Zinc Strips vs Moss Treatment: What Actually Works on a Willamette Valley Roof
Zinc strips suppress new moss in the band of roof directly below them and lose
effect further down the slope. They do not remove moss that is already growing.
A moss treatment kills the existing growth; a strip is a slow preventive for the
area it can reach. Most Willamette Valley roofs need the treatment first.
$2,000,000 general liability insured · 5.0 from 13 Google reviews ·
serving the Willamette Valley since 2022
Do zinc strips work for roof moss?
Partly, and only where the runoff reaches. Rain carries dissolved zinc down
from the strip. That inhibits new growth in a band below it. Oregon State
University research finds strips ineffective for eliminating moss, and notes the
metal dilutes as it travels down the roof.
Two Oregon State publications look at this. They reach different emphases.
OSU’s Solve Pest Problems guidance on moss
carries a section headed “Zinc Strips Are Not Recommended”. It states plainly that
“installation of metallic zinc or copper-coated strip products on your roof is not
effective to eliminate moss.”
The peer-reviewed
PNW 733, The Care and Maintenance of Wood Shingle and Shake Roofs
is more favourable to the mechanism. Oregon State, Washington State and the
University of Idaho publish it jointly. It describes the same limit: “larger roofs
need multiple strips, since the metal concentration is diluted as it moves down
the roof, losing effectiveness.”
Read together, the two sources agree on how a strip behaves. They differ on
whether that behaviour is worth calling a solution. A strip inhibits growth in the
band it can reach. It fades downslope. Neither publication describes it as a way
to clear a roof.
Will zinc strips remove the moss already on my roof?
No. A zinc strip is preventive. It slows new colonisation where its runoff
reaches. It has no effect on established moss, which holds moisture against the
roof and lifts shingle edges. Existing growth has to be killed and weathered off,
or removed by hand.
This is the misunderstanding we are asked about most. A homeowner installs a
ridge strip in September. They look at the roof in February and find the moss
unchanged. The strip was never going to touch it. What a strip can do is slow the
next colonisation, on the part of the roof it protects.
How long do zinc strips take to work?
Seasons, not weeks. A strip acts only when rain dissolves metal and carries it
down the roof. The effect accumulates across a wet season, in the band below the
strip. A wide or complex roof needs several strips before coverage is even.
Rainfall is the delivery mechanism. In this valley that concentrates the effect
in the wet half of the year. Corvallis averages 3.16 inches of rain in October,
6.47 in November and 7.14 in December. Those figures come from the Corvallis State
University station, over an 1889–2012 record. Eugene runs wetter still: 3.52
inches in October, 7.20 in November. A strip installed in spring does very little
until those rains arrive.
Zinc strips vs moss treatment: which is better value?
They solve different problems, so the honest comparison is sequence rather
than either-or. A treatment clears what is growing now. A strip slows what comes
back, in the band it reaches. On a roof that already has moss, treating first and
considering a strip afterwards is the order that makes sense.
| Zinc or copper strip | Moss treatment | |
|---|---|---|
| Removes existing moss | No | Yes — kills growth, which then weathers off |
| Prevents new growth | In the band below the strip only | For a period, then repeats |
| Coverage on a wide roof | Uneven — dilutes downslope, needs multiple strips | Whole roof |
| Time to visible effect | Across a wet season | Weeks, as growth dies back |
| Fixing to the roof | Usually nailed or screwed through roofing | None |
| Runoff | Dissolved metal enters roof runoff | Depends on product and method |
| GreenTree price | Zinc / moss-out treatment from $299. Quoted per job — no per-square-foot rate. | |
What does moss treatment cost in Oregon?
GreenTree’s zinc and moss-out treatment starts at $299. That is also our job
minimum across every service. We quote per job rather than by the square foot.
Roof pitch, access, how much growth is present and the roofing material all change
the work more than area does. Booking three or more services together takes up to
10% off the bundle.
Why Willamette Valley roofs grow moss in the first place
Moss needs moisture and shade. The valley supplies both for most of the year.
Corvallis records 148 days with measurable precipitation in an average year, and
41.06 inches of rain. In Eugene, roughly 90% of annual rainfall arrives between
October and May. North-facing slopes and roofs under Douglas fir stay damp long
after the rest has dried. That is why moss so often appears on one plane of a roof
and not the others.
PNW 733 records the most common mosses on Willamette Valley roofs:
Dicranoweisia cirrata and Bryum capillare. That specificity is
worth noting. The growth on a Corvallis roof is a known, studied organism, not a
generic nuisance.
Oregon State’s
Extension guidance on controlling roof moss
is written from Corvallis and sourced to OSU plant pathologist Jay W. Pscheidt. It
puts the durable fix ahead of any product: “the most effective long-term solution
is to trim nearby trees so more sunlight reaches the roof.” That advice costs
nothing. It also outlasts both a strip and a treatment.
Can you pressure wash moss off a roof?
Not on asphalt shingles. The Asphalt Roofing Manufacturers Association
instructs homeowners never to pressure wash a shingle roof. Washing strips the
granules that protect the shingle and shortens the roof’s life. Moss is killed in
place and left to weather off instead.
The manufacturers’ own trade body is unambiguous. The
Asphalt Roofing Manufacturers Association guidance on cleaning asphalt roofing systems
states: “never use a pressure washer to clean an asphalt shingle roof as this will
cause granule loss and very likely premature failure.” Their technical bulletin
rules out brushes and brooms as well.
Wood shingles and shakes are a different material with the same conclusion.
PNW 733 records that “improper power washing can damage the wood surface and
fracture shingles.” A roof is not a driveway. Our soft wash runs at under 500 PSI.
A roof is treated chemically and left to weather rather than blasted clean.
What we do instead
We treat the growth, clear the debris it has trapped, and leave the roof to
weather. Sometimes a strip makes sense afterwards, on a simple gable roof with a
clear ridge line. We will say so when it does, and say so when it does not. A
strip on a wide, cut-up roof means many fixings through roofing material for an
uneven result.
Book a roof moss treatment
Serving the Willamette Valley since 2022. Registered with the Oregon Secretary of State, Registry #2424331-91. Treatment starts at $299, quoted per job.
Call (971) 280-2861 or request a quote. Book before
the October rains: moss growth accelerates once they arrive.
GreenTree Services · Corvallis, OR 97330 ·
(971) 280-2861 · Mon–Sat 7am–7pm ·
$2,000,000 general liability insured · 5.0 from 13 Google reviews