Rain, Moss and Your Roof in Seattle

Seattle roofs get long, wet winters and lots of shade, which is why moss and slow leaks are the most common roof problems here. Moss holds moisture against the roof and can lift shingles. Remove it gently, never by pressure washing, keep runoff out of storm drains, and stay off the roof.

How much rain does a Seattle roof actually get?

Less in total than many people think, but spread over a lot of days. Based on NOAA's 1991–2020 climate normals for Seattle–Tacoma International Airport (NOAA climate normals, Sea-Tac):

• Annual precipitation averages 39.34 inches.
• About 156 days a year get at least a hundredth of an inch of rain, and about 93 days get at least a tenth of an inch.
• Snow averages 6.3 inches a year, and about 25 nights a year drop to freezing or below.

The rain is also heavily seasonal. NOAA's monthly normals for Sea-Tac show November averaging 6.31 inches, December 5.72 inches and January 5.78 inches, while July averages just 0.60 inches and August 0.97 inches (NOAA monthly normals, Sea-Tac).

What that means for your roof:

• Roofs stay damp for months at a time, especially on shaded, north-facing slopes. That's what moss needs.
• Small leaks show up in fall. A flashing gap that stayed dry all summer can start dripping with the first big November storms.
• Summer is the natural window for inspections, moss removal and roof work.

Does Seattle get big storms too?

Yes, mostly in late fall and winter. The Office of the Washington State Climatologist ranked the December 2006 "Hanukkah Eve" windstorm as the state's top weather event that year. It reports winds of 40 to 70 mph across the western Washington interior, and that close to 1.5 million electrical customers lost power, some for more than a week (Office of the Washington State Climatologist).

The same report notes that Seattle recorded 18.61 inches of rain in fall 2006 (September through November), breaking a record set in 1955.

After a windstorm, check from the ground for missing shingles, lifted ridge caps, damaged flashing and branches on the roof. Wind and long rain together are when weak spots tend to leak.

Why does moss grow so well on Seattle roofs?

Moss needs moisture and shade, and Seattle has both. Oregon State University Extension notes, "There's no changing the rain," but moss also needs shade to thrive (Oregon State University Extension). That's why moss often shows up first on north-facing slopes and on roofs under tall evergreens.

Common reasons a roof gets mossy:

• Overhanging trees that keep the roof shaded and drop needles and leaves.
• North-facing slopes that get little direct sun in winter.
• Debris buildup in valleys, behind chimneys and in gutters, which holds water.

Is roof moss actually a problem?

It can be, over time. OSU Extension explains that the moss itself doesn't directly harm roofing, but it traps debris and holds moisture, which lets fungi take hold and shortens roof life. On asphalt and metal roofs, moss can lift shingles and let water reach the layers underneath (Oregon State University Extension).

Signs moss may be causing trouble:

• Thick mats rather than a thin green film.
• Shingle edges that look lifted or curled where moss is heavy.
• Clogged gutters and downspouts full of moss clumps.
• Stains on ceilings or in the attic under mossy areas, especially in winter.

How should moss be removed?

Gently, and by someone with the right safety gear. Here's what the research-based guidance says:

• Don't pressure wash. OSU Extension warns that "improper technique can damage shingles," and suggests a long-handled scrub brush as a safer option (Oregon State University Extension).
• Physical removal comes first. The University of Washington's Elisabeth C. Miller Library summarizes sources that recommend physical removal of moss, followed by zinc strips as a long-term approach (University of Washington Botanic Gardens).
• Watch out for copper. If you use zinc sulfate products, OSU says not to use them on roofs with copper gutters, downspouts or flashing, because "zinc corrodes copper."
• Trim back trees. OSU calls this the most effective long-term fix, because it lets more sun reach the roof.

OSU's own article includes ladder safety tips, and its bottom line is "When in doubt, hire a professional." For most homeowners, especially on a steep, wet or two-story roof, that's the right call. Stay off the roof yourself.

How do you keep moss from coming back?

Make the roof a worse place for moss to grow. Moss needs shade and moisture, so the long-term steps are about sun, drainage and, sometimes, zinc:

• Let in more sun. OSU Extension calls trimming nearby trees the most effective long-term fix (Oregon State University Extension). Hire a tree service for large limbs over the house.
• Keep debris off. Needles and leaves in valleys and gutters hold water against the roof. Have gutters cleaned before the wet season, ideally in early fall.
• Ask about zinc strips. Sources summarized by the UW library describe zinc strips installed after physical removal as a long-term approach, while cautioning that runoff from them shouldn't go directly to plants, ponds or water supplies (University of Washington Botanic Gardens).
• Schedule regular checks. A quick look from the ground each spring and fall, plus a professional inspection every few years, catches regrowth while it's still thin.

None of these steps needs you on the roof. Anything above the gutter line is a job for a pro.

Why does moss treatment runoff matter in Seattle?

Because rain off your roof often ends up in Puget Sound or a local creek. The Washington State Department of Ecology says a Puget Sound toxics study identified roofs as "a potential significant source of arsenic, cadmium, copper, and zinc released in the Puget Sound basin," and that these metals, in sufficient amounts, can harm or kill fish and other aquatic life (Washington Department of Ecology).

The UW library adds that runoff from zinc strips or flashing should not go directly to surrounding plants, fish ponds or water supplies, and cites the City of Portland's warning that zinc sulfate is toxic to aquatic life (University of Washington Botanic Gardens).

Questions worth asking any moss removal company:

• What product will you use, if any, and is it labeled for roofs?
• How will you keep treatment runoff out of storm drains and nearby water?
• Will you protect plants below the eaves?
• Is my gutter or flashing copper, and does that change what you use?

Where do Seattle roofs usually leak?

The same places as anywhere, but Seattle's long rains find them sooner. Common sources include:

• Flashing around chimneys, vents, skylights and walls.
• Vent pipe boots, whose rubber collars crack with age.
• Clogged gutters and valleys, which back water up under shingles.
• Skylights, at the seals or the flashing.
• Shingles lifted by moss or wind.

If a stain only appears during long or heavy rain, our guide to roof leaks that only happen in heavy rain covers the likely causes. To narrow down where water is getting in, see how to find a roof leak.

If water is dripping near a ceiling light, fan or outlet, stay clear of it, and switch off that circuit at the breaker only if you can do it without standing in water.

What do moss removal and leak repairs cost?

These are national figures, not Seattle prices, as of 2026:

• Roof cleaning: Fixr puts roof cleaning between $450 and $700 (Fixr).
• Moss removal: between $0.20 and $0.70 per square foot, and Fixr notes most cleaning methods already include it (Fixr).
• Soft wash cleaning: $0.30 to $0.75 per square foot (Fixr).
• Flashing repair: usually $200 to $600 (Fixr).
• Clogged gutters: $100 to $250 to unclog and repair (Fixr).
• Leaking skylight: $300 to $800 to fix (Fixr).
• Replacing damaged or missing shingles: typically $150 to $700 (Fixr).

For more on general repair pricing, see our roof repair cost guide.

Should you repair or replace a mossy, leaky roof?

It depends on what's under the moss. If the shingles are still flat and intact once the moss is gone, cleaning and targeted repairs are often enough. If a pro finds widespread curling, cracking, missing granules or soft decking, replacement may make more sense than repeated repairs.

For comparison, Fixr puts full asphalt shingle replacement at $8,000 to $16,000 nationally (Fixr). Ask for photos of the shingles under the moss before you decide, and ask the roofer to explain why they recommend one path over the other.

Do Seattle roofs need to worry about snow and ice?

Sometimes, but far less than in colder cities. NOAA's normals for Sea-Tac show about 6.3 inches of snow a year and about 25 nights at or below freezing (NOAA climate normals, Sea-Tac). When snow does stick, clogged gutters and moss-lifted shingles are the spots where meltwater is most likely to get in. Clearing gutters before winter helps here too.

Do you need a permit to reroof in Seattle?

Often not, for a house. The Seattle Department of Construction and Inspections says a permit is not required for re-roof work at one- and two-family homes and townhouses "if no changes are made to the building envelope other than adding or replacing insulation," as long as the insulation value is equal to or better than before (Seattle Department of Construction and Inspections).

Two things to know:

• Insulation may be required. The department says you need to add insulation to meet current energy code when the existing sheathing or insulation is exposed or the roof cavities aren't insulated.
• Bigger changes need a permit. If the reroof alters the building envelope in other ways, you need a construction permit.

Ask your roofer to confirm which applies to your project in writing.

How do you check a roofing company in Washington?

Washington's Department of Labor & Industries advises homeowners to verify that a contractor is registered, bonded and insured, and to review its history for lawsuits or violations (Washington State Department of Labor & Industries). It also suggests:

• Get at least 3 written bids and compare them carefully.
• Get references, and stay alert for scams.
• Get a written contract and make sure you understand it.
• Consider a performance bond for larger projects.

Other questions to ask a roofer:

1. Is the moss surface growth, or is it lifting shingles and holding water?
2. How will you remove moss without damaging the shingles?
3. Do you recommend trimming trees, and who handles that?
4. Will you check flashing, vent boots and skylights while you're up there?
5. Does a reroof quote include any insulation the city may require?

When should you call a pro?

Call when moss is thick or lifting shingles, when gutters keep clogging, or when a ceiling stain appears during a long rain. Summer is the easiest time to schedule. Call right away after a windstorm that leaves shingles in the yard or a branch on the roof.

Bellwick is a free service that helps you understand the likely problem and can connect you with a local roofing company. Bellwick doesn't do the work. If you'd like a quick read first, check my roof.