Answers · Is something wrong?
What are the black streaks on my roof?
The short answer
Almost always a hardy blue-green algae that feeds on the limestone filler in asphalt shingles. It is a cosmetic problem far more than a structural one, it streaks downward from the ridge, and it is worst on north-facing and shaded slopes.
Why it streaks downward
Spores land on the ridge and wash down with rain, which is why the pattern is vertical and starts at the top. It is also why the shaded, damp slopes are worse: the algae needs moisture, and a slope that dries out quickly in the sun does not hold it.
What to do about it, and what not to
- Do not pressure wash. It strips granules and shortens the roof's life measurably. This is the most common and most expensive mistake in this category.
- Low-pressure cleaning with an appropriate solution is the standard method. Ask what is being used and how the landscaping below will be protected.
- Trim back overhanging branches. Shade and retained moisture are the underlying condition.
- Zinc or copper strips at the ridge release trace metal in rain that inhibits growth on the slope below, which is why you sometimes see a clean band under a metal flashing.
- Algae-resistant shingles exist and are worth specifying at reroof time in humid regions.
Also asked
- Do black streaks mean my roof is failing?
- Not in itself. It is a surface organism, and a heavily streaked roof can have years left. What it does indicate is a slope that stays damp, which is worth attending to for other reasons — moss, which does hold water against the shingle, is a genuine problem and often follows.
General information about roofs and about hiring, not advice about your building. Roofing contractor licensing, permits, insurance practice and deductible rules are set state by state and province by province across the United States and Canada, so your own authority is the current answer.