Every year, homeowners across Kitchener-Waterloo call contractors to clean their roofs. Black streaks, green moss, and grey lichen make an otherwise well-kept home look neglected. The instinct is to blast it away with a pressure washer. It's fast. It looks effective. And it's one of the most damaging things you can do to your roof. This guide explains exactly why pressure washing roofs causes serious harm — and why soft washing delivers better, longer-lasting results without risk.
Why Roofs Are Different from Other Surfaces
Pressure washing is a powerful and effective tool on the right surfaces — concrete, brick, metal, and sealed wood can all tolerate high-pressure water when used correctly. Roofs are fundamentally different. Understanding why requires a quick look at how asphalt shingles actually work.
Asphalt shingles are not solid plastic or stone. They are a fiberglass mat coated in asphalt and then covered with a layer of ceramic granules. Those granules are the shingle's front line of defence. They reflect UV radiation, shed water, provide fire resistance, and protect the asphalt from thermal degradation. The granules are held onto the shingle by the asphalt itself — and that bond, while designed to last 20–30 years under normal weather exposure, is not designed to withstand high-pressure mechanical force.
Beyond shingles, Ontario roofs face specific challenges that make them especially vulnerable. The freeze-thaw cycle from November through March creates micro-cracking in asphalt and grout between tiles. Any high-pressure water forced into those micro-cracks during cleaning accelerates deterioration significantly. Cedar shake roofs are even more susceptible — high pressure splits the wood grain and can buckle or fracture individual shakes.
The organisms growing on Ontario roofs are also worth understanding. The black streaks homeowners commonly mistake for dirt or mould are almost always Gloeocapsa magma, a cyanobacterium (blue-green algae) that forms dark protective sheaths. Green growth is typically algae or moss. Crusty grey, orange, or yellow patches are lichen — the most stubborn of all roof organisms, formed by a symbiotic relationship between fungi and algae. Pressure washing can physically remove the visible growth, but it cannot kill the root systems. Regrowth happens within weeks to months.
The Real Damage Pressure Washing Does to Roofs
The damage from pressure washing a roof isn't always immediately obvious — which makes it especially dangerous. Homeowners often see a clean-looking roof and assume no harm was done. The consequences appear later, and by then, the warranty is void and the damage is done.
Granule Loss
Even at relatively low pressures (800–1200 PSI), a pressure washer directed at an asphalt shingle will dislodge granules. You'll see them accumulating in gutters and downspouts after a cleaning. This isn't minor cosmetic damage — granule loss directly shortens shingle lifespan. Shingles that have lost significant granule coverage become brittle, absorb more heat, and degrade faster. A roof that should have lasted another 15 years may need replacement within 5.
Forced Water Infiltration
Shingles are designed to shed water running downward under gravity. A pressure washer sprays water upward and at angles that can force water underneath shingle edges and into the decking below. This creates exactly the moisture infiltration that the roofing system is designed to prevent. The result: wet decking, potential rot, and mould growth in the attic that may not be discovered until it's extensive.
"Pressure washing forces water underneath shingle edges — the same moisture pathway the entire roofing system is designed to block. You're undoing the most basic function of your roof."
Voided Manufacturer Warranty
This point deserves emphasis because homeowners frequently don't realize it applies to them. Most asphalt shingle manufacturers include specific language prohibiting pressure washing. GAF's limited warranty, for example, states that damage caused by "improper installation, maintenance, or repair" voids coverage. Pressure washing is specifically cited in many warranty documents as improper maintenance. If you have a 30-year architectural shingle that fails at year 15 and an inspector determines it was pressure washed, the manufacturer owes you nothing.
Physical Damage to Underlayment and Flashing
Around chimneys, vents, skylights, and valleys, roofs use metal flashing and waterproof underlayment to seal joints. High-pressure water can dislodge or bend flashing, compromise underlayment edges, and damage pipe boot seals. These failures may not leak immediately — but they create entry points that worsen over time, especially in Ontario winters when ice damming adds additional pressure to already compromised seals.
How Soft Washing Actually Cleans Your Roof
Soft washing uses chemistry rather than mechanical force to clean roofs. Instead of blasting away organisms with high pressure, soft washing applies a diluted cleaning solution that kills them at the biological level — including roots, spores, and reproductive structures. The result is a genuinely clean roof, not just a physically scraped one.
The Solution
Professional soft washing for roofs uses a sodium hypochlorite (bleach) solution diluted to the appropriate concentration for roof surfaces — typically 1.5–3% for asphalt shingles, applied with a surfactant (a specialized soap) that helps the solution cling to vertical and angled surfaces rather than immediately running off.
Sodium hypochlorite is not some exotic chemical. It's the same active ingredient in household bleach, used in concentrations safe for roofing materials when properly diluted. The ARMA (Asphalt Roofing Manufacturers Association) specifically recommends a diluted bleach solution as the approved cleaning method for asphalt shingles. Your shingles are already formulated to resist bleach degradation — it's the method endorsed by the same companies that make your shingles.
The Application Process
A professional soft wash of a residential roof in Ontario follows a consistent process:
- Site preparation: Landscaping below the roof line is pre-wetted with water. This protects plants from any solution runoff. Downspouts may be temporarily diverted if the driveway is freshly sealed.
- Solution mixing and pre-test: The cleaning solution is mixed to the appropriate concentration for the specific roof type. A small test patch on an inconspicuous area confirms no adverse reaction with staining, paint, or unusual coatings.
- Low-pressure application: The solution is applied using a dedicated soft washing pump and wide-fan nozzle at under 100 PSI — essentially a controlled stream, not a spray. The goal is even coverage with adequate dwell time.
- Dwell time: The solution is allowed to dwell on the roof surface for 15–30 minutes. During this time, the sodium hypochlorite actively kills algae, moss, lichen, and bacteria at the cellular level. The surfactant helps the solution penetrate into biological growth rather than just coating the surface.
- Rinse: A gentle low-pressure rinse removes the solution, dead organisms, and any loosened debris. In many cases, light rainfall in the following days completes the rinsing process naturally.
- Post-treatment inspection: Stubborn lichen spots may require a second application. Lichen rhizines (root-like structures) can remain anchored in shingle surfaces even after the organism dies, and may require additional dwell time or a follow-up treatment 30–60 days later.
Is Soft Washing Safe for Gardens and Landscaping?
Yes, when properly managed. The pre-wetting of surrounding plants, use of appropriate dilutions, and post-rinse procedures prevent the solution from causing plant damage. Professional soft washers understand that protecting your landscaping is part of the job. DIY attempts with improper dilutions or without pre-wetting are where plant damage occurs.
Treatment Longevity: 6 Months vs 3–5 Years
This is where the soft washing advantage becomes undeniable from a practical standpoint. Pressure washing mechanically removes visible growth — but leaves the spores, root systems, and environmental conditions that caused growth in place. Regrowth begins almost immediately after treatment. Most homeowners who pressure wash their roofs find the same organisms returning within 6–12 months, sometimes within a single growing season.
Soft washing kills the organisms at the biological level. With no living root system remaining, and with the sodium hypochlorite leaving a residual effect that inhibits re-colonization, re-growth is dramatically slowed. A professionally soft-washed roof in Ontario typically remains visibly clean for 3–5 years before retreatment is needed. In drier, less shaded areas, results can last even longer.
| Factor | Pressure Washing | Soft Washing |
|---|---|---|
| Operating pressure | 1,000–3,000+ PSI | Under 100 PSI |
| Kills organisms at root | No — mechanical removal only | Yes — chemical kill including spores |
| Granule damage risk | High — granules dislodged with each treatment | None — no mechanical force on granules |
| Warranty impact | Voids most shingle warranties | ARMA-approved — warranty preserved |
| Water infiltration risk | High — forced water under shingles | Minimal — low-pressure rinse only |
| Results longevity | 6–12 months before regrowth | 3–5 years typical |
| Effective on lichen | Partially (physical removal only) | Yes — kills rhizines and organism fully |
| Safe for cedar shake | No — splits wood grain | Yes — with appropriate dilution |
When you account for how often retreatment is needed, the economics strongly favour soft washing. Pressure washing annually at $200–$300 per treatment costs $600–$900 over three years while causing cumulative granule loss and warranty damage. A professional soft wash at $350–$600 that lasts 3–5 years costs less over the same period — and protects the roof rather than degrading it.
What to Ask Any Roof Cleaning Contractor
Not every contractor who advertises "roof cleaning" or even "soft washing" actually uses proper low-pressure techniques. Some apply a cleaning solution and then use elevated pressure to rinse — still high enough to cause granule loss. Before hiring anyone to clean your roof in Ontario, ask these specific questions.
1. What pressure do you operate at during application and rinse?
The answer should be under 100 PSI for application, and under 200 PSI for any final rinse. If the contractor mentions PSI in the hundreds or above for anything other than surface flatwork, they are not soft washing your roof regardless of what they call it.
2. What is your cleaning solution, and at what concentration?
A professional soft washer should be able to tell you they use sodium hypochlorite at a specific dilution for roofing (typically 1.5–3%) combined with a surfactant. Vague answers like "eco-friendly detergent" or "biodegradable cleaner" without specifics on active ingredients and concentrations should prompt follow-up questions.
3. What do you do to protect my landscaping?
Pre-wetting surrounding plants before application and rinsing them after should be standard procedure. Any contractor who doesn't mention this has likely caused plant damage in the past.
4. Are you insured for roof work specifically?
Working on roofs carries liability. Ask specifically about general liability and whether their policy covers roof cleaning operations. Workers' compensation coverage matters too — if a technician falls from your roof without proper coverage, the liability exposure can fall on you as the property owner.
5. Can you show me before-and-after photos from previous Ontario roofs?
Legitimate soft washing contractors have a portfolio of completed work. Photos should show the gradual clearing that's characteristic of a proper biological treatment — not a suddenly spotless roof that suggests high-pressure cleaning.
Cost Comparison: Ontario Roof Cleaning Prices
Roof cleaning pricing in the Kitchener-Waterloo region and broader Ontario market varies by roof size, pitch, organism density, and number of stories. Here's a realistic guide to what you should expect to pay for professional soft washing in 2025–2026.
| Home Size | Roof Area (approx.) | Light Growth | Heavy Moss / Lichen |
|---|---|---|---|
| Small bungalow | 1,000–1,400 sq ft | $250–$350 | $350–$500 |
| Average 2-storey (1,500–2,200 sq ft) | 1,600–2,400 sq ft | $350–$500 | $500–$700 |
| Larger home or complex rooflines | 2,500+ sq ft | $500–$650 | $700–$950 |
| Cedar shake (any size) | — | Add $50–$100 | Add $100–$200 |
These prices reflect professional soft washing using appropriate-concentration biocidal solutions, proper site protection, and experienced technicians. Quotes significantly below this range often indicate either DIY-grade equipment, incorrect dilutions, or high-pressure methods being marketed as soft washing.
What Does the Price Include?
A properly quoted roof soft washing service should include: pre-wetting and protection of landscaping, application of the cleaning solution, appropriate dwell time, low-pressure rinse, and a follow-up inspection. Some contractors offer a 12-month re-treatment guarantee — if visible growth returns within a year, they come back at no charge. This is a sign of confidence in their process and worth asking about.
Bundling with Other Exterior Cleaning
Scheduling roof soft washing alongside siding cleaning, gutter cleaning, or driveway pressure washing typically reduces the per-service cost. Contractors already have their equipment on-site, and setup and travel costs are shared. If you're budgeting for exterior cleaning before selling your home or after a harsh Ontario winter, bundling services with D&D's soft washing and roof cleaning packages can offer meaningful savings.
Key Takeaways
- Pressure washing roofs removes protective granules, forces water under shingles, and voids most manufacturer warranties — never an appropriate roof cleaning method.
- Soft washing uses under 100 PSI and a sodium hypochlorite solution that kills algae, moss, lichen, and bacteria at the biological level including root systems.
- Soft washing results last 3–5 years; pressure washing results last 6–12 months at most before regrowth.
- ARMA (Asphalt Roofing Manufacturers Association) specifically endorses diluted bleach solution as the approved roof cleaning method.
- Ask contractors specifically about operating pressure, solution concentration, and landscaping protection protocols before hiring.
- Professional soft washing for an average Ontario home costs $350–$600 and protects the roof's remaining lifespan.
