Cracked concrete slab (dalle): what it is, how serious, what it costs

The slab is the concrete floor of the basement or garage, poured on a bed of crushed stone, inside the foundation walls. In a house, it usually does not carry the exterior walls: those rest on the foundation footing.

A French report says « dalle », with words like « fissures » (cracks), « dénivelé » (uneven), « dalle soulevée » (heaved), or « dalle non visible » when a finished floor covers it.

Like the foundation, the slab concrete shrinks as it cures: thin, flat cracks with no change in level are common and harmless.

What should worry you: a slab that lifts or bulges, star-shaped cracks or cracks that keep opening, one side higher than the other, water coming up through the cracks. In Quebec, a heaving slab brings pyrite to mind, and a lab test settles it. So I place it by the signs: “Can wait” for thin cracks to caulk, “To check” when the cause is unknown or the slab is hidden, “Fix first” when there is a change in level or the inspector asks for a specialist.

Sealing a crack in the slab: $300 to $800, or $700 to $1,800 for several cracks. Redoing a damaged or poorly poured section of slab: $800 to $2,000.

If a test confirms pyrite, the fix means removing the slab and the fill beneath it: that is a different order of magnitude, explained in the pyrite guide.

These are ballpark figures for Quebec, to be confirmed with quotes.

1. Are the cracks thin and flat, or is there a change in level, a bulge or star-shaped cracking?
2. How much of the slab could you see, and what covers it?
3. Did you see water or moisture coming up through the cracks?

Ask the seller whether a pyrite test was ever done, and for the report if there is one.

Does your report mention cracks in the slab? Upload it: in five minutes, I tell you what comes first, what to have checked and what to ask the seller.

Thin cracks, yes: concrete shrinks, and an unheated garage goes through freezing. A change in level, a crack that keeps opening or pieces that lift deserve a look.