Black Streaks on Your Roof: What Causes Them and How to Remove Them

By the Peak Clean team · October 13, 2026

If your roof has dark streaks running down it, you're seeing a very common Houston problem. It looks like dirt, but it's actually something growing on your shingles, and how you remove it matters.

What the black streaks are

The dark streaks on most Houston-area asphalt roofs are an algae called Gloeocapsa magma. It thrives in humid climates and feeds on the limestone filler in asphalt shingles, which is why it spreads in dark streaks down the slope of the roof, often starting on shaded sides.

Why you should care

Beyond the look, growth can hold moisture against the roof and, left alone, may shorten the life of your shingles. Moss and lichen can lift edges and trap even more moisture.

Why not just pressure wash it

High-pressure water can strip the protective granules off asphalt shingles and force water under them. Most shingle manufacturers advise against it. Soft washing, with low pressure and a cleaning solution, is the safer approach.

What a soft wash roof cleaning involves

• Inspection: checking the roofline for algae, moss, lichen, and drainage trouble spots
• Treatment: applying the solution so it can dwell long enough to kill growth at the source
• Gentle rinse: a low-pressure rinse that clears residue without stripping granules
• Protection: pre-wetting and covering landscaping, with minimal foot traffic on the roof

What to expect afterward

Results can keep improving over days or weeks as treated growth fades, and heavy moss or lichen may need more than one treatment. Growth can return over time depending on shade, moisture, and nearby trees, so a clean roof isn't permanent. Typically results last a few years before you notice streaking again.

Other roof types

Tile and metal roofs also collect growth and staining, and each needs a method matched to the material. Metal roofing, for example, is cleaned to remove organic growth while protecting the finish.

Frequently Asked Questions

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