Pyrite under the basement slab: what Montréal-area buyers should know

Almost every basement floor I walk on has a crack or two. Most of them are harmless shrinkage cracks from when the concrete cured. But once in a while the slab is not just cracked, it is lifting, and the cause may be pyrite in the stone underneath. It is not the most common thing I find, but when it is there it is expensive, so it is worth understanding before you buy.

What it is

Basement slabs in most Québec houses are poured over a bed of crushed stone. Some of that stone contains pyrite, an iron sulfide mineral. When pyrite gets moisture and air, it can react and the material swells. The swelling pushes up on the slab from below, slowly, over years. It is a different problem from pyrrhotite in concrete, which affects the foundation walls and footings and is a separate topic. Here the concrete may be fine; it is the fill under it that is changing.

How it shows up in Montréal-area houses

There is no neighbourhood to avoid. It depends on where the stone came from, and nobody can tell that by looking at the house. It concerns houses with a poured slab over a stone bed, so mostly houses built in the last decades of the twentieth century and after, rather than the older plexes with thick, original basement floors. What I see is a slab that has domed or lifted, cracks that spread across the floor and keep widening, and a floor that is no longer level. Upstairs or around it, interior partitions can rise with the slab, doors start to stick, and baseboards separate from the floor. Sometimes there is a whitish deposit in the cracks. None of these signs proves pyrite. Settlement, bad drainage or frost can do some of the same things, which is why the cause has to be confirmed by someone with a lab.

What I check during the inspection

I look at the whole basement floor, not just the corners: cracks and how wide they are, whether they are offset (one side higher than the other), and whether the slab is domed, which I can see by sighting along the floor and with a level. I check where the slab meets the walls and the stairs, and whether partitions, door frames and baseboards have moved. I look at floor coverings that hide the concrete, and ask the seller what was covered and when. I use the moisture tools to see whether water is reaching the slab, since pyrite needs water to react.

What I cannot do is identify pyrite. It sits under a few inches of concrete. I can tell you that a slab is lifting and that it looks like the pattern associated with pyrite, and my report says what I saw, with photos, in line with the BNQ 3009-500 standard.

What it means for you as a buyer

A single hairline crack is not a reason to worry. A lifting slab is different. The repair generally means breaking out the slab, removing the fill that is reacting and pouring a new floor, and the work disrupts the whole basement. I won't give you a price, because it depends on the area and the finishes, but get a number from a contractor before your condition expires. Also ask your insurer how it treats this kind of damage, since coverage is often limited. Read the seller's declaration for anything about the slab, cracks or floor repairs, and ask directly about pyrite. If a floor was recently finished, ask why.

What to do next

If my report flags a lifting or domed slab, ask a structural or materials engineer to look at it. They can take a sample of the stone under the slab and have it tested in a lab, which is the only real way to confirm pyrite. Try to do it before the inspection deadline in your promise to purchase, or ask your broker to extend it. If you are still comparing houses, you can read about what I look at on a pre-purchase inspection or on a larger building inspection.

Buying a house with a basement floor that doesn't look right? Call 514-924-8887.