What Moss Does to a Roof
Moss looks cosmetic. It doesn't stay that way. Moss is a sponge: it absorbs rain and holds it against the shingle surface long after the rest of the roof has dried. That prolonged damp does three kinds of damage. First, it lifts shingle edges — moss under a tab pries it upward, and a lifted tab is a wind target. Second, freeze-thaw turns trapped water into a pry bar, popping protective granules off the mat winter after winter. Third, moss rhizoids work into the shingle surface itself, breaking it down from the outside in.
Moss starts on shaded, north-facing, tree-covered slopes — the places that dry slowest — and spreads from there. By the time it's visible from the street, it's usually been established for seasons. Moss on an otherwise sound roof is treatable; on a roof already past its service life, it just reveals what was already true.
Signs moss is more than cosmetic
- Green patches spreading on shaded slopes — the earlier it's caught, the simpler the treatment.
- Moss thick enough to lift shingle tabs — once edges are pried up, wind damage follows.
- Granule loss visible beneath moss colonies — the protective layer is already compromised there.
- Gutters full of moss debris — colonies shedding means they're mature and well-established.
- Black streaks alongside the green — that's algae, moss's frequent companion, feeding on the same damp conditions.
- Persistent damp smell on the shaded side of the attic — moisture is lingering where it shouldn't.
