That green fuzz creeping across your shingles isn't just ugly — it's actively shortening your roof's life. Moss, lichen, and algae are among the most damaging biological threats to Ontario roofs, and they thrive in our humid climate on north-facing slopes under tree canopy. Here's what they're doing to your roof, why only soft washing can safely remove them, and how to prevent rapid regrowth.
How Moss Damages Shingles
Moss looks relatively harmless — it's soft, green, and almost decorative. The damage it causes is structural and progressive:
Root penetration: Moss develops rhizoids (root-like structures) that physically penetrate between shingle layers and into the shingle surface. These rhizoids anchor the moss and draw water from the shingle, but more importantly, they pry shingle edges upward as the moss colony grows. Lifted shingle edges allow water to infiltrate under the shingle course — directly onto the ice and water shield or the sheathing beneath.
Moisture retention: Moss acts as a sponge. A dense moss colony can hold many times its weight in water. This moisture sits against the shingle surface continuously, accelerating the breakdown of the asphalt layer and the granules embedded in it. In Ontario winters, this retained moisture freezes and expands in micro-cracks in the shingle, breaking down the shingle structure from within.
Weight accumulation: Dense, wet moss can add meaningful weight to a roof. While most residential roofs handle this easily, aged or structurally marginal roof decking may be affected, and the weight concentrates stress on already-aging shingles.
Granule loss: As moss roots penetrate and then the moss is removed (mechanically or chemically), they take shingle granules with them. Granules protect the asphalt layer from UV degradation and provide fire resistance. Their loss accelerates the aging of the underlying asphalt compound dramatically.
Lichen vs. Moss vs. Algae: What's Actually on Your Roof
These three types of biological growth look similar from the ground but behave differently and require different treatment approaches:
| Type | Appearance | Damage Level | Treatment Difficulty |
|---|---|---|---|
| Algae (Gloeocapsa magma) | Black streaks running down the roof | Moderate | Easy — responds to soft washing |
| Moss | Green, cushiony growth (esp. at shingle edges) | High | Moderate — may need re-treatment |
| Lichen | Grey-green crusty, flat patches bonded to shingles | Highest | Difficult — chemically bonded to shingle |
Algae (Gloeocapsa magma) is the source of the black streaks that appear on Ontario roofs, often running vertically from where debris accumulates. It's the least structurally damaging of the three but still accelerates shingle degradation and absorbs heat (dark colouring increases roof surface temperature). Very common in Kitchener-Waterloo on all roof aspects.
Moss is visible as actual plant growth, with the characteristic cushiony green appearance. It's most commonly found on north-facing slopes, under heavy tree canopy, and in valleys where organic debris accumulates and moisture lingers. Moss causes the most active structural damage of the three through root penetration.
Lichen is a symbiotic organism — part fungus, part algae — and is the hardest to remove. It forms a crusty, flat coating that is chemically bonded to the shingle granules. It doesn't respond to a single soft washing treatment the way algae and moss do — lichen requires treatment and then time for the dead organism to release from the shingle surface. Multiple treatments may be required over weeks.
Why You Should Never Pressure Wash a Roof
This is not a cautionary suggestion — it is a hard rule, and violating it has serious consequences:
Granule removal: Asphalt shingles are coated with mineral granules that protect the asphalt compound from UV radiation and provide fire resistance. These granules are pressed into the asphalt surface but are not permanently bonded — they can be dislodged by mechanical force. Pressure washing, even at relatively low pressures, blasts granules off the shingle surface into the gutter and downspout system. You can see this by checking your gutters after pressure washing — they'll be full of granules. Each granule removed permanently reduces the lifespan of the shingle.
Shingle lifting: Pressure washing from below (up the slope) lifts shingle edges, breaking the seal strip that bonds each shingle course to the one below. These broken seals allow wind-driven rain infiltration and are especially problematic in Ontario's stormy spring and fall weather. Once seal strips are broken, they rarely re-seal properly, particularly on older shingles.
Warranty voiding: Most major shingle manufacturers (GAF, CertainTeed, IKO, BP) explicitly state in their warranty documentation that pressure washing voids the product warranty. If you have a shingle warranty remaining on your roof, pressure washing eliminates it immediately. This has real financial consequences when warranty claims arise.
Falls: Walking on wet shingles while operating a pressure washer is extremely dangerous. Roofs are the second-most common location for fatal residential falls. The combination of slippery wet surfaces and pressure washer recoil creates serious fall risk.
Pro Tip: Check your shingle packaging or manufacturer's website for the warranty document. Search for "cleaning" or "washing" in the document. You'll almost certainly find language that prohibits pressure washing. Keep this in mind when evaluating quotes from cleaning companies — a company that recommends pressure washing for your roof is demonstrating a fundamental lack of knowledge about roofing.
Soft Washing Process for Roof Moss and Algae
Soft washing roof moss and algae is the industry-standard safe method recommended by shingle manufacturers and roofing industry organizations. The process:
- Safety setup: Professional roof soft washing requires proper fall protection (harness, anchor points) or equipment that allows the roof to be treated from the ground or from a safe position on the ladder at the eave. Never walk on wet shingles.
- Pre-wet vegetation: All surrounding plantings, grass, and garden beds are thoroughly pre-wetted before solution application. Sodium hypochlorite is effective against plants as well as organisms, and runoff must be managed.
- Apply sodium hypochlorite solution: A solution of sodium hypochlorite (typically 3–6% concentration for roofs) combined with a surfactant is applied to the roof surface using a low-pressure pump. The surfactant helps the solution cling to the shingle surface on the slope rather than immediately running off. Application is typically done from the ridge down, covering all affected areas.
- Dwell time: This is critical and often underestimated. The solution is left on the roof for 30–60 minutes (or sometimes longer for heavy lichen infestations). During this time, the bleach solution is penetrating the cells of the moss, algae, and lichen organisms and killing them. You'll often see colour changes as organisms die during the dwell time.
- Light rinse: After the dwell period, a very light rinse removes the cleaning solution and the surface layer of dead organic material. Pressure stays extremely low — essentially a gentle flow rather than any meaningful pressure. Many professionals allow rain to do the final rinse rather than introducing any additional water pressure to the shingle surface.
- Post-rinse vegetation flush: All surrounding vegetation is thoroughly rinsed with clean water after the roof treatment is complete.
For heavy moss: the moss may not come completely off the roof surface immediately after treatment. This is normal and expected. The treatment kills the moss at the root; the dead plant material then gradually releases from the shingle over the following weeks with rain and wind. After 4–8 weeks, the roof should appear significantly cleaner as the dead material is washed away naturally.
For lichen: multiple treatments may be required over several weeks or months. Lichen's chemical bond to shingle granules requires extended treatment time to fully break down. Results are often progressive rather than immediate.
"The test we use: after a soft wash, check the gutters. After pressure washing, you'll find granules. After soft washing, no granules. That's the difference between cleaning the roof and slowly destroying it."
— David, D&D Home Services Co-Founder
Zinc Strips for Prevention: The Long Game
After treating your roof, the most effective long-term prevention strategy is installing zinc or copper strips near the ridge cap. This is a well-established technique that works through simple chemistry:
When rain flows over zinc or copper, it picks up trace amounts of the metal in solution. These metal ions in the runoff are toxic to algae, moss, and lichen at the very low concentrations carried by rainwater. As rain flows down the roof surface, it continuously deposits these trace metal ions, creating an inhospitable environment for biological growth.
Installation: Zinc strip (sold as "Z-stop" or similar products) is nailed or clipped under the first few rows of shingles below the ridge cap. One strip per side (north and south) on the ridge is typically sufficient for a standard residential roof. The strip is hidden under the shingle course and only slightly visible at the eave edge.
Effectiveness: Zinc strips extend the time between necessary roof treatments from 1–2 years to 3–5+ years on average in Ontario's climate. They're particularly effective at preventing algae — the most common and first-arriving biological colonizer on roofs.
Copper strips perform even better than zinc but are more expensive. Many premium shingle products now incorporate copper granules specifically for this reason.
Ontario Climate Factors and North-Facing Roofs
Ontario's climate creates specific patterns of moss, algae, and lichen growth that homeowners should understand:
North-facing slopes are always worse. In Kitchener-Waterloo's climate, north-facing roof slopes receive significantly less direct sunlight and dry out more slowly after rain and dew events. This persistently moist microenvironment is ideal for all types of biological growth. North-facing slopes may require treatment twice as often as south-facing slopes on the same house.
Overhanging trees accelerate growth dramatically. Trees provide shade (reducing drying) and deposit organic material (leaves, needles, seeds, pollen) that provides nutrients for biological colonizers. Roof sections under significant tree canopy may show growth within a year of treatment without preventive measures.
Valleys accumulate organic debris. The valleys where two roof slopes meet collect leaves, needles, and other organic matter that decomposes and provides an ideal growing medium. Moss growth in valleys often starts here and spreads.
Wet falls and springs trigger growth cycles. September through November and March through May are the highest-risk periods for new biological colonization in Ontario's climate — cool, moist weather without strong UV sunlight. Late summer soft washing takes advantage of the dry conditions while getting ahead of fall's biological activity.
Why Roof Cleaning is a Professional Job
Unlike most exterior cleaning tasks where a capable DIY homeowner can achieve good results with proper knowledge, roof cleaning is almost always better left to professionals for several reasons:
- Fall risk: Roof heights combined with wet surfaces and chemical handling create significant fall hazard. Professional cleaners use proper fall protection systems.
- Chemical handling: Concentrated sodium hypochlorite (pool-grade bleach at 10–12%) is the working stock for roof cleaning solutions. Handling, diluting, and applying this chemical safely requires knowledge and equipment most homeowners don't have.
- Solution ratios: The right concentration for your specific roof situation (severity of growth, shingle age, roof pitch, aspect) requires experience to calibrate correctly. Too weak and the treatment is ineffective; too strong and you risk bleaching dark shingles or over-treating vegetation.
- Vegetation protection: Managing runoff to protect mature trees, valuable shrubs, and garden beds requires preparation and technique developed through experience.
- Equipment: Low-pressure chemical application systems with appropriate chemical lines and pumps are specialized equipment not readily available for consumer rental.
Our professional soft washing service covers roof treatments throughout Kitchener, Waterloo, Cambridge, and Guelph. We use the correct sodium hypochlorite concentrations for your shingle type, protect your landscaping, and take every safety precaution required for safe roof work.
Key Takeaways
- ✓ Moss root penetration lifts shingles and retains moisture — causing accelerated deterioration in Ontario winters
- ✓ Lichen is chemically bonded to shingle granules and requires multiple soft washing treatments
- ✓ Pressure washing roofs is never acceptable — it removes granules and voids manufacturer warranties
- ✓ Soft washing with sodium hypochlorite kills moss at the root for lasting results (3–5 years)
- ✓ Zinc strips below the ridge cap extend treatment intervals by creating a metal ion barrier
- ✓ North-facing slopes under tree canopy need more frequent treatment in Ontario's climate
