The Complete Guide to Soft Washing a Toronto Roof: Moss, Algae and Black Streaks
How soft washing removes moss, algae and Gloeocapsa magma black streaks from Toronto roofs without pressure, protecting shingles, gutters and gardens.
If you live under Toronto’s tree canopy, you have probably seen it: dark streaks running down the roof, green cushions of moss along the bottom edge, and maybe grey crusty patches near the vents. It looks cosmetic, but it shortens the life of your roof. The good news is that it can be removed safely, without a single blast of high pressure.
This guide covers what is growing on your roof, why pressure washing is the wrong fix, and how a proper soft wash works from start to finish.
What Causes Moss, Algae and Black Streaks?
Three organisms do most of the damage on Toronto roofs:
- Gloeocapsa magma, a type of algae, causes the black or dark grey streaks. It feeds on the limestone filler in modern asphalt shingles and spreads on the wind from roof to roof.
- Moss forms green, spongy clumps, usually on shaded slopes and along the lower edges where water lingers.
- Lichen shows up as flat, grey-green crusty patches. It grips the surface tightly and is the slowest to clear.
All three love shade and moisture. Neighbourhoods with mature trees, such as Rosedale, Forest Hill, Hoggs Hollow and Unionville, see the worst of it, especially on north-facing slopes. Lake humidity in The Beaches and wooded lots in Richmond Hill add to the problem.
Moss holds water against the shingles and can lift tab edges, which lets wind-driven rain in. Algae and lichen shorten shingle life and make a well-kept house look neglected.
Why Pressure Washing a Roof Is the Wrong Fix
Asphalt shingles are coated with ceramic granules that protect the asphalt underneath from UV light. A pressure washer strips those granules off. It also drives water up under the tabs and into the roof deck. One afternoon with a pressure washer can take years off a roof.
Scraping moss by hand has similar problems: it tears tabs and dislodges granules. Many shingle manufacturers specifically recommend low-pressure cleaning with a diluted sodium hypochlorite solution instead.
Short version
Never point a pressure washer at a shingle roof. Kill the growth with chemistry and let rain do the rest.
How Roof Soft Washing Works
Soft washing uses our SoftWash low-pressure line, which mixes diluted sodium hypochlorite with a surfactant at a controlled ratio. The surfactant helps the solution cling to the roof long enough to work. Pressure stays close to garden-hose levels.
1. Inspection and protection
We check the shingle type and condition from the ground and ladder, note any loose or damaged tabs, and look at gutters and downspouts. Then we protect the property.
2. Plant pre-wet
Gardens, shrubs and lawn edges below the roof are soaked with clean water. Wet foliage absorbs far less of any overspray. Pets stay indoors.
3. Application and dwell time
The solution is applied from the bottom up at low pressure and left to dwell. Algae streaks begin to lighten within minutes. Moss and lichen are killed on contact.
4. Gutter rinse and plant rinse
Gutters and downspouts are flushed so runoff is diluted, and plants are rinsed again. We work on calm, dry days to keep overspray on the roof.
5. Results over the following weeks
Streaks usually clear the same day. Dead moss releases over the next few rains instead of being scraped off, which is gentler on the shingles. Lichen may take a little longer.
Cost and Timeline
| Roof | Typical price | Typical time |
|---|---|---|
| Bungalow or small roof | $450-$550 | About 2 hours |
| Two-storey home | $550-$750 | 2-3 hours |
| Large or heavily mossed roof | $750-$850 | 3-4 hours |
Very large estate roofs and steep pitches are quoted individually. Full details are on our roof cleaning page.
Keeping the Roof Clean Longer
- Trim overhanging branches to let in more sun and air.
- Keep gutters clear so water does not sit along the eaves.
- Book a soft wash before growth gets thick, which is faster and cheaper.
In winter, roofs face a different problem: ice dams along the eaves. Our ice dam removal service uses a low-pressure steam rig, never hammers or pressure.
Soft Washing Beyond the Roof
The same chemistry cleans vinyl siding, stucco and heritage masonry. See our soft washing service, or read power washing vs pressure washing vs soft washing to understand which method suits each surface. For the driveway below the roof, our guide on removing road salt stains covers hot-water cleaning.
We soft wash roofs as part of our residential and commercial exterior cleaning across the GTA, including the wooded Oak Ridges and Mill Pond neighbourhoods served by our Richmond Hill crew.
Want those black streaks gone without risking your shingles? Request a free written quote and send a photo of your roof.
