Why does algae growth show up on a roof?
Short answer. Algae grows on roofs where moisture lingers, shade blocks the sun, and the shingle surface gives it something to cling to, which describes a lot of Georgia homes. It usually starts on north facing and tree shaded slopes and spreads from roof to roof on wind and wildlife.
The conditions it needs
- Moisture. Dew, humidity and rain that do not dry quickly. Georgia's long humid summers are ideal.
- Shade. North facing slopes and roofs under trees stay damp longer each morning.
- A surface to colonize. Asphalt shingles contain mineral filler, and the common roof algae is often said to feed on limestone in that filler. Debris like pollen and leaf litter adds more food.
- Time. A few seasons after installation is when first streaks typically appear, especially on shingles without algae resistant granules.
Where it starts
Look at the north slope first, then any section under overhanging branches, then the area below a chimney or dormer where runoff concentrates. Growth then follows the water down the slope, which is why it forms streaks.
How it spreads
Algae cells travel on wind, birds and squirrels. Once one roof on a street is heavily streaked, neighbors often follow.
Moss is a later stage
Where shade and debris are heavier, especially under hardwoods and pines, algae can be joined by lichen and moss, which hold far more water. Read moss and lichen on roofs for that side of the problem.
Slowing it down
- Trim branches to let sun and air reach the roof. See trees and your roof.
- Keep valleys and gutters clear of needles and leaves.
- Consider zinc or copper strips near the ridge.
- Choose algae resistant shingles at the next replacement.
For the streaks themselves and how to clean them safely, see black streaks and algae on roofs.