How does moss harm a roof?
Short answer. Moss acts like a sponge, holding water against the roof for days after rain, and it grows into the gaps along shingle edges, lifting them so wind driven rain can get underneath. Over time that moisture can rot decking, and the debris moss sheds clogs gutters and valleys.
The ways moss does damage
- Holds water. A mat of moss stays wet long after the rest of the roof dries, keeping shingles and fasteners damp.
- Lifts edges. Moss grows along the lower edges of shingles and in the slots between tabs, prying them up and breaking the seal strip bond.
- Lets water in. Lifted edges catch wind driven rain and let water run back under the shingle onto the underlayment and nails.
- Feeds rot. Repeated wetting can soften and rot the decking under heavy moss patches.
- Clogs drainage. Dead moss and spores wash into gutters and valleys, causing overflows that soak fascia and soffits.
Material matters
- Asphalt shingles: damage mainly at lifted edges and from granule loss under the mat.
- Wood shakes: the most vulnerable; moss speeds rot of the wood itself.
- Metal: moss grows in debris at seams and laps, holding moisture at fasteners.
Severity
A few small cushions on a shaded slope are an early warning. A carpet covering large areas for several years usually means some shingles and possibly decking need repair. Moss is more aggressive in Georgia under heavy shade from pines and hardwoods.
See moss and lichen on roofs for treatment and trees and your roof for removing the shade that grew it.