Roof Moss in Washington: Removal Options and Damage Risks
Removing moss from a roof is not one job with one method. What takes it off, and what that can cost the covering, is set by the material: the asphalt-shingle association, the wood-roof authorities and the tile industry publish different instructions, and they do not agree. The two things that damage the most roofs — a pressure washer and a stiff brush — are the two the shingle maker forbids outright. What is left is a chemical treatment, gentle physical removal, and the zinc or copper strips that slow its return — and the first decision is which document governs the roof, not which product to buy.
What the covering decides
The same green growth is a cosmetic nuisance on one roof and a slow structural risk on another, and a method that is safe on one material is forbidden on the next. Everything in this table is the covering’s own published instruction.
| Covering | What the growth does | What the maker permits | What the maker forbids |
|---|---|---|---|
| Asphalt shingles (Asphalt Roofing Manufacturers Association) | Algae staining is cosmetic: ARMA states there is no scientific evidence that it damages asphalt shingles. Moss is different — it can cause the leading edges to lift or curl, raising the risk of blow-off, and in severe cases move water sideways into the roof deck. | A bleach-and-water solution, rinsed at low pressure, and a leaf blower for moss the treatment has loosened. | A pressure washer, and any brush or broom. ARMA states high-pressure washing is likely to damage asphalt roofing, and that abrasive scrubbing loosens and removes granules. |
| Wood shakes and shingles (USDA Forest Products Laboratory; Cedar Shake & Shingle Bureau) | Moss and trapped debris hold moisture against the wood and encourage decay, decreasing the roof’s life; heavy moss can back water up the slope and over the shingles. | A bleach or oxygen-bleach cleaner at the weakest effective strength, applied with a soft brush or sprayer and rinsed with a garden hose pointed down the roof. | Aggressive power-washing. The Forest Products Laboratory’s guidance is that it can remove an excessive amount of wood fibre and “drastically” decrease the roof’s life, particularly on cedar and redwood. |
| Tile (Tile Roofing Industry Alliance) | Growth settles on the dust and organic film on the tile rather than the tile itself, dulling the surface; TRI states it “does not deteriorate the roofing tile.” | Treatment, which TRI says can be repeated as often as the growth returns. TRI also states that no manufacturing procedure or job-site coating has a long-term preventative effect. | Doing the work yourself or through a general cleaning company. TRI urges a licensed professional roofing contractor instead, because a wet tile roof is dangerous to walk on. |
| Metal (a panel manufacturer’s own care guide) | Debris accumulates fastest on low-slope panels and those under trees; the guide’s cleaning section covers algae and mildew on the factory coating. | A mild detergent, or a bleach-and-detergent solution, applied with a cloth or very soft bristles and rinsed at low pressure, after testing on a hidden area. | Wire brushes, abrasives, scouring powders and industrial solvents. The guide adds that cleaners it does not recommend may damage the coating and void the warranty. |
The removal options, compared
Three real options exist, and they do different jobs.
Chemical treatment kills the growth so that it can wash or blow away. The recipes in circulation are not universal, and the authorities’ own numbers show why. ARMA’s instruction for asphalt shingles is a 50:50 mix of laundry-strength liquid chlorine bleach and water, left on for 15 to 20 minutes and rinsed at low pressure. For wood, the Forest Products Laboratory starts at one part household bleach to five parts water and says it should not be necessary to go beyond one part to three — while warning that liquid detergent must not be used, because it can react with bleach to form toxic fumes. For moss specifically, Washington State University Extension’s demossing manual names copper compounds such as copper naphthenate, and says they are most effective applied when the moss is actively growing and no rain is expected for several days. A bleach solution is a treatment, not a moss killer with lasting residual action, and every maker attaches the same care: protect landscaping and other surfaces, wear eye and hand protection, and never let the solution dry on the roof.
Gentle physical removal clears what the treatment has loosened. ARMA allows a leaf blower for moss that has come loose, with the airflow directed down the slope so debris is not pushed under the shingle edges. What it does not allow is friction. The shingle maker states plainly that a brush, a broom and a power washer should not be used, because scrubbing loosens the granules that protect the shingle from the sun.
Zinc or copper strips are prevention rather than removal, and they are the one measure with a reach measured in feet. A strip set at the ridge releases a mild solution in the rain that carries a short distance downslope. The Forest Products Laboratory notes that the spacing some literature gives — about 12 feet — is often inadequate, and that recent experience points to about 6 feet, because wide spacing leaves unprotected bands that read as stripes. WSU Extension’s manual places zinc or copper flashings and ridges 10 to 15 feet down from the ridge, and notes that bare copper wire strung across the butt ends of the shingles provides some control through ordinary corrosion. The same authority records the mechanism for free: shakes below a galvanised flashing often stay clear, because a small amount of zinc erodes and washes down the roof — what a strip does deliberately.
How the growth is removed, and how the roof is kept clear of what feeds it, are two halves of one job. The routine off-roof maintenance — trimming back what overhangs, clearing debris, keeping the gutters clear and the ventilation flowing — is set out on the roof-care guide, because it needs no treatment decision and no roof access. What belongs here is the treatment itself.
The damage risks, by method
The failure that reads as “the cleaning ruined my roof” is usually one of four.
- Power washing. This is the one the shingle literature forbids outright, and the reason is mechanical: the water blasts the granules off the surface, and the maker ties that to “very likely premature failure of the roof system.” On wood the Forest Products Laboratory is only slightly gentler — power-washing should not be necessary except in rare cases of heavy build-up, and in the wrong hands it strips an excessive amount of wood fibre. The Cedar Shake & Shingle Bureau calls it the most controversial issue in the trade and states that high-pressure washing by inexperienced people causes significant damage to any material.
- Scrubbing. Abrasion does the same damage more slowly. ARMA’s bulletin states that scrubbing loosens and removes granules and instructs that the surface be left to the solution rather than worked.
- Leaving the growth in place. This is the quiet one. Moss left on a shingle roof can lift and curl the shingle edges and move water sideways into the deck; on wood it holds moisture against the surface and feeds decay; and heavy growth can dam water high enough to run over the shingles.
- Chemical runoff. The treatment protects the roof and can harm everything downstream of it. WSU Extension warns that runoff from moss-control applications damages plants, animals and physical items — copper compounds in particular can damage desirable plants — and that some products are corrosive to metal, so gutters and downspouts should be flushed after treatment, with the flush water captured where a downspout discharges onto a lawn.
Why one recipe will not do
Put the instructions side by side and the point becomes plain. The shingle association and the wood-roof authorities both work from bleach, at strengths that differ several-fold; the tile and metal sources do not start from bleach at all. Add the makers’ warnings — that a strong cleaner damages partially weathered wood, and that a non-approved cleaner voids a coated panel’s warranty — and the conclusion is not the strongest recipe but the correct document. Where a covering’s maker has published nothing, get the instruction in writing before the work is priced.
The limit that matters most in Washington
Washington writes down one consequence of moss that has nothing to do with the roof. The Department of Ecology’s study of copper and zinc in urban runoff found that moss-control products were the single largest source of zinc in its Puget Sound-area study area — 2,527 pounds a year, about 43 percent of the total. WSU Extension’s roof-runoff research adds that roofs of copper, unpainted galvanised metal and treated wood shakes were the least environmentally friendly of the materials tested, and that copper released over long periods from algae-resistant shingles can disrupt a coho salmon’s ability to smell, which it uses to find predators and its spawning grounds. Where a downspout discharges decides where those metals go: water that reaches a driveway runs to a storm drain untreated, while water that reaches a lawn or rain garden is filtered by soil and plants. The strips and the spray both put metal into the water that leaves the roof.
Where the work stops
Every authority on this page ends in the same place. ARMA states that only trained professionals should walk, climb or work on a roof. TRI Alliance directs tile work to a licensed professional roofing contractor. The Forest Products Laboratory recommends a contractor with experience maintaining wood roofs, because walking on aged shakes cracks them. None of the procedures above is a household task, and the choices that are genuinely the homeowner’s are the ones this page has set out: knowing what is on the roof, understanding what each method does to the covering, and refusing the two methods — a pressure washer and a stiff brush — that the makers forbid. How Washington lets you check a contractor’s registration, liability coverage and infractions at no cost is covered on the roof-care guide.
Next step
Before any treatment is quoted, write down the covering and its maker, and ask for that maker’s published cleaning instruction for it. Then compare like for like: what the treatment is, at what strength, how it is rinsed, how the runoff is handled, and how the growth at the ridges and in the valleys is dealt with. A treatment that arrives without a document for your roof is the one to decline.