concrete doing what concrete does.
Poway soil can move. That’s different.
Poway’s Safety Element names fill settlement and expansive soils as main geologic hazards here. It says fill made from the Friars and Mission Valley formations is the main material behind those problems. The California Geological Survey’s map of this area shows the Friars Formation along with conglomerates named for places near Poway.
Clay that swells and shrinks with water, or fill that settles over time, can push and pull on a slab. Those cracks look and act different from shrinkage cracks.
What the code expects from a slab
The state residential code sets a few basics for slabs on the ground:
- At least 3½ inches thick (R506.1).
- Vegetation and topsoil removed first, and fill compacted for even support (R506.3).
- Unless approved otherwise, fill under the slab limited to 24 inches of clean sand or gravel, or 8 inches of earth (R506.3.1).
- Post-tensioned slabs designed to the PTI standard (R506.2).
- On expansive soils, foundations designed per the building code (R403.1.8).
Older slabs were built to the rules of their day. But this tells you what a sound slab is supposed to sit on.
Shrinkage cracks: the usual signs
- Thin. Hairline to narrow.
- Flat. Both sides at the same height. Run a fingernail across. No lip.
- Random or at corners. Often wandering, or starting at an inside corner or a joint.
- Old. Showed up early in the slab’s life.
- Stable. Not getting wider year to year.
- Quiet house. Doors and windows work fine.
Movement cracks: the warning signs
- Offset. One side higher than the other. That’s the big one.
- Widening. Gets bigger over months or years.
- Patterned. Lines up with a slope, a pad edge, or a wet area outside.
- Matched upstairs. Wall cracks, sticking doors, or a sloping floor in the same area. See sloping floors.
- Tile trouble. Tiles cracking along one line, or popping loose.
One sign alone doesn’t settle it. A pattern does.
Do this before you call anyone
Do:
- Photograph the crack with a tape measure across it. Date it.
- Check for offset with a straightedge. Note which side is higher.
- Mark the crack ends with a pencil and date them.
- Check doors and windows nearby.
- Walk outside. Look for drainage problems, wet soil or leaks on that side.
- Check again in three months.
Don’t:
- Don’t fill or patch it before you’ve measured it. You’ll hide the evidence.
- Don’t lay rigid tile over an active crack without a plan.
- Don’t drill or cut a slab marked post-tension. Tendons run through it under high tension.
- Don’t assume the worst from one picture.
Check the joints first
Before you judge a crack, find the joints. In a garage they’re easy to see: straight saw cuts or tooled grooves across the floor. Inside the house they’re hidden under flooring, but you’ll often spot them when the carpet comes up.
A crack running right down a joint is the joint doing its job. That’s success, not failure. A crack that wanders between joints is still usually shrinkage if it’s thin and flat. NRMCA’s guidance ties joint spacing to slab thickness, so wide spacing in an older slab can mean extra random cracks between the joints. Again, mostly cosmetic.
Isolation joints matter too. They separate the slab from columns, walls and other slabs so each can move on its own. If a garage slab and a house slab are tied together with no break, a crack where they meet doesn’t automatically mean the house is moving. It can just mean the two pieces wanted to move differently.
Keep it simple: note where the joints are, then note where the cracks are. Write both on a sketch of the room.
Flooring is a good time to look
Flooring jobs expose the slab, so they’re the best chance you’ll get to check it. If you’re putting down tile or stone, an active crack can telegraph through. A stable shrinkage crack can often be handled with the right underlayment. A movement crack needs the cause sorted first.
When it needs a pro
Get it checked if you see offset, widening, or cracks paired with wall and door problems. Same if you’re on a hillside pad or a lot you know has fill. Our fill settlement repair page explains fill problems. Our older post on reading cracks in Poway homes covers walls and stucco.
On a visit we take floor level readings across the house, map the cracks, check drainage and look at the lot. That tells us if the slab is moving and which way. We coordinate with licensed engineers as needed when soil testing or repair design is called for.
What repair can look like
- Shrinkage cracks: often nothing, or sealing and the right flooring prep. See foundation crack repair.
- Movement cracks: fix the cause first. Drainage, leaks, irrigation. Then repair or support the slab as the findings call for. See slab foundation repair.
Cost depends on factors like how much flooring is in the way, access, soil conditions, whether engineering is needed and any permits. No numbers until someone has measured.
Bottom line
Most slab cracks are shrinkage. Flat, thin, old and stable. Leave them be or prep them for flooring. Cracks with offset, growth or friends upstairs are a different story.
We serve the whole city of Poway. Found a crack you don’t like? Request a Free Quote.