If your roof has dark streaks running down it, especially on the shaded or north-facing side, that is not dirt, and it will not rinse off with a garden hose. It is a living organism, and it is extremely common in humid climates like Florida and much of Texas. Here is what those streaks actually are, whether you need to worry, and how to get rid of them without damaging your roof.
What the streaks actually are
The black streaks are a blue-green algae called Gloeocapsa magma. It lands on your roof as airborne spores and feeds on the limestone filler in asphalt shingles, then grows a dark protective coating that shows up as those familiar streaks. It thrives in humid, shaded areas, which is why it usually appears on the north side first and spreads from there.
Is it actually harmful?
This is where honest sources disagree. The asphalt shingle manufacturers say there is no scientific evidence that algae damages shingles, and they treat it as a cosmetic issue. Roof-cleaning companies argue that the dark colonies trap heat and moisture and can shorten shingle life over years. The fair take is that it is mostly a curb-appeal and home-value problem, but it does not do your roof any favors, and it is worth cleaning off, especially before you sell.
How to remove it safely
The method the manufacturers recommend is a gentle chemical wash, not scrubbing or blasting:
- Mix a 50/50 solution of laundry-strength liquid chlorine bleach and water.
- Wet the plants and landscaping around the house first, so runoff does not harm them.
- Spray the solution on the streaks with a low-pressure sprayer and let it sit for about 15 to 20 minutes.
- Rinse gently with low-pressure water, and rinse the plants again afterward.
Never pressure wash a shingle roof
A pressure washer strips off the protective granules and can drive water under the shingles, which very likely leads to premature failure and can void your warranty. For a steep or large roof, hire a soft-wash professional rather than doing it yourself.
How to keep it from coming back
Once the roof is clean, a few things slow the algae from returning:
- When you next replace the roof, choose algae-resistant shingles, which use copper-infused granules and carry an algae warranty. Brands market these as StainGuard, Scotchgard, or StreakGuard.
- Zinc or copper strips near the ridge release trace metal ions when it rains, which inhibit algae downslope. They work best installed with a new roof.
- Trim back trees to let more sunlight hit the roof, and keep the gutters clean.
A note on adding metal strips to an old roof
Installing zinc or copper strips on an existing roof usually means exposed nails or breaking the shingle seal, which the manufacturers caution against. The cleanest time to add them, or to switch to algae-resistant shingles, is when you replace the roof.
What it costs to have it cleaned
A professional soft wash typically runs about 0.25 to 0.75 dollars per square foot. In practice that is roughly 300 to 500 dollars for a smaller home and 800 to 1,200 dollars for a large or complex roof.
“Black streaks are a living algae, not grime. Soft-wash them off, never pressure wash, and choose algae-resistant shingles next time.”
Algae versus moss
If what you have is green and fuzzy rather than a flat black streak, that is moss, which is a different and more damaging problem. See how to remove moss from a roof. Either way, keeping your roof clean is part of helping it last, which we cover in how to make your roof last longer.
With Hommy you can post a job once and get roof cleaning and inspections from vetted local roofers, each with real ratings from other homeowners, so you can clear the streaks without risking your shingles.
