Last reviewed October 31, 2026.
From the street the roof looks like someone dragged a dirty mop down it, dark stripes running from the ridge to the gutters, worst on the side that never gets sun. It is not soot and it is not the shingles wearing out. It is algae, and it washes off, as long as nobody takes a pressure washer to it.
What the black streaks actually are
The Asphalt Roofing Manufacturers Association says Gloeocapsa magma is probably the most prevalent of several algae species behind roof discoloration, and its dark pigment gradually turns light roofs brown or black. Owens Corning calls it a cyanobacteria carried through the air, and a GAF article on blue-green algae says that because it travels on the wind, if your neighbor has it, your roof has probably been exposed as well.
Owens Corning says the streaks are most widespread in the eastern United States and common on roofs facing north. It says it has not seen evidence that algae affects shingle performance and calls it more of an aesthetic problem. Moss is a different organism. It shows up in the same shady, damp spots, and McMorris Power Washing describes it as holding water against the roof like a sponge.
Long Island roof cleaning prices
| Job | Published price | Source |
|---|---|---|
| Most Long Island homes | $400 to $800 | McMorris Power Washing |
| Small roof or ranch | $300 to $500 | Gulf2Bay Softwash |
| Average Long Island home | $400 to $700 | Gulf2Bay Softwash |
| Large, steep or complex roof | $700 to $1,200+ | Gulf2Bay Softwash |
| Most residential roofs, soft wash | $495 to $1,250, or $0.27 to $0.55 per sq ft | Pressure Wash LI |
| 1,800 sq ft ranch roof, moderate condition | about $550 to $700 | Pressure Wash LI |
| Starting price | from $250 | Beyond Maids |
Pressure Wash LI says pitches of 8:12 or steeper cost more because of the extra safety setup. Gulf2Bay says it backs roof cleanings with a 5-year warranty where applicable, and Pressure Wash LI offers a 3-year no-streaks warranty, so ask any company what its warranty covers and what happens if the streaks come back early.
Soft wash, never pressure wash
Both the industry group and the shingle makers say the same thing. The Asphalt Roofing Manufacturers Association says not to use a power washer, brush or broom on roof algae, and recommends a solution of household bleach and water, left on for 15 to 20 minutes and rinsed off with a gentle spray. Owens Corning gives a 1:1 bleach and water mix for 10 to 20 minutes and warns that scrubbing or high-pressure washing loosens granules and shortens the life of the roof.
Both say the result is temporary and the discoloration can return, and the association recommends leaving the job to trained professionals. That is what “soft wash” means in a cleaning company’s ad: low pressure and a cleaning solution doing the work. Gulf2Bay says it uses no more than 90 PSI where soft washing is appropriate. If a company’s plan involves a pressure washer on your shingles, find another company.
Protecting the shrubs and the rules on chemicals
Bleach does not stop at the gutters. Owens Corning says to cover all ground vegetation under the roof before the solution goes on, then thoroughly flush the roof and the greenery afterward. McMorris Power Washing says it covers plants and shrubs before starting. Ask how the crew handles runoff, especially near a pool, a vegetable garden or a pond.
New York also regulates businesses that apply pesticides for hire. The state Department of Environmental Conservation says a business registration is required to offer, advertise or make pesticide applications for hire, with a certified applicator on staff. If a roof cleaner sells a treatment that kills moss or algae, it is fair to ask whether the product is a registered pesticide and whether the company holds DEC registration.
Cedar roofs need a gentler hand
Cedar shakes do not take the same treatment as asphalt. McMorris Power Washing says it uses low pressure on cedar because wood splits under high force. Dirty-roof.com in Amityville says its own cedar process also rehydrates the wood with an oil treatment, that it is very easy to damage the wood with the wrong process, and that a cedar roof can last 40 to 60 years when it is maintained every 5 to 8 years. We found no Long Island company publishing a separate price for cedar cleaning, so get it quoted on its own.
Keeping the streaks from coming back
Long Island companies disagree on how long a clean roof lasts, from Gulf2Bay’s 1 to 3 years to Pressure Wash LI’s 4 to 7. Shade, trees and moisture decide most of it.
If the roof is near the end anyway, the longer fix is in the shingles. GAF says its StainGuard Plus technology uses capsules infused with copper microsites that release algae-fighting copper over time, and backs it with a 25-year algae warranty. Its Timberline UHDZ carries 10 percent more of that time-release technology than GAF’s best-selling shingles and a 30-year StainGuard Plus PRO warranty. Owens Corning offers a 25-year algae resistance warranty on lines including Oakridge and Duration, as long as an approved Owens Corning hip and ridge shingle is installed with them. CertainTeed’s StreakFighter warranty runs 10 or 15 years depending on the product. Owens Corning says the idea came from experts noticing that algae staining was reduced or absent beneath metal vents and flashing, and that its algae-resistant products do not address green algae or moss.
