What causes moss on a roof?
Short answer. Roof moss is caused by airborne spores landing on a roof that stays damp: shade from trees, north facing slopes, trapped leaves and pine needles, low pitch, and humid weather all keep the surface wet long enough for moss to take hold. Rough surfaces like tile, wood shakes and old granular shingles give it extra grip.
The ingredients moss needs
- Spores: always in the air, especially near wooded areas and other mossy surfaces.
- Moisture: moss has no true roots and absorbs water through its surface, so it needs a roof that stays damp.
- Shade: sun dries roofs and limits growth; shade from oaks, maples and pines keeps them wet.
- A foothold: rough or porous surfaces, granule textures and joints between shingles or tiles.
Conditions that make it worse
- North facing slopes: get the least direct sun in Georgia.
- Debris: leaves and pine straw in valleys and behind chimneys hold water and add organic matter.
- Low pitch: slower drainage and drying.
- Clogged gutters: water backs up at the edge, where moss often starts.
- Runoff from upper roofs onto lower sections.
- Humidity and mild winters: moss keeps growing through Georgia's cool, damp months.
On tile roofs
Clay and concrete tiles are porous and textured, and their overlaps hold water, so moss grows readily on shaded tile roofs.
Where it shows up in Georgia
Wooded lots and mountain homes in North Georgia see it most, along with older neighborhoods with mature trees. Sunny new subdivisions see more algae streaks than moss.
Taking away the causes
Trim back branches, clear debris every fall, keep gutters flowing, and add zinc or copper strips. See moss and lichen on roofs, trees and your roof and pine needles and leaves on the roof.