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Homeowners usually call a roof cleaner when streaks darken a slope, when moss starts filling the gaps between shingles, when gutters keep clogging with debris, or before putting the house on the market. A good job combines careful removal of what's growing, a gentle treatment and practical steps that slow the return of moss.
This page explains why moss and streaks matter, soft washing compared with pressure washing, cleaning products and where their runoff goes, zinc and copper strips, overhanging branches and what safe roof cleaning looks like. Cleaning a roof means working at height on a slippery surface, so it's a job for insured professionals with fall protection, not for a homeowner on a ladder.
Why roof moss, algae and black streaks matter
Not everything growing on a roof is equally serious, and a good roof cleaner starts by identifying what's there. What matters is how much moisture each kind of growth holds against the roof and whether it's working its way under the covering.
- Black or dark streaks: usually algae living on the surface in shade and damp; mostly cosmetic, but a sign that the slope stays wet.
- Moss: thick green cushions that act like a sponge, keeping shingles wet and growing into the joints until it lifts their edges.
- Lichen: flat, crusty patches that grip tightly; forcing them off can tear the surface, so they're usually treated and left to loosen.
- Needles, leaves and seeds: debris that settles in valleys, behind chimneys and at the eaves, damming water and feeding new growth.
- Clogged gutters: moss and debris washed down from the roof make gutters overflow against the fascia and add to ice at the eaves in winter.
Soft washing vs pressure washing a roof: why shingles need low pressure
Soft washing applies a cleaning solution at low pressure, much like a garden sprayer, gives it time to work on moss and algae, then rinses gently or lets the weather finish the job. Thick moss is usually loosened first by hand, working down the slope with soft brushes or plastic tools so nothing is scraped or pried up. The aim is to kill growth at the root rather than blast away what's visible.
Pressure washing does the opposite. As our pressure washing page cautions, a jet on asphalt shingles strips their protective granules and can drive water up under them; it can also crack brittle shingles, gouge cedar and break slate. If a company offers to pressure wash your shingle roof, look elsewhere. Cedar, slate and metal roofs each call for methods suited to the material, so ask what the crew plans to use on yours.
Roof cleaning products and runoff: protecting plants, gutters and storm drains
Treatments strong enough to kill moss and algae can also harm lawns, shrubs and garden beds, and some can mark metal, painted trim or stone. Whatever runs off the roof goes into the gutters, onto the plants below and often toward the street. Ask the cleaner which product they use, ask to see its label, and ask how they'll control the runoff.
Runoff rules are set by municipalities. Toronto's Sewers By-law is a clear example: the City says only rain and melted snow may flow into an outdoor catch basin, that wash water must not reach one, and that even washing with water alone carries contaminants. Check your own municipality's rules before the job is booked.
- Wetting or covering the plants and beds below the eaves before and after treatment.
- Blocking or redirecting downspouts so treated water is collected instead of reaching the lawn or the street.
- Keeping solution away from catch basins, ditches, streams and wells.
- Rinsing or shielding metal gutters, flashing and siding the solution may reach.
Zinc and copper strips and tree trimming: keeping moss from coming back
Once a roof is clean, the goal is to make it a poor place for moss to return. Strips of zinc or copper, installed by a roofer along the ridge or near the top of a slope, shed a trace of metal every time it rains, and that film discourages moss and algae below. The effect is strongest just downslope of the strip and fades farther down, so long or broken-up slopes may need more than one row. Strips are prevention for a clean roof, not a way to remove thick moss. They must be fastened so no exposed nail becomes a leak, and the metal they shed ends up in the runoff, which is worth weighing near a pond or stream.
Shade and damp matter more than any product. Overhanging branches keep a roof from drying, drop leaves and needles into valleys and gutters, and scrape shingles in the wind. Cutting them back lets sun and air reach the roof. Work on trees near a house or power lines belongs to an arborist; see our tree trimming page, and ask whether a municipal permit is needed first.
How a professional roof cleaning is done, with fall protection
Roof cleaning combines height, a wet and slippery surface, and chemicals. Ask the company how its workers are protected from falls, whether it carries liability insurance, and whether its crew is covered by your province's workplace injury insurance. A typical job runs like this:
- Assessment: the cleaner identifies the roofing material and the growth, and notes damaged or lifted shingles before touching anything.
- Protection: plants, walkways, siding and outdoor furniture below are wetted or covered, and downspouts are set up to manage runoff.
- Access: the crew works from secured ladders or lifts, or on the roof in harnesses tied to anchors, following its fall protection plan.
- Removal: thick moss and debris are loosened by hand, working down the slope, and valleys and gutters are cleared.
- Treatment: a low-pressure solution suited to the material is applied, then rinsed or left to work as the label directs.
- Follow-up: gutters are cleared again, surfaces the solution touched are rinsed, and any repairs the roof needs are reported to you.
When to book roof cleaning: spring and fall weather windows
Most cleaners work from spring to fall, when the roof is clear of snow and ice and the weather suits the product. Many labels call for dry weather after application so rain doesn't wash the solution away before it works, which makes the forecast part of the booking. Spring is a good time to deal with growth that thrived under winter snow; late fall, once the leaves are down, is the time to clear valleys and gutters before freeze-up. A frozen or snow-covered roof is too dangerous to clean.
When roof cleaning isn't enough: moss damage that needs a roofer
Cleaning removes growth but can't undo the damage it has already done. If the cleaner reports any of the following, have a roofer assess the roof, and see our roof repair and roof inspection pages:
- Shingles lifted, curled or loosened where moss grew between them.
- Bare patches where granules are gone, especially after an earlier pressure washing.
- Soft spots, or stains and damp in the attic, suggesting water has reached the deck.
- Cracked slates, split cedar or rusted metal hidden under the growth.
- A roof near the end of its life, where cleaning would only buy a little time.
Roof Cleaning: frequently asked questions
Can moss be pressure washed off a roof?
No. High pressure strips granules from asphalt shingles, forces water under them and can break older roofing, on top of the risk of a fall. Hire a roof cleaner who uses low-pressure methods suited to your roof.
Are black streaks on my roof a sign of damage?
Usually not on their own. Dark streaks are typically algae growing in shade and damp, and they're mostly cosmetic. Moss, lifted shingles or bare patches are the signs worth having a professional look at.
Do zinc or copper strips get rid of existing moss?
Not really. They discourage new growth below the strip once the roof is clean and work best as prevention. Thick moss still has to be removed and treated first.
Will roof cleaning products harm my garden?
They can. Ask the cleaner to name the product and show you its label, and to protect plants and control runoff by wetting or covering beds and managing the downspouts.
How often does a roof need cleaning?
There's no set schedule; it depends on shade, nearby trees and how quickly the roof dries. Look from the ground each spring and fall, and book a cleaning when moss starts to build rather than waiting for thick cushions.
Is it safe to remove roof moss myself?
No. Moss and cleaning solution make a roof slippery, and a fall can be fatal. Leave it to a crew with fall protection and insurance, and keep your own checks to the ground.
Sources
Written by the LokalMatch editorial team. Reviewed by Zubair Afzal on September 10, 2026. How we write and check our guides