Mould after a leak, without the panic and without the sales pitch

What the guidance actually says

Health Canada, in line with other public-health bodies, does not recommend testing the air for mould, and sets no numerical safe-exposure limit. The reasoning is practical: if you can see or smell mould, you already have the answer, and the response is the same regardless of species: find the moisture, fix the moisture, remove the growth. Anyone selling you an air test before a moisture inspection has the order backwards.

The size thresholds that decide who does the work

Canadian occupational-health guidance breaks visible growth into three tiers:

• Small: one to three patches, each under one square metre. Typically a careful DIY or handyman-scale cleanup with basic precautions.
• Medium: more than three patches, or patches between one and three square metres. Containment and proper protective procedure territory.
• Large: contiguous growth beyond roughly three square metres. Professional remediation with full containment.

The honest corollary: a single palm-sized patch behind a toilet is not a four-figure emergency, and a wall-sized bloom behind old panelling is not a spray-and-paint job.

"Mould is the only thing I demolish that people apologize for. Do not. It means water got somewhere it should not, full stop. My job is to find where, fix why, and rebuild dry. The species of the mould has never once changed what I do next; the source of the water decides everything."

Mitch Fraser, owner, Ontario Contracting Co.

Where insurance fits, and where it does not

Coverage typically follows the sudden-versus-gradual line: mould downstream of a sudden, accidental covered event (the burst supply line) is often addressed, frequently within sub-limits, while mould from slow leaks, chronic seepage and condensation is generally excluded as gradual. The full picture, including the vacancy and notice traps, is in our renovation-insurance guide. Where the answer is "not covered", the fix is a construction scope, and pretending otherwise burns weeks.

How remediation fits into a renovation

1. Source first. Roof, plumbing, grading, sewer backup or condensation: diagnose before demolition, the same discipline as the water-problem triage we run.
2. In pre-1980 homes, test the materials, not the air. Disturbing old drywall compound, plaster or flooring can trigger Ontario's asbestos survey requirement; that legal layer is covered in our older-home hazards material.
3. Remove and dry to the tier-appropriate standard, containment where the size demands it.
4. Rebuild with the lesson: ventilation that reaches outside, insulation sequenced air-seal-first per our retrofit practice, and materials suited to the location.