Soft washing: the right call for most roofs
Soft washing uses water pressure no stronger than a garden hose, paired with a diluted cleaning solution that kills moss, algae, and lichen at the root. The chemistry does the work instead of water force, which is exactly why it's the method the shingle manufacturers recommend. Your granules stay put, your shingles don't get blasted, and your warranty stays intact.
The process is straightforward: apply the solution, let it sit long enough to kill the growth, then rinse gently. The catch is that the mix and the dwell time have to be right. Too weak and the moss is back by fall. Too strong and the runoff takes out the hydrangeas below. That balance is most of what you're paying a professional for, along with the safety gear to be up there in the first place.
Before you hire anyone, ask three questions: What pressure do you use? What cleaning agent and concentration? How long does it dwell before rinsing? If a contractor can't answer all three without hesitating, keep looking. We cover this in more detail on our roof cleaning page.
Pressure washing: great for driveways, rough on shingles
A pressure washer delivers water with real force, and on concrete or a deck, that's useful. On asphalt shingles, it's a problem. High pressure knocks loose the granules embedded in the shingle surface, and those granules aren't decorative. They're what protects the asphalt underneath from sun, weather, and impact. Once they're gone, the shingle ages fast, and most manufacturers, CertainTeed included, will point to pressure washing as grounds to void the warranty.
There are places pressure washing still makes sense: metal roofs, where there are no granules to lose, dense concrete tile, and certain flat commercial membranes designed for it. Even then, the person holding the wand needs to know the right pressure for that specific surface.
The classic homeowner mistake is renting a pressure washer on Saturday morning and going at the roof without knowing the output or the nozzle angle. The usual results are granule loss, lifted shingles, or water driven up under the flashing. If a contractor shows up to clean your asphalt roof with a standard pressure wand and no soft wash rig, that's your cue to confirm the method in writing or send them home.
Brushing, sprays, and zinc strips: the hands-on options
Manual removal and chemical treatments have their place, mostly as light maintenance between proper cleanings. Here's how each one breaks down:
- Roof raking and brushing works for loose debris, pine needles, and light moss. Use a soft-bristle brush, work downward with the shingles, and go easy. A stiff brush and enthusiasm will scrape granules off just as effectively as a pressure washer.
- Moss and algae sprays, like zinc sulfate or potassium-salt products, can be applied from the ground or the roof and kill growth over days or weeks with no rinsing. A decent lower-risk option if you'd rather not walk your roof, which honestly is most people, and that's fine.
- Zinc or copper strips installed near the ridge release metal ions when it rains, which slows moss and algae from coming back. Prevention only. They won't clear the moss that's already up there.
A word on runoff before you spray anything
Chemical runoff from roof treatments can damage plants, lawns, and drainage areas. If your roof drains toward a garden or a creek, pick products labeled biodegradable and rinse the surrounding plants with clean water before and after you apply. Your roof getting cleaner shouldn't cost you the flower beds.
Keep in mind these hands-on methods complement soft washing, they don't replace it. Heavy moss and lichen that have bonded to the shingle surface need the full treatment. For the long version of the moss battle we all fight out here, see our guide on moss on Oregon roofs.
Which method fits which roof?
Match the method to the material first and the price second. A cheap pressure wash that voids the warranty on a five-figure shingle roof is not a bargain, it's just a slower way to buy a new roof.
The short version: soft washing for asphalt shingles, cedar shake, and most tile. Pressure washing only for metal and concrete tile, in the right hands. Brushing and sprays for light upkeep on any roof. Zinc strips for prevention everywhere.
For the Pacific Northwest specifically, my standing recommendation for an asphalt roof is a soft wash every two to three years, zinc strips near the ridge, and gutters kept clear in between. Our wet climate means moss and algae never fully clock out, so the goal is management, not victory.
DIY or hire it out? What's actually at stake
DIY roof cleaning is possible, but the risk isn't only doing the cleaning wrong. Falls from roofs cause some of the worst injuries homeowners ever take, and most people don't own the harnesses, roof jacks, or footwear that professional crews treat as standard equipment.
The DIY mistakes are expensive too. Wrong chemical concentration bleaches shingles. A pressure washer strips granules and voids the warranty. Spraying treatment without protecting the landscaping kills plants. These aren't hypotheticals. They're the calls we get after someone tried to save a few hundred dollars.
Always hire a professional if your roof is steep, if the moss or lichen has bonded to the shingles, if you have cedar shake, or if keeping your manufacturer warranty matters to you. Handle the light stuff yourself: gentle brushing, ground-level sprays, and keeping gutters and valleys clear. Professional cleaning costs a small fraction of a replacement, and pairing it with a free roof inspection every few years means problems get caught while they're still cheap.
What I've learned after years of cleaning roofs in Oregon
The most common misconception I run into is that a pressure washer is just a faster, stronger version of soft washing. It isn't. They're fundamentally different approaches, and using the wrong one on asphalt shingles causes real, lasting damage you won't notice until years later when the shingles start failing early.
The second thing I see constantly: homeowners hiring whoever quotes the lowest number without asking what method they use. The guy with the pressure wand will clean your roof fast and cheap. He'll also strip your granules on the way out.
My honest advice: verify the method before you sign anything, ask whether they follow the manufacturer guidelines, and treat routine soft washing every couple of years as normal home maintenance, like servicing the furnace. Boring, yes. Cheaper than a new roof, also yes.