One of the most common things we find in Puget Sound attics is dark, blotchy staining on the underside of the roof sheathing. Sometimes it's a few spots. Sometimes a whole side of the roof is black. Homeowners are usually surprised, because the roof doesn't leak.
That's the point. Most attic mold here isn't from roof leaks. It's from moisture coming up from inside the house.
How it happens
In winter, the roof deck is cold. Warm, moist air from the house finds its way into the attic through gaps in the ceiling, bathroom fans that vent into the attic instead of outside, and kitchen fans that do the same. When that air hits the cold plywood, the moisture condenses. Do that night after night from October to April, and mold grows.
Poor ventilation makes it worse. If soffit vents are blocked by insulation or never existed, damp air has nowhere to go.
Is it active?
- Fuzzy or raised growth, rather than flat staining, is usually active.
- Damp wood or frost on nail tips on cold mornings means moisture is still getting in.
- Staining that's concentrated above bathrooms points to fans venting into the attic.
- A musty smell in upstairs rooms is another clue.
Fixing it for good
Cleaning or treating the mold is only half the job. If the moisture source stays, it comes back. The usual fixes are ducting bath and kitchen fans all the way outside, sealing the ceiling, and making sure the attic has clear intake vents at the eaves and exhaust near the ridge.
We handle mold remediation and the attic work that keeps it from returning.