Those dark streaks running down your shingles aren't dirt or soot. They're alive — and pressure washing them the wrong way can cost you the whole roof.
If you've looked up at a roof anywhere between Mexico Beach and Gulfport and noticed dark, streaky stains running down from the ridge, you've seen Gloeocapsa magma — a blue-green algae that feeds on the limestone filler in asphalt shingles and produces a dark, UV-resistant coating as a byproduct. It's not mold, it's not soot, and it's not just a cosmetic problem. It's a living organism, and on the Gulf Coast's combination of humidity, shade and salt-laden moisture, it spreads faster than almost anywhere else in the country.
Algae needs three things to take hold on a roof: moisture, warmth and something to eat. The Gulf Coast delivers all three on a near-permanent basis. Humidity rarely drops low enough to fully dry out a roof overnight, afternoon storms keep shingles damp through most of the summer, and salt air adds a layer of organic residue that gives algae spores something extra to grip onto. North-facing roof planes and anything shaded by oak or pine canopy are almost always the first to show streaking, because they hold moisture the longest after each rain.
Black streaks, algae and moss killed at the root — no shingle damage.
This is the part that causes real damage. Asphalt shingles are coated in a layer of protective granules, and those granules are what's actually holding the algae stain. A pressure washer wand aimed at a roof doesn't clean the algae — it strips the granules, taking the stain with them and leaving behind a roof that looks clean for a few months and then fails years ahead of schedule. Most shingle manufacturers explicitly void their warranty if high-pressure equipment is used on the roof, and insurance adjusters know the difference between a soft-washed roof and a pressure-blasted one on sight.
Soft washing uses a biodegradable, algaecidal solution applied at garden-hose pressure. It doesn't blast the algae off — it kills it at the root, including the spores sitting below the surface that would otherwise regrow within a season. The solution dwells for a period, breaking down the organism, then gets rinsed at low pressure. Done correctly, a soft-washed roof stays clean for two to three years rather than two to three months, and the shingles underneath are untouched.
Algae streaking is mostly cosmetic in its early stages, but left alone for years it does start to trap additional moisture against the shingle surface, which shortens roof life. A good rule of thumb for this climate: have a roof soft washed every two to three years, or sooner if you're selling the house and the streaking is visible from the street. If a company offers to pressure wash your roof instead of soft wash it, that's the one piece of advice from this whole article worth remembering — say no.
Free assessment, honest answer on whether it needs cleaning or just needs watching.
