What it usually means, the likely causes in order, what to safely try first, what a pro charges in Canada, and when to stop and call one.
This guide covers the common cases. Describe your exact problem and get a personalized diagnosis — free to start, with a DIY vs. call-a-pro verdict and a real cost estimate.
Visible mould spots on a wall, ceiling, or corner mean one thing with certainty: that surface has been staying damp enough, for long enough, for mould to grow on it. The growth is the symptom, and the localised moisture or humidity feeding it is the disease, whether that is a slow plumbing leak, condensation on a cold surface, a bathroom fan that is not doing its job, or water coming through from outside. Mould does not grow on dry building materials, so every patch has a water story behind it. Finding and ending that story matters more than anything that is done to the spots themselves, because growth on a still-damp wall simply returns.
This worsens if delayed, since the moisture feeding the growth is usually also damaging the material underneath and the patch will keep expanding while conditions suit it. Small, recent spots over a hard, non-porous surface with an obvious, fixable cause, such as bathroom condensation, can be addressed promptly at home. Larger areas, growth on drywall, flooring, or other porous materials, growth that keeps returning, or growth in a bedroom or in the room of anyone with asthma or a weakened immune system should be assessed professionally without a long delay.
A plumbing joint weeping inside a wall, a failed seal around a tub or shower, or a roof or flashing leak can keep drywall and framing damp for months while the only surface evidence is a growing patch in a corner. Mould concentrated in one defined area, especially low on a wall or under a bathroom or roof line, points to a leak pattern rather than room humidity.
Warm, moist indoor air condensing on cold exterior walls, corners, window frames, and uninsulated closets produces the classic winter pattern: mould along ceiling corners, behind furniture against outside walls, and around windows. Closets and corners grow it first because air barely circulates there, so surfaces stay coldest and dampest longest.
Showers and cooking load a home air with moisture daily, and a fan that is missing, weak, ducted nowhere useful, or simply never run long enough leaves that moisture to settle on ceilings and grout. Mould on a bathroom ceiling and in grout lines, worst after the heating season begins and windows stay shut, is usually this cause introducing itself.
A failed exterior caulk line, cracked masonry, poor grading, or an eavestrough fault can wet one wall from outside, growing mould on its interior face. If the patch lines up with an exterior feature, gets worse after driven rain, or sits below a known exterior problem, the moisture source is weather, not plumbing or condensation.
| Job size | Typical estimate (CAD) |
|---|---|
| Simple fix — the straightforward version of this repair | $500–$1,500 |
| Major job — complex, hidden, or replacement-level work | $1,500–$6,000 |
Estimates only, calibrated to Greater Toronto Area trade rates and adjusted by region on this site — not a quote. Your exact price depends on the cause, parts, and access. A personalized diagnosis narrows this range using your specific symptom.
A small patch of surface growth on a hard, non-porous surface, such as tile or a sealed surface, with a moisture cause you have found and fixed, is reasonable homeowner territory, cleaned carefully while ventilated and then kept dry. Call a professional for growth covering a large area, growth on or in drywall, flooring, or insulation, growth that returns after cleaning and drying, or any case where the moisture source is hidden. Be honest with yourself about the porous-material rule: bleaching-and-forgetting porous material does not work, because the surface colour can fade while growth remains inside the damp material, ready to reappear.
Only if the moisture is solved too. Mould is a growth on damp material, so a cleaned wall that stays damp, from a leak, condensation, or humidity, will regrow, often within weeks. The durable sequence is source first, drying second, cleaning or removal third. If the same patch returns in the same place after a careful clean and the surface has genuinely been kept dry, that tells you the moisture is arriving from inside the wall, and it is time for a professional look.
Not reliably. Moulds come in many colours, common household growth can be dark without being the specific type people fear, and lighter growth is not automatically harmless. Colour is not a home test for risk. What matters practically is the extent of growth, whether the material is saturated, whether it keeps returning, and who in the home is sensitive to it. Large or persistent growth deserves professional assessment on those facts alone, not on its colour.
Because those are the coldest, stillest pockets in the room. Furniture tight against an exterior wall traps a layer of air that barely circulates, so the wall behind it stays colder and collects condensation, and a closet on an outside wall combines the same cold surface with almost no airflow. Pulling furniture slightly away from exterior walls, keeping closet doors ajar in winter, and lowering indoor humidity address the actual mechanism, which cleaning alone cannot.
For most household situations, testing changes little: the response to visible growth is the same regardless of the exact species, which is to fix the moisture, remove or clean the affected material appropriately to its size and type, and keep it dry. Testing earns its place in disputes, medical investigations, or verifying a remediation. Money is usually better spent on the moisture diagnosis first, since no test result makes a damp wall stop growing mould.
Describe what's actually happening and get a personalized diagnosis — DIY vs. call-a-pro verdict, real cost estimate, and top-rated local pros.