What we document by eye, what we cannot confirm through visual inspection, and when laboratory petrographic analysis becomes necessary. Educational page — not engineering expertise or legal advice.
Pyrite (FeS₂, sometimes called "fool's gold") is a sulphide mineral naturally present in certain stone and granular backfill. In Greater Montreal, the pyrite-related risk concerns mainly crushed stone backfill used under basement and garage concrete slabs between roughly 1985 and 1995. When pyrite in such backfill comes into contact with air and moisture, it can slowly oxidize and form sulphates. This chemical reaction is accompanied by volumetric expansion of the backfill. Over several years, this pressure may heave the slab and cause cracks in the slab, foundation walls or adjacent walls. Not all backfill contains pyrite, and not all pyrite-bearing backfill necessarily causes damage — the concentration, mineral type and site conditions determine the actual risk.
During a pre-purchase inspection, we document the visible signs that may suggest the need for further investigation. None of these indicators, taken alone, confirms the presence of pyrite — each may have other causes. It is their combination, location and the property's context that guide the recommendation.
The basement or garage slab is no longer level, or shows visible doming at the centre. Measurable with a spirit level or by observing interior baseboards and door frames that are no longer aligned.
Star, spider-web or straight-line cracks running across the slab. Width, orientation and evolution are documented. An active crack (moving, widening) is more concerning than a stabilized crack.
Vertical or diagonal cracks at the ends of the slab, horizontal displacement of the wall's lower courses, cracks at the slab–wall junction.
Unheated garage slabs are often the first to show signs — exposure to thermal swings, absence of finishing that could mask movement.
As heaving progresses, door frames deform slightly. Interior doors may start to rub or fail to close properly, particularly those located above the affected slab.
Cracks in stucco or brick at the exterior foundation level, mirroring the cracks observed inside. Useful complementary documentation for evaluating the movement's trajectory.
Important: these visible indicators are not specific to pyrite. Heaving, cracks and displacement can also be caused by differential settlement, frost action, thermal stress, concrete placement defects, tree roots or hydrostatic pressure. Only a laboratory petrographic analysis can confirm or rule out pyrite as the cause.
Our visual inspection follows the InterNACHI standard. For pyrite specifically, there are strict limitations that are important to understand before purchasing a property.
We recommend considering laboratory petrographic analysis in the following situations, ideally before removing the conditions on the purchase offer.
These regions have documented cases, but this does not mean every property is affected. Each construction site depended on the supplier and backfill batch used.
Other Greater Montreal areas may include affected properties without being publicly documented. Case-by-case verification remains recommended depending on each purchase context.
If a purchase offer has a tight condition-removal deadline and visible pyrite indicators are observed during the pre-purchase inspection, you have several options depending on your risk tolerance and the seller's cooperation.
What is documented during an inspection, what is not, and how to confirm.
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