What Are the Black Streaks on My Roof? (And How to Remove Them Safely)

It's usually algae

The black streaks on asphalt shingle roofs are most often caused by a type of airborne algae commonly known as Gloeocapsa magma. Its spores travel on the wind, settle on roofs, and feed in part on the limestone filler used in many shingles. As the algae builds up a dark protective coating, it shows up as streaks, usually starting on the shaded, north-facing side and running down with rainwater.

Can it damage my roof?

The streaks themselves are mostly cosmetic at first, but they aren't harmless. Organic growth holds moisture against the shingles, and when moss or lichen join in, they can lift shingle edges and speed up wear. A stained roof also absorbs more heat and drags down curb appeal, which matters if you're selling.

Why you should never pressure wash a roof

High pressure knocks the protective granules off asphalt shingles, can break seals, and can drive water under the shingles. It may also void your shingle warranty. Walking on a wet roof with a pressure wand is also dangerous.

How soft wash roof cleaning works

1. Protect the property. Plants and landscaping are pre-wet, and downspouts and sensitive areas are managed.
2. Apply at low pressure. A roof-safe cleaning solution is applied gently to kill the algae, moss and lichen.
3. Let it work. The solution breaks down the growth at the root. Streaks often fade visibly on the spot.
4. Rinse and clean up. Landscaping and surfaces below are rinsed.

Because the growth is killed rather than just blasted, a soft washed roof stays clean much longer.

Keeping streaks from coming back

Trim branches that shade the roof, keep gutters flowing, and plan on a maintenance roof wash every few years depending on shade and moisture.