The short answer
Black streaks on a roof are a blue-green algae called Gloeocapsa magma. It feeds on the limestone filler in asphalt shingles, it is cosmetic rather than structural, and it does not by itself mean the roof is failing. It spreads on north-facing and shaded slopes first because those stay damp longest.
What causes it
| Cause | How you can tell | How common |
|---|---|---|
| Algae (Gloeocapsa magma) | Dark streaks running downhill, worst on north slopes | Cosmetic |
| Moss | Raised green clumps, not flat streaks | Needs attention |
| Lichen | Crusty patches that look fused to the shingle | Harder to treat |
| Soot or industrial fallout | Uniform grey rather than streaked | Rare here |
| Shadowing from granule loss | Looks like streaks but does not wash off | Age indicator |
How fast do you need to act
Not urgent. Worth addressing before selling the house, since it reads as neglect to a buyer, and worth distinguishing from moss, which does hold moisture against the roof and does cause damage.
What to do right now
- Confirm it is flat staining rather than raised growth, which would be moss
- Do not pressure wash it, which strips granules and voids most shingle warranties
- Have it soft washed with a manufacturer-approved solution if appearance matters
- Consider zinc or copper strips at the ridge, which inhibit regrowth as rain carries ions down
- If the next roof is going on soon, ask for algae-resistant shingles
What we see in the Hill Country
Humidity after summer storms plus heavy tree cover on a lot of Hill Country properties makes this common, especially on homes tucked under live oaks. It is almost always worse on the north and east slopes where morning shade holds moisture into the day.
When to call us
We inspect for this at no charge across Boerne, San Antonio, New Braunfels, Kerrville, Fair Oaks Ranch, Bulverde, Helotes, Spring Branch, Austin and Stone Oak. You get photographs of what we find and a written explanation, including when the honest answer is that nothing needs doing yet. Call (210) 245-4125 or book an inspection.
