Why is moss on a roof bad?

Asphalt shingles

Moss grows between courses, lifts the lower edges and breaks the seal strips. Lifted shingles blow off more easily, and water that gets under them soaks the underlayment and decking. Granules loosen under the moss, so the shingles age faster once it is removed. This is the most common case in Georgia. See moss and lichen on roofs.

Wood shakes and shingles

Wood stays damp under moss and rots, splits and cups much sooner. Moss on wood roofs is especially destructive in humid climates.

Clay and concrete tile

Tiles themselves tolerate moisture, but moss clogs the water channels and overlaps, pushing water sideways and under the tiles to the underlayment. Removing it means walking on tiles that can crack.

Metal

Moss is less common on metal, but where debris collects, trapped moisture can attack coatings and fasteners and lead to rust spots.

Everything around the roof

• Gutters clog with moss debris and overflow onto fascia and soffits.
• The roof becomes slippery and dangerous for anyone who needs to work on it.
• Moss holds seeds and debris, so other plants can start growing on the roof.

The bottom line

Moss is a slow problem, but it only goes one direction. Treat it early, keep the roof clear of debris, and let in more sunlight. See trees and your roof.