Soffit To Ridge
An attic in Toronto has two jobs in opposite seasons: stay cold in winter so the roof does not melt its snow, and flush out heat in summer.
Short answer
Ventilation needs intake low at the soffits and exhaust high at the ridge, with a clear path between them and roughly balanced areas. In practice the intake is blocked by insulation, the soffit panel was never cut through, bathroom fans discharge into the attic, and the result is ice dams in winter and a baked attic in summer.
What goes wrong
Insulation stuffed into the eaves, blocking the soffit intake. The most common defect and the cheapest to correct.
Perforated soffit panel with no opening cut through the sheathing behind it, so it looks like a vent and admits nothing.
No exhaust at the top, or exhaust that was covered over by a later addition or reroof.
Bathroom and kitchen fans terminating in the attic instead of through the roof or wall.
Pot lights and the attic hatch leaking warm, moist air into the attic continuously.
And mixing vent types — ridge vent plus a powered fan, or ridge plus gable — which short-circuits the airflow and leaves part of the attic unventilated.
What it looks like
Frost on the underside of the sheathing in January, then staining on the same area in spring. Mould heaviest on the north slope and in the lowest few feet above the eave. Rusted nail tips. That pattern is interior air plus poor venting, not a roof leak — a leak makes a local stain, this makes an even film over a whole plane.
The order to fix it
Air seal the ceiling plane first. This is the step that stops the moisture arriving and it matters more than the vent area.
Duct every bathroom and kitchen fan outside, in insulated duct with a short run and a working damper.
Clear the soffit intake and fit baffles.
Confirm the soffit panel actually has an opening behind it.
Add exhaust high and balance it against the intake rather than guessing.
Then insulate to current depth, and only then clean and treat any mould on the sheathing.
Common misfires
Adding exhaust with no intake, which makes the attic pull its replacement air out from inside the house, which raises the moisture it has to deal with.
A powered attic fan on a leaky ceiling, which depressurizes the house and in the worst case backdrafts a gas appliance.
Mixing ridge and gable or ridge and powered venting.
Cleaning the sheathing and leaving the bathroom fan discharging into the attic, which guarantees a return visit.
Replacing sheathing that only needed cleaning, which is where attic quotes inflate most.
And adding insulation over blocked soffits, which traps the problem under a thicker blanket.
Common questions
Is attic mould usually a roof leak?
No. Interior air plus blocked venting accounts for most of it in Toronto.
Will an attic fan help?
Frequently it makes things worse by pulling indoor air up into the attic. Seal the ceiling and open the intake first.
Can I just add more insulation?
Not over blocked soffits. Clear the intake and fit baffles first.