What is the impact of moss on a roof?
Short answer. Moss holds water against the roof like a sponge, lifts shingle edges as it grows underneath them, and loosens granules, which speeds aging and can let wind-driven rain get under the shingles. A thin patch is a maintenance issue, but thick moss left for years can shorten a roof's life and cause leaks.
How moss damages a roof
- Constant moisture: a moss mat stays damp for days after rain, keeping the shingle surface wet and feeding algae and wood rot on shake roofs.
- Lifted edges: moss grows into the gaps between courses. As it thickens, it pries shingle edges up, breaks the sealant bond, and opens a path for water to wick uphill under the shingle.
- Granule loss: moss anchors itself to the granule surface. When it is scraped off or blows away, it takes granules with it, exposing asphalt to the sun.
- Debris dams: moss traps needles and leaves in valleys and behind chimneys, slowing drainage where the roof is already most leak-prone.
- Freeze damage: in the North Georgia mountains, water held in moss can freeze and expand at the shingle edges.
Where it shows up in Georgia
North-facing slopes, roofs under hardwoods and pines, and lower edges near gutters that stay full of leaves. Shade and humidity are the drivers, so moss is often a symptom of tree cover more than roof age.
What to do about it
Treat it with a moss killer labeled for roofs and let it die and loosen before removing it gently, working downhill. Avoid pressure washers and aggressive scraping. Then fix the conditions: trim back branches, keep gutters clear, and consider zinc or copper strips near the ridge. Insurance generally treats moss damage as maintenance, not a covered loss, so check your policy before counting on it.