Not all cracks are equal. A fine shrinkage crack is common, while a heaved slab or a wall with map cracking can signal major work. In Québec, two known problems affect thousands of homes: pyrite in the fill under slabs, mainly in greater Montréal, and pyrrhotite in foundation concrete, mainly in Mauricie and part of Centre-du-Québec.
Reading a foundation crack
The inspector examines each crack's shape, width, direction and location. A fine vertical crack near a basement window is often a shrinkage crack. A horizontal crack, stair-step cracking in blocks, or an offset between the two edges can signal soil pressure or settlement.
They also look for signs of activity: leaks, efflorescence, recent caulking that has reopened, sticking doors or sloping floors. Depending on what they see, they recommend monitoring, a waterproofing repair or an assessment by a structural engineer.
Pyrite: fill that swells
Pyrite is a mineral found in some aggregates used as fill under concrete slabs. Exposed to air and moisture, it oxidizes and makes the fill swell, lifting the slab. Typical signs are star-shaped or networked cracks in the basement slab, a bulging floor, partitions rubbing on the ceiling or floor and distorted door frames.
In Québec, a fill's swelling potential is assessed in the lab with a petrographic swelling-potential index, following a standardized method. The inspector spots the visual clues and tells you whether fill sampling by a specialized firm is warranted.
Pyrrhotite: concrete that deteriorates
Pyrrhotite is an iron sulphide found in the aggregate of some concretes. As it oxidizes, it causes reactions that make the concrete itself swell and crack, in walls, footings and slabs. Typical signs are a spider-web network of cracks on foundation walls, often with rust staining.
Affected homes were mostly built from 1996 to 2008, mainly in Trois-Rivières, Shawinigan, Bécancour, Nicolet and the Maskinongé and Des Chenaux RCMs. Only a concrete core analyzed in a lab can confirm its presence and level. The absence of visible cracks is not enough to rule it out.
Buying an at-risk home knowingly
If the home is in an at-risk area and construction period, lab tests should be done before conditions are waived. The results, and any repair reports, are the only reliable references for you, your lender and a future buyer.
If work has already been done, such as fill or foundation replacement, ask for the expert reports, invoices and warranties. A well-repaired, documented home can be a good buy. An undocumented home in an at-risk area calls for much more caution.
