Attic Mould in GTA Homes: What Causes It and What Stops It
Attic mould is almost always a moisture and ventilation problem, not a cleaning problem. The three usual sources are bathroom or kitchen fans venting into the attic, air leaking up from the house, and blocked soffit ventilation. Treating the surface without correcting the source means it comes back.
Where the moisture comes from
Warm indoor air holds far more moisture than cold attic air can. When that air leaks upward through top plates, pot lights and the attic hatch, it condenses on the cold underside of the roof sheathing — and that damp surface is where growth starts.
A bathroom fan ducted into the attic instead of outside does the same thing deliberately, several times a day. It is a building code violation and the most common single cause we find.
Why treatment alone fails
Products that clean the staining do nothing about the humidity that produced it. Within a season or two of the same conditions, the growth returns to the same sheathing.
A durable fix has four parts: identify the source, contain and treat the affected area, correct ventilation and air sealing, and replace insulation that has been contaminated or matted by moisture.
Signs, and what they usually mean
| What you notice | Usual cause |
|---|---|
| Musty smell near the attic hatch | Active growth, moisture present |
| Dark staining on sheathing or trusses | Repeated condensation cycles |
| Frost on nails or sheathing in winter | Warm humid air leaking upward |
| Damp or matted insulation | Condensation or a fan venting into the space |
| Recurring window condensation | High indoor humidity, often with poor ventilation |
Follow-up questions
Is attic mould a health risk?
It is a health and building concern, and it also degrades insulation and wood. We contain the work area so spores are not spread through the house.
Do we have to leave the house?
Usually not. The work area is isolated and the extraction vents directly outside.
General guidance for homes in the Greater Toronto Area. Every building is different — treat this as background, not as a specification for your project.
Want this checked on your own house?
The assessment is free and there is no obligation. You get the findings explained on site and the quote in writing afterwards.
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