Three different things grow on a Quebec roof, and they get confused constantly. They are not remotely equal in severity.
Black streaks: algae, mostly cosmetic
Those long dark runs, usually oriented down-slope, are caused by a cyanobacteria that feeds on the calcium carbonate in shingle granules. It is unsightly, it makes a roof look far older than it is, and it hurts perceived value at resale.
The immediate structural risk is low. But a darker surface absorbs more heat, which accelerates thermal ageing of the shingle. It is not an emergency, it is a clock running slightly faster.
Moss: the real problem
Moss is an organism that holds water. It settles in shaded, damp zones, north face, under trees, around chimneys.
Two damage mechanisms:
- It keeps the shingle permanently wet. In Saguenay, with over 100 freeze-thaw cycles a year, that trapped water freezes, expands and works the material from within.
- It physically lifts shingles. As it thickens under the edge it creates a gap where water can travel backwards, exactly what a roof is built to prevent.
Moss is the priority.
Lichen: the most stubborn
Lichen is a symbiosis between an alga and a fungus. It anchors into the surface with structures that penetrate the granule layer. Pull it off mechanically and it takes granules with it, and granules are precisely the layer protecting the asphalt from UV.
For that reason, established lichen must never be scraped or blasted off.
Why pressure washing must be avoided
This is probably the most important advice in this article. On asphalt shingle, a pressure washer must never be used, under any circumstances. Industry guidance is unambiguous on this point.
The reason is simple: the mineral granule layer is what shields the asphalt from UV. A high-pressure jet strips it. You get a roof that looks clean for one season and whose life expectancy has just been cut by years. It is also a classic cause of manufacturer warranty voiding.
The correct method is low-pressure cleaning with a solution that acts chemically on the organisms, letting rain finish the work over several weeks.
Prevention, in order
- Trim branches overhanging the roof. Less shade, less standing moisture, less organic debris.
- Keep gutters clear. A blocked gutter keeps the eaves permanently damp.
- Check attic ventilation. A poorly ventilated attic heats the roof from below, creates condensation and encourages ice dams.
- Have it cleaned the right way. Low pressure, never high pressure.
- Consider a hydrophobic treatment if your roof regrows moss every year: a surface that dries faster is markedly less hospitable.
When to actually worry
Call someone if you see: thick moss visibly lifting shingles, significant granule accumulation in the gutters, shingles curling at the corners, or a damp stain in the attic. These four signals often appear together, and they change the conversation.