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

In short

Pyrite is a mineral found in some crushed-stone backfill placed under concrete slabs (garage, 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.

How it shows

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.

The 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.

What it changes for the purchase

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

A slab 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.

Orders of magnitude

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 to confirm with quotes.

What to do before buying

Ask the seller for the pyrite test certificate, if any, and the year of construction.
If the house dates from 1975 to 1999 and no test exists, have one done during the inspection condition, allowing for laboratory turnaround.
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.
Negotiate on the result: correction before closing, price reduction, or walking away if the seller refuses everything.

What Clarity does with it

Clarity flags any mention of pyrite, a heaved slab or a missing test in the report, files it under “have checked with the inspector” when it is a sign to confirm, and adds it to the questions for the seller and the negotiation strategy.

Does your report mention this? Clarity tells you in five minutes what is priority, what to have checked and what to ask the seller.

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Frequently asked questions

Which parts of Quebec are affected by pyrite?

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.

Is a pyrite test mandatory?

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.

Is pyrite a health hazard?

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

Sources