Dark streaks running down a roof are common across the Houston area. Many homeowners assume it is dirt, mildew, or a sign the roof is failing. In most cases, it is algae, and it thrives in warm, humid climates like ours.
What Causes Black Streaks
The streaks are usually caused by a type of algae often identified as Gloeocapsa magma. It spreads by airborne spores and feeds on moisture and, on older shingles, the limestone filler in some asphalt shingles. It shows up first on the shady, north-facing slopes, which stay damp longer. As it grows, it develops a dark protective coating that creates the black streaks you see.
Is Algae Harmful to Your Roof?
Algae is mostly a cosmetic problem, but it is not completely harmless:
- Dark streaks absorb more heat, which can slightly increase attic temperatures.
- It hurts curb appeal, which matters if you are selling.
- Heavy growth can hold moisture against the shingles.
- Moss and lichen, which are different from algae, can lift shingles and cause real damage.
How to Clean Algae Safely
Never pressure wash shingles. High pressure strips off granules and can void your warranty. It can also force water under the shingles.
- Use a cleaning solution made for roofs, or a diluted mix recommended by the shingle manufacturer. Many manufacturers publish cleaning guidance.
- Apply with a low-pressure sprayer and let it work, then rinse gently.
- Wet the plants and landscaping around the house first, and rinse them after, to protect them from runoff.
- Work from a ladder or hire a professional. A wet, slick roof is dangerous.
Preventing Algae From Coming Back
- Algae-resistant shingles. Many shingle lines include copper-coated granules that slow algae growth. If you are replacing your roof in Houston, this is worth asking about.
- Zinc or copper strips. Metal strips installed near the ridge release small amounts of metal when it rains, which helps keep algae from growing below them.
- Trim trees. More sun and airflow means the roof dries faster.
- Keep gutters clean so water does not sit along the roof edge.
When Streaks Mean Something More
If streaks come with curling shingles, heavy granule loss, or leaks, the roof may be near the end of its life. See 10 signs you need a new roof.
Want a professional opinion? JD Roofing Solutions offers free roof inspections across Houston. Call 832-295-3277.
