Roof washing starts with identifying what you’re working with. The material determines how much pressure the surface can take, which chemicals are safe to use, and how often the job needs repeating. Cleaning a roof that actually needs replacing also just wastes time and money — so it’s worth checking the roof’s condition before booking a clean.
Here are the four roof types most common in Canada, how each is typically cleaned, and how often it needs it.
Asphalt shingles
Asphalt roofs are flat and sleek in appearance, with sand granules embedded in the shingle surface. They’re cost-effective, reflect sunlight well, and are fire-resistant — one reason they’re common in wildfire-prone areas and closely spaced neighbourhoods. A new asphalt roof has intact granules and shingles free of cracks or curling; wear shows up over time as those granules wash off and shingles start to curl.
Cleaning typically involves:
- Brushing off heavy moss buildup at the shingle overlaps
- Leaf-blowing (in dry conditions) or gently rinsing the loosened moss into the gutters, then clearing the gutters
- Applying a diluted sodium hypochlorite and roof-washing soap mixture across the roof and letting it work over the following days
Brushing does put some wear on the shingle surface, which is part of why re-treating on a schedule — rather than brushing repeatedly — is the better long-term approach. Most asphalt roofs need re-treating every 2–4 years, with gutters cleared before each application.
Cedar shakes
Cedar roofs are easy to identify by their large wooden shingles, which vary in size. A new cedar roof has rich, even colour with no lifting or curling.
Cleaning typically involves removing loose debris first, clearing the gutters, then applying a roof-washing detergent with a downstream injector and letting it dwell for 15–20 minutes. A pressure washer rated for the shake and capped at a low setting, with a wide-angle nozzle to spread the force, rinses the surface — more pressure than that strips the wood’s surface fibres along with the grime. A diluted oxalic acid treatment brightens the wood back toward its natural colour, and a roof coating afterward helps protect it from ongoing weather exposure.
Cedar needs cleaning far less often than asphalt. When it’s due, the same process applies — ideally on a dry, sunny day so the wood and debris aren’t already saturated.
Metal roofing
Metal roofs — large corrugated sheets, common in snowy regions for how well snow sheds off them — don’t accumulate moss the way other materials do, but they do collect debris from nearby trees. A well-maintained metal roof has no bent panels or loose screws and an even paint coating.
Cleaning is a diluted sodium hypochlorite and roof-washing soap mixture, rinsed off with low pressure for an immediate clean look. Re-treating every 2–4 years is typical, with steeper roofs needing it less often than shallow-pitched ones.
Concrete and tile
Concrete and tile roofs are heavy, often painted, and recognizable by their uniform, identically-sized tiles. They’re also energy-efficient, helping regulate indoor temperature. Before cleaning, check for cracked or broken tiles and replace them first — pressure and chemical treatment on a damaged tile can make things worse.
Cleaning follows the same basic pattern: clear debris, clean the gutters, apply the sodium hypochlorite and roof-washing soap mixture, let it dwell 10–15 minutes, then rinse with a pressure washer at the same capped, wide-angle settings used for cedar — enough to lift the grime without cracking or dislodging a tile. Gutter guards installed afterward help keep the gutters clear between cleanings. Re-treating every 4–5 years is typical.
What determines your actual schedule
Local climate — UV exposure, rainfall, tree cover — affects how fast moss and algae return, so treat these intervals as starting points rather than fixed rules. A shaded, moss-prone roof under conifers will need attention more often than the same material in full sun.
If you’d rather have this handled than DIY it, roof cleaning & moss removal is one of ours.