Moss comes off a Lower Mainland roof by hand or soft brush followed by a registered moss-control product, never a pressure washer, and a professional roof cleaning costs about $350 to $900. Keeping it off means more light, less debris and a zinc strip or an annual treatment. Here is the whole picture: why moss grows here, what it does to a roof, how to remove it safely and what the law says about what goes down the drain.
Why moss grows on roofs here
Moss needs moisture and shade. Vancouver gets about 1,160 mm of precipitation on 170 days a year, more in the valley and on the North Shore, and from October to April a shaded north slope under trees never fully dries. Add the organic debris that falls from cedars and firs and a shingle roof becomes a moss bed within a few years. South slopes in full sun rarely have a problem; the north slope of the same house can be green.
Does moss damage a roof?
Yes, unlike algae. The black streaks on shingles are algae, which the Canadian shingle industry says is cosmetic and doesn't shorten shingle life. Moss is a plant with roots. It grows under the shingle edges and lifts them, holds water against the shingle and the sheathing, and freezes in cold snaps, prying the tabs further. On cedar, it keeps the wood wet and speeds decay. A mossy shingle roof fails years earlier than a clean one, and a thick carpet of moss can hide damage until it leaks.
How to remove moss from a roof
Don't pressure wash. The Asphalt Roofing Manufacturers Association and its Canadian counterpart both say high-pressure washing damages asphalt shingles and should never be used on them. It strips granules, forces water under the shingles and can void the warranty. On cedar it tears the surface fibres.
Do this instead:
- Wait for a dry spell when the moss has dried and loosened.
- Remove the bulk by hand or with a soft brush, working downward with the shingle laps, never upward. Clear the gutters of what comes down.
- Apply a moss-control product registered for roofs in Canada (it will have a Pest Control Products Act registration number on the label). Common active ingredients are potassium salts of fatty acids and zinc sulphate. Don't use zinc products on a roof with copper gutters or flashing, because zinc corrodes copper.
- Let the treatment work; dead moss dries and blows or washes off over the following weeks.
- Rinse gently, if at all, with a garden hose from above.
Most homeowners hire this out. Working on a wet, mossy, sloped roof is how people get hurt, and WorkSafeBC requires fall protection for workers at 3 m or more for a reason.
What about bleach?
Diluted bleach (one part to one part water, 15 to 20 minutes, gentle rinse) is the manufacturers' recommended cleaning for algae stains. For moss it's less effective than a registered product, and in BC it comes with a legal problem: the Province's roof cleaning guidance notes that bleach and soaps kill fish and that many storm drains lead straight to creeks. The Environmental Management Act prohibits polluting discharges. If a cleaner proposes bleach, ask how they'll keep it out of the drains.
What does roof moss removal cost?
| Service | Typical price (Lower Mainland) |
|---|---|
| Moss removal and soft-wash cleaning, typical house | $350 to $900 |
| Zinc strips at the ridge | $3 to $6 per linear ft |
| Cleaning plus zinc strips package | $550 to $1,100 |
| Gutter cleaning at the same time | $150 to $400 |
| Roof inspection with written report | $150 to $400 |
Prices from Lower Mainland roofers' published 2026 rates; large, steep or heavily treed roofs cost more. The zinc strip figures come from a single company and should be confirmed in your quote.
How to keep moss off
- Light. Trim branches that shade the roof. The Oregon State Extension service calls more sunlight the most effective long-term fix, and it's true here too.
- Debris. Blow or brush needles and leaves off the roof and out of the valleys each autumn. Debris holds water and feeds moss.
- Zinc or copper strips under the ridge cap release metal ions in rain that inhibit moss for the few courses below them. They don't protect the whole slope; the cedar industry says the effect reaches only a few courses down.
- Annual or biennial treatment with a registered product on the north slope, applied in a dry spell.
- Algae-resistant shingles when you re-roof. They inhibit algae, and help a little with moss, but shade and debris matter more.
- Metal roofing when you re-roof a chronically mossy house. Moss has nothing to root into on metal. See the metal versus shingles guide.
When cleaning isn't enough
If the moss has lifted and cracked the shingles, or the roof is past 20 years, cleaning buys a season or two and costs most of a repair. Have a roofer look at it; the repair cost guide and when to replace your roof will help you judge the answer. Roof cleaning is a maintenance service we don't match for, but if the roof turns out to need repair or replacement, request quotes and we'll send your details to up to three vetted roofers in your area.
Common questions
Does moss damage a roof?
Yes, over time. It lifts shingle edges and cedar shakes, holds water against the roof and traps debris. Black algae streaks, by contrast, are mostly cosmetic and don't shorten shingle life according to the shingle industry.
Can I use bleach or dish soap on the roof?
The shingle industry's cleaning method is a 1:1 chlorine bleach and water solution, rinsed with low pressure. The BC government asks that runoff not reach storm drains or watercourses, where bleach and soap harm fish, and that products used be registered for roofs in Canada.