Local know-how
Soft Washing vs Pressure Washing: What Your Roof and Siding Want
Those black streaks on the roof are algae, and the people who make shingles are clear about how to clean them. A pressure washer isn't it.
Every roof in the Lowcountry eventually grows a beard. It starts as a faint shadow on the north side, turns into long black streaks running down from the ridge, and before long makes a perfectly good house look like it’s been through something. University of Georgia Extension once put it at five to seven years in high-humidity areas. (UGA Extension)
Mostly, it’s algae.
The Culprit Has a Name
The Asphalt Roofing Manufacturers Association says an algae called Gloeocapsa magma is probably the most prevalent of the species behind those streaks. It’s most widespread in the Gulf States and along the Eastern and Northwest seaboards, and it travels through the air. (ARMA) In other words, it’s here because we’re here.
What the Shingle Makers Say
On cleaning, the industry speaks with one voice. ARMA: “DO NOT use a power washer or any type of brush or broom to clean algae from the roof surface. High-pressure washing systems are likely to damage asphalt roofing.” (ARMA)
GAF agrees, warning that power washing “may dislodge granules, which can lead to premature shingle failure.” Its recommended mix is four gallons of water, one gallon of bleach and a cup of TSP, left on for up to 20 minutes and rinsed “with low-pressure water,” with the shrubs below protected. (GAF Technical Bulletin TAB-R-102)
Those granules are what protect the shingle from the sun. Blast them off and you’ve traded a cosmetic problem for a real one.
So What Is Soft Washing?
Soft washing is the professional version of what the manufacturers describe: low pressure, a lot of water, and a cleaning solution, typically sodium hypochlorite, which is the active ingredient in bleach, plus a surfactant to help it cling. One equipment supplier recommends a maximum of 300 to 500 psi. (Kleen-Rite) The solution does the cleaning. The water just carries it.
The trade-off is your landscaping. Bleach runoff is hard on plants, so a careful crew wets everything down first, covers what it can, bags the downspouts and keeps rinsing the beds while it works. (powerwash.com) Ask how they protect the azaleas before they start, not after.
Siding Is a Different Story
Vinyl siding is more forgiving. The siding trade group says a power washer “can be used, but with caution,” and to read both the siding and the washer instructions first. For mildew it suggests 30 percent vinegar and 70 percent water, and says to avoid undiluted bleach. (Polymeric Exterior Products Association)
What It Costs
National figures: HomeAdvisor puts a typical roof cleaning at $295 to $597, and soft washing at $0.20 to $0.60 per square foot, and notes that most contractors don’t recommend pressure washing a roof. (HomeAdvisor) Angi puts the average cost to pressure wash a house at $312. (Angi, August 2026)
Before you hire, check whether your shingles carry an algae-resistance warranty. GAF’s StainGuard coverage, for example, applies only to shingles from packages bearing that logo. (GAF)
Gentle Wins
There’s something almost un-American about it: the right way to clean a roof is the gentle way. Low pressure, a little patience, and a crew that cares about your shrubs. The black streaks will come back eventually. That’s the price of living somewhere this green. At least the roof will still be there when they do.