If your asphalt shingle roof has developed dark streaks or patches — particularly on north-facing or shaded slopes — your roof has an algae problem. The species is called Gloeocapsa Magma, and left untreated, it shortens your roof's lifespan and eventually forces premature replacement. Soft washing is the industry-approved solution.
What Are Those Black Streaks on Your Roof?
The dark streaks, patches, and staining that develop on asphalt shingles — especially in Ontario's humid climate — are caused by a cyanobacteria called Gloeocapsa Magma. This organism feeds on the crushed limestone used as filler in asphalt shingles. The dark colour comes from a UV-protective pigment the bacteria produces to shield itself from sunlight.
The bacteria spread by spores carried on wind and birds. Once established on one section of roof, it gradually spreads across the entire surface — typically taking 3–5 years to become widely visible. Homes near trees and in shaded areas experience faster colonization.
Soft washing removes algae staining from shingles without stripping protective granules.
The Damage They Cause
This is more than a cosmetic problem. Gloeocapsa Magma:
- Feeds on limestone filler, gradually degrading shingle composition
- Retains moisture against the shingle surface, accelerating granule loss
- Increases moisture retention under shingles, contributing to wood deck rot
- Significantly increases solar heat absorption (dark colour), increasing cooling costs by up to 20%
- Reduces the reflective capacity of lighter-coloured shingles
A shingle manufacturer study found that algae-colonized shingles deteriorate up to 40% faster than clean shingles. In a region with Ontario's humidity and temperature swings, this translates to meaningful reduction in actual shingle lifespan.
"Roof algae isn't just ugly — it's actively degrading your most expensive exterior asset. Treatment costs a fraction of premature replacement."
— D&D Home Services Roof Cleaning Team
The Soft Wash Method for Roofs
Professional roof soft washing involves applying a low-pressure mixture of diluted sodium hypochlorite and a biodegradable surfactant to the roof surface. The chemistry kills the algae at the cellular level — not just bleaching it white temporarily. The dead organisms then break down and are washed away by rain over the following weeks.
The process:
- Pre-wet all landscaping below the roof perimeter
- Apply soft wash solution at low pressure (100–200 PSI — comparable to a garden hose)
- Allow the solution to dwell 10–15 minutes for full penetration
- Rinse with low-pressure water
- Flush all landscaping and hard surfaces with fresh water
Results are visible immediately — streaks lighten and often disappear during the treatment. Full brightening occurs over 1–4 weeks as dead biological material weathers away.
Why You Cannot Pressure Wash Asphalt Shingles
This is an important distinction. Pressure washing at high PSI:
- Strips protective granules from shingles — equivalent to 5–10 years of normal weathering in a single pass
- Voids most shingle manufacturer warranties immediately
- Can dislodge fastener seals and cause shingle lifting
- Doesn't kill algae — it temporarily removes it, leading to rapid regrowth
The ARMA (Asphalt Roofing Manufacturers Association) specifically recommends against pressure washing asphalt shingles. Soft washing is the only ARMA-compliant method for removing biological growth from shingle roofs.
Important: If a contractor quotes you roof cleaning and mentions using a pressure washer, find a different contractor. This is a clear indicator of inexperience or indifference to manufacturer guidelines and your roof warranty.
How Long Do Results Last?
Soft washing results on asphalt shingles typically last 2–4 years. Factors affecting longevity:
- Tree coverage: Heavy shade accelerates algae regrowth
- Roof pitch: Steeper roofs shed water faster, which slows regrowth
- Algae-resistant shingles: Some modern shingles contain copper granules that inhibit algae — these need less frequent treatment
- Whether a biocide inhibitor was applied post-cleaning
When to Clean Your Roof
Best times in the Ontario climate: Late spring or early fall, when temperatures are moderate and rain is expected within a few days (rain helps rinse away dead biological material). Avoid mid-summer cleaning on very hot days — the solution evaporates too quickly for full effectiveness.
What Does Professional Roof Soft Washing Cost?
Typical Roof Soft Washing Costs
- Small residential roof (1,000 sq ft): $350–$500
- Average residential roof (1,500–2,000 sq ft): $450–$750
- Large estate roof (3,000+ sq ft): $700–$1,200+
- Add-on biocide inhibitor treatment: +$50–$100
- Compare to: Premature roof replacement = $8,000–$20,000+
