The short version. Moss is treated on a dry day between October and May. That is a long season, and it is the dry stretch rather than the month that matters — the product needs to go down dry and then be carried through the moss by the rain that follows. Late winter and spring is when the results show fastest, because the moss is at full growth and dies back visibly.
What matters more than the calendar is which stage your roof is at. Two of the five need nothing at all. One of them should not wait for spring.
Call (253) 366-8066 and we’ll connect you with a licensed local specialist who works Steilacoom.
Why Steilacoom roofs, specifically
the median year a Steilacoom detached home was built. 19% of them predate 1960.
Pierce County Assessor, Aug 2026of Steilacoom’s 2,013 detached homes have a roof moss grows on.
Pierce County Assessor, Aug 2026of those roofs are on homes built before 1990 — 71%, against 54% for Pierce County as a whole.
Pierce County Assessor, Aug 2026contributing properties in the National Register historic district. Roofing work on them runs through a review path.
Steilacoom Municipal Code Ch. 2.14Moss does not need a bad roof. It needs shade, moisture and something to hold onto, and Steilacoom supplies all three. What is worth knowing is the shape of the housing stock underneath.
Every single-family detached home in the city, from the county tax roll. Thirty-five predate 1890 and are left off the scale. Hover any bar for the count.
Steilacoom’s numbers describe two towns at once. The 1970s is the largest single decade at 449 homes, and the median build year is 1975 — but beneath that sits the oldest continuously settled townsite in the state, with a National Register district of 68 contributing properties and houses dating to the 1850s.
What a treatment actually is
Not a wash. The standard product is a zinc sulfate powder or granule, applied dry along the ridge and upper courses, which then works down the slope with the rain over the following weeks. It kills the moss where it sits and leaves a residue that discourages regrowth for a season or two.
The label sets the rate, and the county tells us the roof size, so the quantity for a typical Steilacoom house is arithmetic rather than guesswork. EPA Reg. No. 10699-1 specifies 3 lb per 600 sq ft of roof.
| Roof pitch | Surface area | Zinc sulfate |
|---|---|---|
| Footprint only (flat proxy) | 1,300 sq ft | 6.5 lb |
| Low pitch, about 4:12 | 1,495 sq ft | 7.5 lb |
| Steeper, about 8:12 | 1,690 sq ft | 8.5 lb |
Footprint is recorded floor area divided by story count, across 1,248 pre-1990 Steilacoom homes with a moss-susceptible roof. The pitch multipliers are standard geometry. This is an order of magnitude with the arithmetic shown — not a quote.
It is useful mostly as a sanity check. A treatment for a house this size is a small quantity of product and a couple of hours of careful work on a ladder; most of what you are paying for is the access, the care taken not to damage the granules, and the fact that whoever is doing it is licensed to apply it at all.
The review question, if your house is in the district
Steilacoom is a Certified Local Government with a seven-member Historic Preservation and Review Board, and its code reaches exterior work on properties on the Steilacoom Register or inside a Steilacoom historic district.
The line that matters for roofs is SMC § 2.14.061(C). Replacing roofing with in-kind materials on a contributing property is handled administratively, as staff review. Replacing it with substantially different materials, or in a way that affects the design of the resource, goes to the full Board. Ordinary repair and maintenance is exempt, and so is exterior work on buildings outside the district that are not individually listed.
The license behind the work
This is the part that surprises people, and it is the single most useful thing on this page.
Washington law treats your roof as “land.” The Pesticide Application Act defines land broadly enough to include a structure, which puts moss treatment squarely inside it — so applying a moss killer on someone else’s roof for money is a commercial pesticide application, and it requires a WSDA commercial applicator license carrying the Structural and Turf category.
- Licensed applicator, with the category that covers roofs — every specialist we refer holds it
- Debris and needles cleared first, by hand, so the product reaches the shingle
- Dry product along the ridge and upper courses, not a pressure wash
- Gutters and downspouts flushed afterwards, so the runoff does not sit in the trough
- Plants below protected — zinc sulfate runoff is hard on what it lands in
- Realistic expectations set — moss goes brown and washes off over weeks, it does not disappear the same afternoon
The one thing not to do
Do not let anyone pressure-wash an asphalt shingle roof. The granules on the surface are the roof’s ultraviolet protection and they are held on with asphalt, not glue — a pressure washer takes them off, and a roof stripped of granules ages far faster than one with moss on it.
Every shingle manufacturer says the same thing, and the specialists we work with treat it as settled: on composition shingle, soft methods only. It is one more thing you do not have to think about when the introduction comes through us.
Where we work in Steilacoom
Across the town, with the historic core and the streets along the bluff the ones that need it most often.
Find out which stage your roof is at
Tell us your neighborhood and what is above the roof, and send a photo from the driveway if you have one. We will tell you whether it needs treating this spring or can wait — and if it needs work, connect you with a licensed local specialist. No cost to you, no obligation.
Call (253) 366-8066Get a Free Quote Cascade Roof & Gutter is not a contractor and does not perform work. We connect Pierce County homeowners with independent licensed specialists.Nearby
We cover the whole of Pierce County. If you are closer to one of these, start there:
Sources
- Pierce County Assessor-Treasurer, improvement_builtas and tax_account extracts, August 2026, joined on parcel number — 2,013 Steilacoom single-family detached homes, roof cover, year built and floor area.
- NOAA NCEI, U.S. Climate Normals 1991–2020, Tacoma No. 1 — 40.81 in annual, 6.50 in November.
- Reukema, D.L. (1964), USDA Forest Service Research Note PNW-14 — Douglas fir litterfall by month.
- Steilacoom Municipal Code Ch. 2.14 — Historic Preservation and Review Board; § 2.14.061(C) on roofing replacement and materials.
- Town of Steilacoom Comprehensive Plan, 2015 — elevation, Chambers Creek fishery, eelgrass beds.
- Washington Dept. of Ecology, Western Washington Phase II Municipal Stormwater Permit (2024) — conditionally allowable building washdown.