Those dark streaks running down your roof aren't dirt, soot, or a sign the shingles are worn out. They're a living blue-green algae called Gloeocapsa magma, and in Florida's heat and humidity it spreads fast. The good news: it comes off completely with the right method — and the right method is not a pressure washer.
What causes black streaks on a roof?
The algae lands on your roof as airborne spores, then feeds on the limestone filler in asphalt shingles. It holds moisture against the roof and slowly breaks down the granules that protect the shingle, which is why a streaked roof isn't just ugly — it ages faster. Treasure Coast roofs are hit especially hard because we have three things algae loves: constant humidity, warm nights, and salt air that keeps surfaces damp.
Why you should never pressure wash a roof
High pressure blasts the protective granules right off your shingles and can force water under them, voiding many manufacturer warranties. The correct approach is soft washing: low pressure (about the force of a garden hose) combined with a biodegradable cleaning solution that kills the algae at the root. The streaks lift within minutes and rinse away, and because the treatment kills the organism rather than just rinsing the surface, the results last far longer.
Can you remove roof algae yourself?
You can, but think hard before climbing up. Roof work is the leading cause of DIY exterior-cleaning injuries, wet shingles are slippery, and the cleaning chemicals will kill your landscaping if they aren't pre-wet and rinsed properly. A professional soft wash is done almost entirely from a ladder or the ground, with plants protected and the mix dialed in for your specific roof. If you do tackle it yourself, never use a pressure washer, work on a cool overcast day, and wet down every plant below first.
How to keep the streaks from coming back
A quality soft wash typically keeps a Florida roof clean for 18–24 months. You can stretch that by keeping gutters clear so water drains properly, trimming branches that shade and drip on the roof, and scheduling a maintenance wash before the streaks become obvious again. Homes near the water usually need service a little more often than inland homes.
Seeing streaks on your roof? We soft wash roofs across the Treasure Coast and can tell you exactly what your roof needs. Get a free estimate or call (772) 559-2666.
