Pyrite in the backfill: how serious is it, and what does it cost?

Pyrite is a mineral found in some crushed-stone backfill placed under concrete slabs, in a garage or a basement. As it oxidizes in contact with moisture, it swells, lifting and cracking the slab.

The problem is common in Quebec homes built from the 1970s to the late 1990s. It is measured by a laboratory test, and the fix is to replace the backfill and the slab.

Visible signs are a garage or basement slab heaving in the middle, star-shaped or network cracks, floor tiles that “pop”, a whitish powder in the cracks, and foundation walls pushed outward at slab level.

Your building inspector can only note these signs: the backfill itself is not visible. Only a pyrite test, done by a laboratory from a core sample of the slab, settles the question. The result is expressed as the petrographic swelling potential index (IPPG). An IPPG of 10 or less is generally considered free of significant swelling risk. Above that, risk rises with the value.

A recent test with a low IPPG is reassuring. On a 1975 to 1999 house in an affected area, having no test is an unknown to resolve before the inspection condition expires, not after.

A slab that has already heaved is not a hidden defect: it is visible. Backfill with a high swelling potential and no visible damage, however, may fall under the legal warranty of quality if the seller did not disclose it and nothing allowed it to be seen.

A pyrite test costs a few hundred dollars. A complete garage correction (demolition of the slab, removal of the backfill, certified new backfill, new slab) most often runs in the tens of thousands, depending on area and access.

These are orders of magnitude. Confirm them with quotes.

Before you buy, here is what I would do.

1. Ask the seller for the pyrite test certificate, if there is one, and the year of construction.
2. If the house dates from 1975 to 1999 and no test exists, have one done during the inspection condition, allowing for laboratory turnaround.
3. Check that the garage slab was actually inspected. If it was covered (rugs, boxes, a car), the inspector will have noted it as a limitation.
4. Negotiate on the result: correction before closing, price reduction, or walking away if the seller refuses everything.

I flag any mention of pyrite, a heaved slab or a missing test in your report. When it is a sign to confirm, I file it under “To check”, and I add it to the questions for the seller and to the negotiation strategy.

Does your report mention this? Send it to me. In five minutes, I tell you what is priority, what to have checked and what to ask the seller.

Known cases cluster where quarries supplied pyrite-bearing backfill, mainly the Montérégie and some Montreal suburbs, for construction from the 1970s to the late 1990s. A house outside these areas is not excluded: only the test answers.

No. It is not required by law, but many brokers recommend it, and some lenders or insurers may ask for it on a house from the at-risk period.

No, the problem is structural: heaving and cracking of the slab. It has no known effect on air quality.