What Causes Black Streaks on Roofs? (And How to Remove Them Safely)
If you drive through any neighborhood in Brandon, Riverview or Valrico, you’ll see it: dark black streaks running down roofs, usually starting on the north side or under trees. Homeowners often think it’s dirt, roof damage or mold from old shingles.
It’s none of those. It’s alive, and it’s very common in Florida.
What causes black streaks on roofs?
The black streaks on most Tampa Bay roofs are caused by Gloeocapsa magma, a type of algae (technically a cyanobacteria). It spreads by airborne spores and settles on roofs, where it grows in long dark lines as rain and dew run down the slope.
The dark color comes from a protective coating the algae builds up to shield itself from the sun. That’s why the streaks get darker over time instead of fading.
Why it loves Florida roofs
Gloeocapsa magma thrives in exactly the conditions we have most of the year:
- Heat and humidity almost year-round.
- Morning dew that keeps roof surfaces wet for hours.
- Shade from oaks and other trees, which slows drying.
- Limestone filler in many asphalt shingles, and minerals in concrete tile, that the algae feeds on.
North-facing slopes and shaded areas almost always show it first.
Is it just algae, or something else?
Black streaks are usually algae, but roofs in our area often have more than one problem at once:
- Moss: green, fuzzy growth, usually in shaded spots.
- Lichen: crusty, flat spots that stick tightly to the surface.
- Mildew and dirt film: a dull gray haze across the roof.
- Tannin stains: brown stains from leaves and debris sitting in valleys.
All of these respond to the same safe cleaning method.
Why you shouldn’t pressure wash a roof
It’s tempting to grab a pressure washer and blast the streaks away. Please don’t. High pressure on a roof can:
- Strip protective granules off asphalt shingles.
- Crack or chip roof tile.
- Force water under flashing and into the underlayment.
- Leave the algae alive below the surface, so it grows right back.
Roofing manufacturers and the Asphalt Roofing Manufacturers Association (ARMA) recommend against high-pressure washing on shingle roofs. A low-pressure method is the safer way to go.
It’s also risky to get on a roof yourself. Wet, algae-covered roofs are slippery, and falls are a real danger.
How soft wash roof cleaning works
Soft washing is the low-pressure method we use for roof cleaning. Instead of blasting the surface, we apply a cleaning solution that kills the algae, mildew, moss and lichen, then rinse gently.
Here’s what the process looks like:
- Protect the property. We pre-wet plants and landscaping and plan around them.
- Apply the solution at low pressure. The solution is applied to the roof, not blasted into it.
- Let it dwell. The solution needs time to kill the growth at the root.
- Rinse and check. We rinse and look over the roof, gutters and walls below.
Because the algae is killed rather than just knocked loose, the streaks fade and the roof looks clean again without the wear of high pressure.
Shingle, tile and metal roofs
Soft washing works on the roof types common around Tampa: asphalt shingle, concrete and clay tile, and metal. Each one gets the right mix and approach for the material.
Learn more about our roof cleaning service, including what we look for before we start.
What about the rest of the house?
If your roof has black streaks, your walls, soffits and gutters often have algae and mildew too. Many homeowners pair roof cleaning with a house soft wash and gutter whitening so the whole exterior matches.
Not sure which method your home needs? Read our guide to pressure washing vs. soft washing.
How to keep streaks from coming back fast
You can’t stop algae spores from landing on your roof, but you can slow regrowth:
- Trim back overhanging branches so the roof dries faster.
- Keep gutters clear so water and debris don’t sit on the roof edge.
- Clean the roof before growth gets heavy. Lighter growth is easier to remove.
Get a free roof cleaning estimate
Peter’s Pressure Washing has been cleaning roofs around Tampa Bay since 2012. We’re family-owned, licensed and insured, and rated 4.9 stars from 782 Google reviews. We serve Valrico, Brandon, Riverview, Tampa and nearby areas within about 25 miles.
Free same-day estimates are available by phone, and we can quote from photos if you’re not home.
Call 813-455-3670 for a free same-day estimate, Monday to Friday, 9 AM to 5 PM.
Frequently asked questions
What are the black streaks on my roof?
In most cases they are Gloeocapsa magma, a type of algae (cyanobacteria) that grows on shingle and tile roofs in warm, humid climates like Tampa Bay. It is not dirt or mold from the shingles themselves.
Can I pressure wash my roof to remove black streaks?
High-pressure washing is not recommended for roofs. It can strip shingle granules, crack tile and force water under flashing. Soft washing uses low pressure and a cleaning solution to kill the algae instead.
Is roof algae harmful?
The streaks are mainly a cosmetic problem at first, but algae, moss and lichen hold moisture against the roof and can contribute to wear over time. Cleaning also helps your home’s curb appeal.
How long does a soft wash roof cleaning last?
It depends on shade, trees and moisture around your home. Shaded roofs under oak trees tend to see growth return sooner than roofs in full sun.