What Causes Black Streaks on Houston-Area Roofs (and How to Remove Them)
Key takeaways
- The black streaks on most Houston-area roofs are Gloeocapsa magma, a type of algae, not dirt or mold from the shingles themselves.
- Humidity and shade make Spring, The Woodlands, Magnolia and the rest of north Houston ideal for it.
- The safe way to remove it is a low-pressure soft wash that kills the algae. Pressure washing damages shingles.
- A cleaned roof usually stays clean for two to three years in this climate.
What are the black streaks?
The dark streaks running down so many roofs are a blue-green algae called Gloeocapsa magma. Its spores travel on the wind and land on roofs, where the algae feeds on the limestone filler used in many asphalt shingles. As it grows, it forms a dark protective coating, which is why the streaks look black rather than green.
The streaks usually start on the north- and west-facing slopes and on any part of the roof that stays shaded and damp. They often run downward from the ridge because rain carries spores down the slope.
Why are they so common in north Houston?
Algae needs moisture and shade, and the Houston area provides plenty of both. Humid air, frequent rain, warm temperatures and tall pines and oaks keep roofs damp for long stretches. Neighborhoods built among the trees, like those in The Woodlands, Spring and Magnolia, see it especially often. Once one roof in a neighborhood has it, spores spread easily to the roofs around it.
Do black streaks damage your roof?
The streaks are more than cosmetic over time. The algae holds moisture against the shingles, and dark roofs absorb more heat. Moss and lichen often follow on shaded slopes, and lichen in particular attaches firmly and can damage the shingle surface. Cleaning the roof early keeps the growth from getting established.
How to remove black streaks safely
The safe, effective method is a soft wash: a cleaning solution applied at low pressure that kills the algae, moss and lichen. The streaks fade as the growth dies and rinses away, and heavier moss and lichen finish releasing over the next few weeks of rain.
Pressure washing is the wrong tool for a shingle roof. It strips the protective granules, can lift shingle edges and shortens the roof's life. Read more in soft wash vs. pressure wash.
How long does a roof cleaning last?
In the Houston climate, a soft-washed roof usually stays clean for two to three years, sometimes longer. Roofs in heavy shade or under overhanging branches tend to need it sooner. Trimming back branches that hang over the roof helps it dry faster and stay clean longer.
When to schedule a roof cleaning
Good times to clean a roof are when streaks first appear, before listing a home for sale, and after receiving an HOA or deed-restriction notice about roof staining. Many homeowners pair roof washing with a house wash and gutter cleaning so the whole exterior is done in one visit.
Common questions
Is the black stuff on my roof mold?
It's usually algae (Gloeocapsa magma), not mold. It's common on asphalt shingle roofs in humid climates like Houston's.
Will the streaks come back after cleaning?
Eventually, because the spores are in the air. A soft wash kills the existing growth at the root, so a cleaned roof usually stays clean for two to three years here.
Can I just pressure wash the streaks off?
No. Pressure washing strips the protective granules from shingles and can damage the roof. A low-pressure soft wash is the safe way to remove roof algae.
Get a free custom quote
Every job is quoted for your property. Call or text, or send us the details online.