What is moss growth on a roof?
Moss, algae or lichen?
- Moss: green, raised, soft clumps you could pinch, often along shingle edges and in the gaps between tabs.
- Algae: flat dark streaks running down the slope with no texture. See black streaks on roofs.
- Lichen: crusty gray green or pale circles glued tight to the granules.
Why it grows where it does
Moss needs moisture and shade more than anything. In Georgia that means north facing slopes, roofs under hardwoods and pines, and spots where leaves and needles pile up in valleys or behind chimneys. Debris holds rain and gives spores something to root into. Moss is most active in the cool, wet months from late fall through spring, and often looks brown and dormant in the summer heat.
Why it matters
Moss acts like a sponge. Water held against the shingle for days speeds up granule loss, and the root-like strands work under the leading edges of shingles, lifting them so wind-driven rain can get underneath. Thick mats in valleys can dam water and push it sideways under the shingles. On an older roof, heavy moss is often what turns slow aging into an actual leak.
What to do about it
Kill it first with a product labeled for roofs, let it dry and loosen, then remove what is left gently from the top down. Never pressure wash shingles. Trimming branches to let in sun and air prevents most regrowth. Our moss and lichen guide goes deeper, and trees and your roof covers the pruning side.