Is it safe to pressure wash a roof to remove moss?
No. Oregon State University Extension warns that pressure washing can damage shingles. High pressure strips the protective granules, forces water under tabs and shortens the roof's life. A roofer who offers it should be asked how they protect the shingles, and for a gentler alternative.
It is tempting because it works fast and looks dramatic. A roof that looks clean the same afternoon may have lost years of service. Hand brushing is slower and more expensive but leaves the surface intact.
Do zinc strips work, and what about copper gutters?
Zinc strips and zinc sulfate are commonly used to slow moss, but Oregon State University Extension advises against zinc on roofs with copper gutters or flashing. The Washington State Energy Office found no study verifying how long zinc treatments last. Ask the roofer what metal is already on the house.
Zinc washes down the roof with the rain and is the active ingredient in most treatments. On a roof with copper, the reaction between the metals is the concern. On an all-asphalt roof with aluminum or steel gutters, the caution is smaller.
Because longevity is unverified, it is sensible to treat zinc as one part of a program that also includes trimming, cleaning and monitoring. The guide on roof moss in the Skagit Valley goes through the evidence.
How often does moss come back?
In a valley with roughly 70 inches of rain a year and plenty of shade, it can come back within a season or two. Frequency depends on shade, roof pitch, the treatment used and how clean the roof is kept. Most owners find a yearly look after the wet season is enough to catch it early.
Sun is the best natural treatment. Trimming branches to let light reach the north slope often does more than any product. Keep gutters clear too, because a gutter that overflows soaks the lower courses of shingles and encourages growth there.
When is moss a sign the roof is finished?
When the shingles under it are brittle, bare of granules or lifted across a whole slope. Moss on a sound roof is a maintenance problem. Moss on a failing roof is a symptom, and cleaning will not rescue the shingles. A roofer can usually tell by gently testing a few tabs.
If more than a few tabs snap or crumble when lifted, ask for a written comparison of cleaning and replacing. The page on asphalt shingle roofing and the guide on signs an older roof is near the end can help you weigh it.
Who should do the work, and when?
A roofer experienced with moss, working in a dry spell, usually in late spring through early autumn. Moss is easiest to lift when dry and brittle, and a dry roof is safer to stand on. Avoid anyone who proposes pressure washing, and ask what treatment they will leave on the roof.
The best sequence is to clear gutters and valleys first, remove the moss, make any repairs it revealed and then treat the surface. Doing it in that order means a treatment is applied to a roof that is ready for it.
Ask the roofer to explain what metals are on your roof, what product they would use and what they would do to protect landscaping below. Washing moss killer into a garden bed or a creek is not harmless, and a careful crew plans for runoff.
