Overhead
Moss on BC roofs
Moss loves cool, damp, shaded roofs. Coastal BC has all three for months at a time — which is why north slopes and tree-covered runs turn green long before roofs inland do.
Moss is not only cosmetic. Thick growth holds moisture against the surface, lifts edges, and dams water in valleys. On asphalt shingles it shortens life. On cedar it keeps the wood wet.
Where it shows up first
- North-facing slopes
- Under overhanging cedars and maples
- Valleys and behind chimneys where debris collects
- Older roofs with less remaining granule protection
If one plane is green and the opposite plane is clean, shade and drying time are usually the difference — not a different product.
What actually helps
Removal and prevention are different jobs.
- Clear bulk growth carefully. Aggressive scraping that strips granules or gouges wood creates a worse problem than the moss.
- Keep valleys and gutters clear so water can leave.
- Improve drying where you can: trim branches that keep the roof wet after every rain.
- Treatments (zinc or copper strips, professional soft wash) are maintenance tools, not a one-time cure under heavy tree cover.
Pressure washing is a common request and a common way to damage asphalt shingles. If you hire, ask what method they use and what they will not do.
Timing
Dry spells in late summer and early fall are when many Island and coastal crews prefer to work. Slippery surfaces and continuous rain make the job slower and more hazardous.
When moss is a symptom
If moss returns in months, the roof may stay too shaded and too wet for any treatment to hold. Sometimes the honest answer is more trimming, more frequent clearing, or accepting a shorter roof life under heavy canopy.