I spend a lot of time looking at things that were supposed to be done right the first time. Over about six years now, I’ve reviewed a few hundred home service jobs a year—roofing, gutters, window installations, that kind of thing. You start to see patterns. One of the more frustrating ones? Moss removal. Specifically here in the Pacific Northwest. In Otis Orchards, it’s not a question of if you’ll get moss on your roof, but when. And the real issue isn’t the moss itself—it’s the solution.
There is no universal answer for how to handle roof moss. Anyone who tells you there’s one best way probably only knows one way. The right approach depends on your roof’s material, its age, and your tolerance for risk. Let me break it down by the three most common situations I’ve seen.
Situation A: The Asphalt Shingle Roof (The Most Common, and the Trickiest)
If your home in Otis Orchards has a standard asphalt shingle roof, this applies to you. Asphalt shingles are designed to shed water, not handle the weight and moisture retention of a moss colony. I’ve seen roofs that look like a lawn from the street. It’s not just unsightly—moss lifts the edges of the shingles, exposing the underlying material to wind and rain. That’s what leads to leaks.
The low-pressure wash route. This is what most companies offer. It works, but poorly. The top layer of the shingle's granules gets blasted off. When I was auditing a series of jobs for a homeowner in 2021, we reviewed a had a roof that was power-washed for moss. The moss was gone, but the shingles looked bald in spots. That roof was maybe 10 years old and it looked 25. The moss came back faster the next year because the granules weren't there to protect the asphalt.
A better alternative: chemical treatment and hand removal. This is a slower process, and it costs more. But in the long run, it preserves the shingle life. You apply a zinc or copper sulfate mix, let it sit, and then gently rake the moss out. No pressure washing.
Which brings me to a cost consideration. If you're looking at a service that quotes you $300 for a full roof moss removal by pressure washing—that’s a red flag. They're likely cutting corners on chemical treatment, or they're planning to blast your roof. I've learned to ask 'what is NOT included' in a quote for this. The vendor who lists the chemical treatment and gentle removal upfront, even if their total is $700, usually costs less in the end. You avoid the re-roofing cost down the road.
Situation B: The Cedar Shake or Slate Roof (Premium Materials, Premium Care)
If you have a cedar shake roof, your options narrow considerably. Cedar is naturally resistant to insects and rot, but moss can still thrive in the dampness. The issue here is the high-pressure wash. It can literally shred the wood fibers. I saw this happen on a project in 2022. A homeowner went with the lowest bid to clean his aged cedar shake roof. The company used a pressure washer set at a standard residential pressure. It tore grooves into the wood, channeling water under the shakes. That $400 moss removal job ended with a $18,000 full re-roof.
For cedar, the only real solution is a gentle hand-scraping with plastic tools and a copper-based fungicide spray that prevents re-growth for a few years. Expect to pay a premium—$0.50 to $1.00 per square foot. For a 1,500 sq ft roof, that's $750 to $1,500. But you’re preserving an expensive roof.
Same goes for slate, though slate is much more durable. Pressure washing is okay, but only at a very low pressure and with a wide fan tip. Slate can still crack if the pressure is misdirected at the edges.
Situation C: The D.I.Y. Approach (The Riskiest, But Sometimes the Only Option)
I've seen homeowners take matters into their own hands, and the results are… mixed. The trigger event that changes how I think about DIY roof work was a situation in spring 2023. A couple in Otis Orchards decided to climb up there with a leaf blower and a garden hose. They dislodged a ton of moss, but they also dislodged a chimney cap.
If you’re going to DIY, you need to know the limits. Don't use a pressure washer. You cannot control the pressure from a ladder. Don't use a metal rake. You will scratch the shingles or shake edges.
What works: a plastic scoop, a spray bottle of moss killer, and patience. Apply the moss killer (zinc sulfate is good). Wait 24 hours. Scoop the dead moss into a tarp. Then rinse with a garden hose—not a nozzle jet, just a flow of water. That's it. Not exciting. But safe.
Also, a small point of caution: cleaning moss off a roof is slippery work. If you're not comfortable on a ladder, you probably shouldn't be on a roof. The risk of a fall is far more expensive than paying a pro. It's like the old saying: 'Speed, quality, price. Pick two.' Well, with moss removal, it's 'safe, effective, cheap. Pick two.'
How Do You Know Which Situation You're In?
Check your roof's material first. If it's asphalt, go with chemical treatment and hand removal. If it's cedar or thick slate, hire a specialist who works with those materials. If you don't know, look at the edge of a shingle from the ground. Asphalt has a textured granular surface. Cedar looks like wood. Slate is stone.
Check the age. If your asphalt roof is over 15 years old, even a gentle chemical treatment is risky. The shingles are brittle and can crack when walked on. At that point, you're better off planning for a replacement than spending $700 on a treatment that might not hold.
One more thing: Before you hire anyone, ask if they're insured for roof work and whether they'll guarantee the integrity of your chimney cap and flashing. I had a case last summer where a crew accidentally knocked a chimney cap loose, and the homeowner didn't notice until rainwater leaked onto their ceiling. A lot of companies will say they cover that, but it's worth clarifying. The numbers said go with the cheaper company. My gut said stick with the one who listed every item on the quote. We went with my gut.
The older I get in this work, the more I realize that most problems aren't caused by a lack of options. They're caused by choosing the option that looks easiest today, without considering what it costs tomorrow. Roof moss is a perfect example. You can blast it off for cheap, or you can do it right for a bit more. The moss doesn't care about your deadline. It just grows. And the only thing that stops it is proper care.
