Why the Cleaning Method Matters
A pressure washer can clean concrete, pavement and other durable exterior surfaces. That does not make it suitable for a roof.
High-pressure washing should not be used on asphalt shingles. The force can loosen protective granules and damage the roofing surface. Other roof systems, including metal, tile and commercial membranes, also require cleaning methods based on the material, coating, condition and manufacturer’s instructions.
Soft washing is often used for roof cleaning because it relies on low-pressure application and an appropriate cleaning solution instead of aggressive water pressure. However, “soft washing” is not one universal process for every roof. The method still needs to be selected carefully.
The Short Answer: Should You Power Wash a Roof?
Asphalt shingle roofs should not be cleaned with a power washer. The Asphalt Roofing Manufacturers Association warns that high-pressure washing is likely to damage asphalt roofing and should not be used to remove algae, dirt or other stains.
For metal, tile, slate and commercial roof systems, the answer depends on the manufacturer’s cleaning instructions and the condition of the roof. A method that is acceptable for one system may damage the coating, seams, fasteners or membrane of another.
Before any roof cleaning begins, the roof material and condition need to be confirmed.
Those Dark Roof Streaks May Not Be Dirt
Dark streaks are often described as dirt, mould or mildew without any inspection. In many cases, black or brown discolouration on asphalt shingles is caused by an airborne blue-green algae commonly known as Gloeocapsa magma.
The Asphalt Roofing Manufacturers Association explains that algae discolouration can appear as individual spots, broad dark areas or streaks running down the roof. It is particularly noticeable on lighter-coloured shingles.
Identifying the buildup matters because algae, moss, lichen, dirt and leaf debris do not behave the same way.
Algae
Algae commonly creates dark staining. According to ARMA, there is no scientific evidence that algae discolouration damages conventional asphalt shingles, although it clearly affects their appearance.
This means roof algae should not automatically be described as eating through shingles or causing roof failure. Cleaning may improve appearance, but exaggerated structural claims should be avoided.
Moss
Moss is different from algae. It grows as a thicker plant-like layer and tends to develop in damp, shaded areas.
Moss can interfere with asphalt shingles by lifting or curling their leading edges. Severe growth may increase the risk of wind damage or allow water to move sideways beneath the roofing material.
Moss should not be pulled, scraped or blasted from shingles. Aggressive removal may damage the roof before the growth has had time to loosen.
Lichen
Lichen can attach firmly to the roofing surface. Trying to force immediate removal may dislodge granules or damage the finish underneath.
Treatment may loosen the growth gradually rather than producing a completely clean appearance during the first visit.
Leaves and Debris
Leaves, branches and organic debris often collect in valleys, behind rooftop equipment and near gutters. Removing loose debris is different from treating algae or moss and may require a non-abrasive method.
Why High Pressure Is Risky for Asphalt Shingles
Asphalt shingles have a layer of mineral granules that contributes to their appearance and protects the asphalt from sunlight and weather exposure.
Direct high-pressure washing can:
- Loosen or remove surface granules
- Lift shingle edges
- Disturb already weakened shingles
- Drive water into vulnerable areas
- Damage flashing or surrounding components
- Make existing roof deterioration worse
ARMA specifically states that a power washer should not be used on asphalt roofing. Its guidance also warns against scrubbing with brushes or brooms because friction can loosen granules.
The roof may look cleaner immediately after aggressive washing, but that does not mean the correct method was used.
What Soft Washing Actually Means
Soft washing uses low-pressure application with a cleaning solution selected for the roof material and the type of organic growth. The cleaning product does most of the work, reducing the need for mechanical force.
A roof-cleaning process may include:
- Identifying the roof material and visible buildup
- Checking for obvious damage or loose components
- Reviewing access and fall-protection requirements
- Protecting landscaping and nearby building surfaces
- Applying a compatible cleaning solution at low pressure
- Allowing the required dwell time
- Rinsing according to the product and roof manufacturer’s instructions
- Reviewing the roof and surrounding property afterward
This process should not be reduced to “spray it with chemicals.” Product selection, concentration, application, runoff control and surface compatibility all matter.
Royal Wash provides soft-washing services for exterior surfaces that may not tolerate conventional pressure washing.
Roof-Cleaning Results May Take Time
A successful roof cleaning does not always mean every trace of moss or lichen disappears before the crew leaves.
ARMA notes that algae may continue to wash away with subsequent rain. Moss can loosen gradually, and severe growth may require more than one treatment.
Trying to force an instant result can damage the roofing material. This is particularly important when moss or lichen is firmly attached to asphalt shingles.
Property owners should ask what improvement to expect immediately, what may change over the following weeks and whether another assessment will be necessary.
Different Roof Systems Require Different Decisions
Soft washing is commonly associated with asphalt shingles, but no single roof-cleaning procedure should be applied to every building.
Asphalt Shingles
High-pressure washing should be avoided. The cleaning method should follow the shingle manufacturer’s instructions and recognized asphalt-roofing guidance.
Roof age, granule loss, curling, missing shingles and previous repairs should be considered before treatment.
Metal Roofing
Metal roofing can have factory-applied coatings, exposed or concealed fasteners, seams and penetrations. Cleaning products that are incompatible with the coating may cause discolouration or deterioration.
Water should not be forced into seams, flashing or rooftop openings. The roof manufacturer’s care instructions should guide the cleaning process.
Tile and Slate
Tile and slate may be durable, but individual pieces can crack or loosen. Walking on the roof can also cause damage when access is not properly planned.
The technician should inspect the surface and confirm the appropriate method before cleaning.
Flat and Low-Slope Commercial Roofs
Commercial roofs may use single-ply membranes, modified bitumen, built-up roofing or other specialized systems. These roofs also contain drains, seams, flashing, rooftop equipment and mechanical penetrations.
Cleaning should not proceed until the membrane and manufacturer requirements are identified. A generic residential soft-washing process may not be appropriate.
Roof Cleaning Is Not Roof Repair
Cleaning can remove or treat visible buildup, but it cannot correct defects in the roofing system.
Roof cleaning will not repair:
- Missing or broken shingles
- Failed flashing
- Open seams
- Membrane punctures
- Active leaks
- Wet insulation
- Damaged roof decking
- Blocked internal drainage caused by a structural problem
- Deteriorated sealants around rooftop equipment
If the inspection reveals significant deterioration, the cleaning may need to be postponed until a qualified roofing contractor evaluates the roof.
Royal Wash provides cleaning services. Structural roofing repairs, leak investigations and replacement work should be handled by an appropriate roofing professional.
Cleaning Products and Runoff Require Planning
Low pressure reduces the risk of physical surface damage, but it does not remove the need to manage cleaning products responsibly.
Before treatment, the crew should consider:
- Roof drainage and downspout locations
- Nearby plants and landscaping
- Building entrances and pedestrian areas
- Vehicles and neighbouring property
- Exterior metals, painted surfaces and glass
- Catch basins and storm drains
- The cleaning product’s label and safety information
ARMA advises protecting the building and surrounding landscaping during asphalt-shingle treatment. The City of Toronto also states that outdoor washing water containing cleaning products or dislodged contaminants must not reach catch basins. Review Toronto’s outdoor washing requirements.
Calling a cleaner biodegradable does not remove these responsibilities.
Roof Access Is a Major Safety Consideration
Roof cleaning introduces hazards that are not present during ground-level exterior washing. Roof pitch, wet surfaces, hoses, cleaning solutions, edges and changing weather can all increase the risk of a fall.
ARMA recommends that only trained professionals perform this type of work. Appropriate access equipment and fall protection depend on the building, roof and work being completed.
Property owners should not climb onto a wet roof to inspect cleaning progress or remove treated moss themselves.
What Can Help Reduce Future Buildup?
Cleaning addresses existing growth, but surrounding conditions can influence how quickly it returns.
Practical maintenance may include:
- Keeping gutters and drainage paths clear
- Removing loose roof debris without abrasive tools
- Trimming overhanging branches through a qualified contractor
- Reducing areas where leaves and moisture collect
- Monitoring shaded roof sections
- Inspecting the roof after severe weather
- Following the roofing manufacturer’s maintenance guidance
No cleaning service can guarantee that algae, moss or staining will never return. Moisture, shade, tree coverage, roof orientation and surrounding conditions continue to affect the roof after cleaning.
Questions to Ask Before Hiring a Roof-Cleaning Company
Before approving the work, ask:
- Have you identified the roofing material?
- Will high pressure be used anywhere on the roof?
- Does the cleaning method follow the manufacturer’s guidance?
- How will landscaping and surrounding surfaces be protected?
- How will cleaning solution and runoff be managed?
- What safety and access equipment will be used?
- What results should I expect immediately?
- Will moss or lichen need time to loosen?
- What happens if damaged roofing is found?
The contractor should be able to explain the process without promising instant removal, permanent results or structural repairs.
Arrange Roof Cleaning in Toronto
Roof cleaning requires a different approach from washing concrete, pavement or other hard exterior surfaces. Asphalt shingles should not be exposed to high-pressure washing, and other roof systems need methods suited to their materials and manufacturer requirements.
Royal Wash provides roof-cleaning services in Toronto for residential, commercial and multi-unit properties. Our team assesses the roof surface and visible buildup before recommending a cleaning method.
Contact Royal Wash to discuss algae, moss, staining or debris on your roof.
Frequently Asked Questions
Can asphalt shingles be pressure washed?
No. The Asphalt Roofing Manufacturers Association advises against using power washers on asphalt roofing because high pressure can damage the shingles and remove protective granules.
Are black roof streaks mould?
Not necessarily. Black or brown streaks on asphalt shingles are often caused by blue-green algae. Moss, lichen, soot and other contaminants can also cause discolouration.
Does algae damage asphalt shingles?
ARMA states that there is no scientific evidence that algae discolouration damages conventional asphalt shingles. Moss is different and can affect shingle performance when heavy growth lifts or curls the edges.
Will all moss disappear immediately after soft washing?
Not always. Treated moss may loosen gradually and weather away over time. Severe growth may require additional treatment. Trying to force it off can damage shingles.
Can roof cleaning repair a leak?
No. Cleaning does not repair failed flashing, damaged shingles, open seams, punctured membranes or other defects. A qualified roofing contractor should assess active leaks or structural concerns.
Can power washing void a roof warranty?
Warranty terms vary by manufacturer. Improper cleaning or damage caused by high-pressure washing may affect coverage. Review the specific roofing manufacturer’s instructions before cleaning.
