· by Michael Blair, Nashville's Pressure Washing
When we soft wash, we put a cleaning solution on the surface at low pressure, let it kill the growth, and rinse it away instead of blasting it off. On a shingle roof it is the only way we clean, and on most of a house it is how our house washing is done.
The difference in one sentence
A pressure washer cleans by force, stripping off whatever is on the surface with the speed of the water, and some of the surface too if the tip gets close. A soft wash puts the solution on, gives it time to work, and finishes with a low-pressure rinse.
The distinction matters because a lot of what discolors a house is alive. In a 1968 survey of discolored roofs in the United States and Canada, the University of Minnesota phycologist A. J. Brook found that roof shingles are often colonized and stained by blue-green and green algae. Growth like that is something we kill, and scrubbing the top of it off leaves the rest in place.
Where does high pressure do real damage?
Roofs first. We clean shingles with solution only and never put a pressure wand on them, and our roof cleaning is built around that. If your roof is still under the shingle manufacturer’s warranty, read what it says about cleaning before anyone climbs up, and our post on why roof washing is never done with pressure explains how we handle the roof itself.
Off the roof, we keep the pressure down on siding, wood, older mortar, window seals, screens and light fixtures, and let the solution carry the work. Our post on what pressure washing can permanently damage goes through those surfaces one at a time. This is how we approach each one:
- Roof shingles: soft wash only
- Vinyl and fiber cement siding: low pressure with solution
- Wood siding, decks and fences: low pressure, careful tips
- Stucco, dryvit and older mortar: soft wash
- Concrete, brick paving and other hard surfaces: where we use pressure
What are the black streaks on a Nashville roof?
The dark streaks running down a roof are usually living growth. Gloeocapsa, an airborne cyanobacterium, is known for causing black streaks on asphalt roofs, according to a 2026 University of Waterloo study in the journal Biofouling, and Brook’s survey found green algae on shingles as well as blue-green.
A soft wash kills what is living on the shingle, and we rinse it off at low pressure. Because the cells travel through the air, a clean roof can be colonized again. That is the problem copper-containing roof granules were developed to solve, the Waterloo researchers write, and in their lab tests the granules made with copper prevented growth while the ones without copper did not. The dark stripes on the gutter face below the roof are a different problem, and the soft wash that clears the shingles and siding usually leaves them behind; our post on tiger stripes on gutters and why washing fails explains what we use on them instead.
What should you ask a soft washing company?
Ask what solution they use and at what strength, how they protect the landscaping, and whether they wet the plants down before and rinse them after. We soak the beds before we start and rinse them when we finish. The solution deserves respect on the crew’s side too: a 2020 case report in JAAD Case Reports describes a skin disease triggered by a pressure washing bleach solution.
Also ask whether they are insured for roof work specifically. OSHA’s fall protection page lists falls among the most common causes of serious work-related injuries and deaths, and standing on a pitched roof is a different job from standing on a driveway. We would not hire a company that proposes to pressure wash a shingle roof, and our questions to ask a pressure washing company cover the rest. If you want us to look at yours, ask us for a free estimate.
Sources
- A. J. Brook, University of Minnesota, The discoloration of roofs in the United States and Canada by algae, Journal of Phycology (1968) — Roof shingles are often colonized and discolored by blue-green and green algae
- Yasoubi and Ward, University of Waterloo, In vitro anti-cyanobacterial activity of copper roof granules towards Gloeocapsa alpicola, Biofouling 42(9) (2026) — Gloeocapsa is an airborne cyanobacterium known for black streaks on asphalt roofs; copper granules developed to prevent the growth and shown to prevent it in lab tests
- Madray, Liles and Davis, Chemical-induced sclerodermoid disease triggered by pressure washing bleach solution, JAAD Case Reports (2020) — A case of skin disease triggered by a pressure washing bleach solution
- OSHA, Fall Protection — Falls are among the most common causes of serious work-related injuries and deaths
FAQ
Soft Washing and When It Is the Right Call — questions
Does soft washing damage plants?
It can if the crew is careless, because the solution is made to kill living growth. We wet the surrounding plants before we start, keep them wet while we work, and rinse them afterward. Ask any company how they handle the plants before they set up.
How long does a soft wash last?
There is no fixed number, so we do not promise one. The growth that streaks a roof comes from cells carried in the air, which means a cleaned surface can be colonized again, and how quickly depends on the roof and the house around it. Shingles made with copper-containing granules were developed to hold that regrowth back.
Can I soft wash my own roof?
The chemistry is the easy part and the roof is not. OSHA counts falls among the most common causes of serious injuries and deaths at work, and the other risks are mixing the solution at the wrong strength and leaving the plants unprotected. In our judgment a low, walkable single-story roof is a reasonable do-it-yourself job if you are careful; a steep or two-story roof is not.
Is soft wash better than pressure washing?
It depends on the surface. Shingle roofs, stucco and older mortar get a soft wash from us, and vinyl or fiber cement siding gets low pressure with solution, since force marks those surfaces and the solution kills the growth anyway. Concrete, brick paving and other hard surfaces are the places where we use pressure.