Roofs and siding collect growth that looks alarming from the street: dark streaks, green fuzz, patchy grey film. The instinct is to hit it with a pressure washer, yet the people who make shingles and siding tend to say the opposite. This page explains what the stains are, why gentler methods are favoured for these materials, how low-pressure cleaning with a solution works at a general level, and what the public safety agencies say about working on ladders and roofs. For the wider picture of methods and equipment, see our overview of pressure washing and soft washing.
What the black streaks are
On many asphalt roofs the dark streaking is algae. The Asphalt Roofing Manufacturers Association (ARMA) says algae species such as Gloeocapsa magma are the most common contributors, that they travel through the air and collect on roofs, and that their natural pigments can turn light roofs brown or black, uniformly or in streaks. The condition is most common in the Gulf States and along the Northwest and Eastern Seaboards, especially in warm, humid conditions, and it is more visible on light-colored roofs. ARMA adds that it is often mistaken for fungus, soot, dirt, moss or tree droppings (Asphalt Roofing Manufacturers Association).
Moss is a separate problem. Oregon State University Extension describes moss as a shade-loving growth that holds soil, debris and moisture, which encourages fungi and shortens roof life, and says that on asphalt and metal roofs it can lift shingles and let water reach underlying layers. It does not harm the roofing material directly, and the main long-term fix is trimming nearby trees so sunlight reaches the roof: "You've got to let the sun in or it will return" (Oregon State University Extension).
Why high pressure is discouraged on shingles
Asphalt shingles carry a layer of mineral granules that protects the surface from weather. Force can dislodge them. ARMA's guidance is blunt: do not use a power washer or any type of brush or broom to clean algae from the roof surface, because high-pressure washing is likely to damage asphalt roofing and should not be used for algae or other stains (Asphalt Roofing Manufacturers Association, above).
Not every source is quite so absolute. Oregon State Extension says experienced users may use a pressure washer on a roof, but that poor technique can damage shingles, and it suggests a professional or a long-handled scrub brush if you are unsure. ARMA's position is the stricter one and comes from the shingle makers themselves, so it is the more careful reading, and the shingle manufacturer's own care instructions should settle any specific case. Anyone commissioning roof work can ask whether the proposed method matches those instructions, one of the questions in our guide to choosing and vetting a cleaning company.
How low-pressure cleaning with a solution works
The idea is to let a solution do what force would otherwise do. ARMA recommends a solution of liquid household bleach (sodium hypochlorite) and water, with mixing directions that may vary by shingle manufacturer and the amount of discoloration. The solution is dispersed gently, without scrubbing because friction may loosen granules, left on for at least 15 minutes but no more than 20, and then rinsed gently with water. ARMA also says the results are temporary and discoloration may return, and that no other chemicals or methods should be used without the shingle manufacturer's approval.
Oregon State Extension describes alternatives for moss, which are not cleaners for algae streaks. Zinc sulfate is applied with a pump sprayer, and the page warns against it if the roof has copper gutters, downspouts or flashing because zinc corrodes copper. Potassium salts of fatty acids are soap-based, biodegradable and noncorrosive and kill moss by breaking down cell membranes. The extension page advises cleaning the roof before applying either treatment and following all product label instructions.
Two practical points follow from ARMA's wording. First, the cleaning is chemical and gentle, so the contact time and the rinse matter more than the water pressure. Second, work is done from a ladder or walkboards when possible, to avoid direct contact with the roof, with landscaping and nearby building parts protected. For a longer-term view, ARMA suggests considering algae-resistant shingles when a roof is replaced, which it says are designed to inhibit algae growth for extended periods.
Runoff and plant protection
Whatever runs off a roof lands on gutters, beds and paving. ARMA advises protecting nearby landscaping, and Oregon State Extension says to avoid spraying nearby plants with either moss treatment, rinse any plants that get exposed and clean tools afterward. Our overview explains how wash water is treated under stormwater rules. The key point is that a cleaning solution on a roof does not stay on the roof. Where it goes depends on gutters, downspouts and the ground below, and local stormwater rules vary.
Siding care, with vinyl as the worked example
The Polymeric Exterior Products Association (the vinyl siding trade body) says a power washer can be used on vinyl siding, but with caution. The wand should be held level, because spray angled upward can force water behind the siding panels. Some manufacturers advise against power washing entirely, and others set pressure limits and restrict which cleaners can be used, so the installer or manufacturer instructions should be read first. The association also advises caution around windows, doors, electrical wiring and plumbing, recommends a soft cloth or a long-handled soft-bristle brush, and lists solvent-based cleaners, undiluted chlorine bleach, liquid grease remover, nail polish remover and furniture polish among products that can damage the surface (Polymeric Exterior Products Association).
We did not open an association or extension page with specific cleaning guidance for fiber-cement or wood siding, so we do not make claims about them. The general pattern holds across materials: read the manufacturer's instructions, test a small hidden area, and prefer the gentlest method that works.
Working at height safely
Roof cleaning is dangerous work. ARMA says cleaning can make the roof slippery and hazardous to walk on, and recommends that only trained professionals walk, climb or work on roofs while following government safety regulations and manufacturer instructions. It also advises eye protection and rubber gloves when handling bleach.
OSHA's fact sheet on re-roofing notes that more than one-third of fall deaths in residential construction are caused by falls from roofs, and lists surfaces that become slick when wet among the unpredictable fall hazards. It says employers must provide fall protection equipment that meets OSHA requirements whenever workers are 6 feet or more above a lower level, and describes a personal fall arrest system as an anchorage, a full body harness and a connector such as a lanyard or lifeline. The document prohibits body belts in fall arrest systems. It is written for roofers on construction sites, but it shows what professional fall protection looks like (OSHA: Reducing Falls During Residential Construction, Re-Roofing).
For ladders, NIOSH reports that more than 100 people die each year in the United States from ladder-related falls and recommends three points of contact, a level and firm base, an angle of about 75 degrees and a ladder that extends at least three feet above the roof edge (NIOSH: Ladder Safety). Oregon State Extension adds that tools should be raised with a bucket and rope instead of carried up the ladder, and that safety belts and lines are advised on steep roofs, near edges or where eaves exceed 16 feet. Our guide to pressure washing safety covers the equipment hazards that come on top of the fall risk.
Frequently asked questions
Are black roof streaks harmful to the house?
The sources we opened describe algae as a cause of discoloration, and we found no statement in them that algae structurally damage shingles. Moss is different, because Oregon State Extension says it holds moisture and can lift shingles. Checking what is actually growing matters before choosing a response.
Will the streaks come back after cleaning?
ARMA says results are temporary and discoloration may return. The cause is airborne algae in warm, humid conditions, plus shade, so cleaning does not change the conditions that allowed the growth. Algae-resistant shingles at replacement are ARMA's longer-term suggestion.
Is a gentle rinse the same as soft washing?
Not exactly. In the sources, the low-pressure approach pairs a cleaning solution applied gently with a set contact time and a light rinse. The solution does the work, and the label or the shingle maker's instructions decide the details.
The short version
Shingle makers advise against power washing algae off asphalt roofs and favour a gentle application of a bleach and water solution with a timed contact and a light rinse, while vinyl siding trade guidance asks for a level wand, manufacturer limits and care around openings. Moss needs shade control as much as treatment. Whatever the material, the height, wet surfaces and chemicals make this work for trained people with fall protection, and manufacturer instructions come first.