Dark streaks on a roof can be unsettling, but the right first step is identification—not a pressure washer. Roof materials and stains differ, and an aggressive cleaning method can cause damage. Here are a few practical ways to think about roof care before you book or begin a cleaning project.
What are those dark streaks?
Dark streaks on asphalt shingles may be algae discoloration. The Asphalt Roofing Manufacturers Association (ARMA), in a technical bulletin revised June 2024, notes that algae discoloration can look like soot, dirt, moss, or tree droppings. Moss and debris are different conditions, so a visual check from the ground may not tell the whole story. If you are unsure what you are seeing, ask a roofing professional to assess it.
Why pressure washing is the wrong choice for asphalt shingles
ARMA advises against using a power washer on asphalt shingles because high-pressure washing can damage the roofing surface. GAF gives the same warning in its technical bulletin on blue-green algae staining: power-washing can dislodge protective granules. For asphalt shingles, don’t assume a method used on concrete or siding is appropriate for the roof.
Cleaning guidance can differ by shingle product and manufacturer. Before any treatment, identify the roofing material and check the manufacturer’s current instructions. ARMA says other chemicals or methods should not be used on asphalt roofing without the shingle manufacturer’s approval. Avoid improvised mixes and follow product labels exactly.
Roof access and safety come first
Wet roofs can become dangerously slippery. ARMA recommends trained professionals for roof cleaning and notes that ladder or walkway methods can reduce direct contact with the roof surface where appropriate. OSHA’s residential-roof guidance says employers must use fall-protection systems at unprotected roof edges four feet or more above a lower level, subject to the applicable rule and conditions. Homeowners should not climb onto a roof to inspect or clean it if they lack the training and safety equipment.
A homeowner’s roof-care checklist
- Look from the ground for streaks, moss, missing shingles, sagging, and debris.
- Keep gutters clear from a safe position or hire a qualified professional.
- Trim back branches only when it can be done safely; avoid roof access for routine inspection.
- Ask what the roof material is and what the manufacturer recommends before authorizing a cleaning method.
- Confirm how the contractor plans to protect nearby landscaping and manage runoff.
- Never accept pressure washing as the default treatment for asphalt shingles.
Good roof care is careful roof care
Every roof is different. A careful assessment, manufacturer-based guidance, and a safe work plan matter more than a one-size-fits-all cleaning recipe. The goal is to address the concern while respecting the roofing material and keeping people safe.
Looking ahead to Spring 2027
United We Shine is preparing to offer roof care along with house siding and soft washing, concrete and driveway cleaning, fleet and truck washing, vehicle detailing, and window washing. Our full-service launch is planned for Spring 2027, and online booking is coming soon. To leave a message or ask about a future estimate, visit our estimate and contact form.
Thanks for following our launch journey. We’ll keep sharing practical exterior-care information as we get ready to serve the community.
— Robert E. Allen Jr.
Sources
- Asphalt Roofing Manufacturers Association, “Algae Discoloration of Roofs,” revised June 2024.
- GAF, “Blue-Green Algae Staining on Shingled Roof Surfaces,” Technical Advisory Bulletin R-102.
- OSHA, Residential roof fall-protection guidance.
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