Black Streaks and Moss on Your Roof

What the black streaks are

Brown or black streaks running down a shingle roof, usually starting near the top and darkest on the shadier slopes, are almost always algae. It spreads on the wind and settles where the roof stays damp longest. It is ugly, but on its own it is generally an appearance problem rather than damage: the shingle underneath is usually sound. It shows up most on lighter-coloured roofs simply because it is easier to see.

Moss is the one that does damage

Moss is different. It grows in green, raised clumps, mostly on the north-facing slopes, under trees, and wherever leaves and needles collect. Unlike algae it:

• holds water against the shingles like a sponge, keeping the roof wet long after the rain stops;
• grows into the gaps between shingles and can lift their edges, letting wind-driven rain get underneath;
• traps leaves and debris, which hold more water and grow more moss;
• freezes — and in a Connecticut winter, wet moss freezing and thawing in the joints between shingles works them apart.

Lichen, the flat crusty growth in pale green or grey patches, clings tightly and can be stubborn to remove, and it usually shows a roof that has stayed damp for years.

Why you should never pressure wash a roof

A pressure washer will take the streaks off. It will also take off the granules — the gritty top layer of every asphalt shingle that protects it from the sun — and it can drive water up under the shingles and break their seals. The roofing industry warns against scrubbing shingles or pressure washing them for exactly this reason. A roof that has been pressure washed often looks clean for a season and has lost years of its life. We do not pressure wash roofs.

Scraping moss off with a hard tool does the same kind of damage on a smaller scale, and it means walking on a wet, slippery roof, which NRCA and ARMA both tell homeowners not to do.

What actually keeps a roof clean

• Let in the sun. Trimming back overhanging branches is the single most effective thing you can do. A roof that dries quickly does not grow moss.
• Keep the debris off. Leaves and needles piled in valleys and behind chimneys are where moss starts.
• Keep the gutters clear so the lower edge of the roof drains. See gutters.
• Choose algae-resistant shingles at replacement. The major manufacturers make shingles with algae-resistant granules, which are designed to keep streaks from forming in the first place. On a Connecticut roof under trees, they are worth asking for. See asphalt shingle roofs.
• Fix the attic. A roof that stays damp from underneath because the attic is not ventilated grows more on top as well. See attic ventilation.

Cedar and slate roofs

Moss is especially hard on cedar, which rots when it stays wet. On a cedar roof under trees, keeping the keyways and valleys clear and the branches back matters more than anything else. Slate is stone and shrugs off algae, though moss can still trap water around the nails and flashing. See slate and cedar roofing.

When moss means more than cleaning

A roof with heavy moss that has been there for years often has lifted shingles, wet decking at the edges, or both. That is when it is worth having someone look properly — from a ladder and from the attic — to see whether it is a repair or a roof near the end of its life. See roof inspection and how long a roof lasts.

Common questions about roof streaks, moss and algae