Asphalt roofs west of the Cascades typically last 20 to 25 years, shorter than the national average, and moss is the main reason.
What moss actually does
It grows in the gaps between shingle courses and expands. That lifts the lower edge of the shingle above it, breaking the tight overlap that makes a roof shed water.
Once that overlap is compromised, water runs sideways under the courses rather than down the slope. Moss also holds moisture against the surface continuously, so the roof stays wet far longer after rain than it otherwise would.
The combination is why a fifteen-year-old roof under heavy canopy can be in worse condition than a twenty-year-old roof on an exposed lot.
The thing not to do
Do not pressure wash an asphalt roof. This is the most important sentence in this article.
Asphalt shingles are protected by a layer of mineral granules that shields the asphalt from ultraviolet degradation. Pressure washing strips them. The roof looks dramatically better immediately afterwards and has lost years of life.
If a contractor proposes pressure washing your asphalt roof, that tells you something about them. Metal roofs are a different matter, but asphalt should never see a pressure washer.
Also avoid aggressive scraping
Vigorous scraping and stiff wire brushes damage granules too, and they lift shingle edges further. Gentle removal of thick accumulation is reasonable, but the goal is not a roof that looks scrubbed.
What actually works
Treatment products designed for roof moss kill the growth, which then dries and weathers away over subsequent months. It is slower and less satisfying than physical removal, and it is considerably better for the roof.
Applied from a ladder at the edge, or by someone with proper fall protection working on the roof. Follow the product directions, particularly regarding runoff — some products affect plants below.
Zinc and copper strips
Metal strips installed near the ridge release trace amounts of metal ions as rain runs over them, inhibiting growth on the slope below. They work reasonably well as prevention and do nothing about existing growth.
The same principle explains algae-resistant shingles, which incorporate copper or zinc granules. In Western Washington these are worth specifying on any new roof — usually a modest upgrade that meaningfully extends life.
Reduce the conditions
Moss needs shade and moisture. Trimming branches overhanging the roof increases sunlight and airflow, and reduces the organic debris that gives moss something to establish in.
Keeping gutters and valleys clear of needles matters for the same reason. Accumulated debris holds moisture and creates a bed for growth.
These do more over the roof’s life than any treatment, because they address the conditions rather than the symptom.
Timing
Late spring, once growth is at its heaviest after the wet season and there is a reasonable dry window for treatment to work. Autumn is also reasonable, ahead of the wet months.
Safety
Roof falls cause serious injuries. A wet Washington roof with moss on it is genuinely slippery, and a steep pitch multiplies the risk.
Treating from a ladder at the edge is reasonable for many homeowners. Working across the roof surface, on a steep pitch, or on a wet day is worth paying someone with proper equipment to do.