What causes moss to grow on a roof?
Short answer. Moss grows where a roof stays damp long enough for airborne spores to settle and take hold: shaded slopes, roofs under trees, north faces that never dry out, and spots where leaves and pine needles trap moisture. In North Georgia's humid climate, any roof surface that stays wet for hours after rain is a candidate.
The recipe moss needs
- Moisture that lingers: moss has no roots and no way to store much water, so it only survives where the surface stays damp. Dew, humidity and slow drying after rain all count.
- Shade: direct afternoon sun dries shingles fast. Shade from hardwoods, pines or a taller neighboring house keeps them wet.
- Something to grip: the rough granule surface, the gaps between shingle tabs and the edges of each course give spores a foothold.
- Organic debris: leaves and needles piled in valleys or behind chimneys hold water and break down into a thin soil.
Where it shows up first
- North and east facing slopes, which see the least sun.
- Below overhanging branches, where drip keeps the roof wet.
- Valleys, the uphill side of chimneys and skylights, and along the lowest courses near clogged gutters.
- Low slope porch roofs, which drain slowly.
Why Georgia roofs get it
Long humid summers, heavy tree cover on many North Georgia lots, and mild winters give moss a long growing season. Mountain counties with more fog and shade see it most, but shaded lots across metro Atlanta get it too.
Cutting off the cause
Removing moss without changing the conditions means it returns. Trim branches back so sun and air reach the roof, keep valleys and gutters clear, and consider zinc or copper strips near the ridge, which shed small amounts of metal that discourage regrowth below them. More in moss and lichen on roofs and trees and your roof.