Black streaks, moss and pine needles — what they mean and what to do

Black streaks: algae

The dark streaks running down north- and east-facing slopes are Gloeocapsa magma, a cyanobacteria that feeds on the limestone filler in asphalt shingles. It's everywhere in the humid Southeast and nearly universal on shaded Upstate roofs over about eight years old. It's cosmetic — it doesn't shorten the roof's life meaningfully — but it looks like the roof is failing and it hurts resale. Modern algae-resistant shingles (copper-coated granules) prevent it for 10 to 20 years, which is why we install them as standard.

Moss and lichen: not cosmetic

Moss grows on the shaded, damp slopes under trees and holds water against the shingles, lifts the edges, and freezes in winter. Lichen bonds to the granules and pulls them off when removed. Both shorten the roof and both should be treated — gently.

Pine needles: the real damage

Needles collect in valleys, behind chimneys and skylights, in the gutters and at the eaves. They hold water, rot the metal and decking beneath, dam the valleys so water runs sideways under the shingles, and overflow the gutters onto the fascia. Most of the valley and chimney leaks we repair in Greer started as a pile of needles. Clean them off twice a year, or put a cricket behind the chimney and micro-mesh guards on the gutters and cut the work in half.

How to clean a roof (and how not to)

Never pressure-wash asphalt shingles. It strips the granules and takes years off the roof; the 'before and after' looks great for a season. The right method is a soft wash: a 50/50 solution of household bleach (sodium hypochlorite) and water with a little surfactant, applied at low pressure from a sprayer, allowed to dwell 15 to 20 minutes, and rinsed with a garden hose. Wet the plants below first and rinse them after. Moss dies and washes off over a few weeks; don't scrape it. For a steep or two-story roof, hire a soft-wash company — falling off is the most common roof injury in the Upstate.

Prevention

• Algae-resistant shingles at replacement
• Zinc or copper strips along the ridge — rain carries the ions down the slope and inhibits growth
• Trim overhanging limbs back 8 to 10 feet for sun and airflow
• Valleys, chimney crickets and gutters cleaned twice a year under pines
• Micro-mesh gutter guards

When it's more than cleaning

If the granules are gone under the moss, the shingles are brittle, the valley metal is rusted through, or the decking under a needle dam is soft — that's a repair or replacement, not a cleaning. An inspection tells you which.

Common questions

Related: Roof inspection · Gutters · Chimney & flashing · Roof lifespan