Why does moss build up on a roof?
What moss needs
- Shade: less sun means the roof dries slowly after rain and dew.
- Moisture: Georgia's humidity and frequent rain supply plenty.
- Something to root in: loose granules, decayed leaves and the gaps between shingles.
- Time: a few undisturbed seasons.
Where it shows up on Georgia homes
Look first at north facing slopes and roofs under hardwoods and pines. Valleys and the uphill side of chimneys and dormers collect debris that holds water like a mulch bed. The lower edges near overflowing gutters stay wet longest. Lower roofs that catch runoff from an upper roof, such as porch roofs, get extra water and often extra debris.
Why it snowballs
Moss acts like a sponge, so each clump keeps the shingles around it damp, which helps the next clump start. It also catches windblown leaves and grit, building its own soil. As root-like strands creep under the shingle edges, they lift them slightly, creating sheltered pockets that hold even more moisture.
Roof factors that invite it
- Older shingles that have shed granules and have a rougher, more porous surface.
- Low slopes that drain slowly.
- Wood shakes, which hold moisture far more than asphalt.
How to change the conditions
Trim limbs to let in sun and air, keep valleys and gutters clear, and treat early growth before it thickens. Zinc or copper strips near the ridge can slow regrowth below them. See trees and your roof and pine needles and leaves on the roof.