We get this question constantly: "Can I just pressure wash my roof to get rid of those black streaks?" The short answer is no — and doing so could cost you tens of thousands of dollars in premature roof replacement. Here's why.
What Happens When You Pressure Wash a Roof
Asphalt shingles are coated with ceramic granules that protect the underlying asphalt from UV radiation, rain, and wind. These granules are the shingle's armor. When you hit them with 2,500+ PSI of water pressure:
1. Granules get blasted off — You'll see them collecting in your gutters. Each granule lost is protection stripped away. 2. Shingle tabs lift and crack — High pressure can get under shingle edges and lift them, creating entry points for water. 3. Sealant strips break — The adhesive strips that bond shingle layers together can be broken by high pressure, leading to wind damage. 4. Warranty voided — Every major shingle manufacturer (GAF, Owens Corning, CertainTeed, IKO) explicitly states that high-pressure cleaning voids the warranty.
What Are Those Black Streaks Anyway?
Those dark streaks running down your roof aren't dirt, soot, or tree sap. They're Gloeocapsa magma — a cyanobacteria (blue-green algae) that feeds on the limestone filler used in asphalt shingles.
It starts as tiny spots and spreads across the roof over time, creating the signature dark streaks that run vertically down the roof face. It's more than cosmetic:
- The bacteria slowly digest the limestone, weakening the shingle structure - Dark-colored growth absorbs more heat, increasing attic temperatures and energy bills - Left untreated for years, it can reduce shingle lifespan by 5-10 years
The Right Way: Soft Washing
Professional roof cleaning uses soft washing — a method recommended by the Asphalt Roofing Manufacturers Association (ARMA). Here's how it works:
1. We apply a specialized cleaning solution at low pressure (under 100 PSI — less than a garden hose) 2. The solution kills the Gloeocapsa magma bacteria at the cellular level 3. Rain naturally washes away the dead organic matter over the following days 4. No granule loss, no shingle damage, no warranty concerns
The results are dramatic — roofs go from streaked and dark to clean and uniform, often looking decades newer.
DIY vs. Professional Roof Cleaning
Can you soft wash your own roof? Technically, yes — but we strongly advise against it for several reasons:
- Safety: Working on a wet roof is extremely dangerous. Falls from roofs account for thousands of ER visits annually. - Chemical handling: The cleaning solutions used in soft washing require proper mixing ratios and PPE. Too strong and it damages vegetation; too weak and it doesn't kill the bacteria. - Coverage: Professional equipment ensures even coverage across the entire roof surface. Consumer-grade sprayers often leave streaks and missed spots. - Landscaping protection: We pre-wet and protect all landscaping below the roofline before application. DIY attempts often kill plants and grass.
For a job that costs $250-$600 professionally, the risk-to-reward ratio of DIY doesn't make sense.
How Often Should You Clean Your Roof?
In Northwest Indiana, we recommend roof cleaning every 2 to 3 years. Our humid summers create perfect conditions for Gloeocapsa magma, and once it establishes itself, it spreads aggressively.
Homes with heavy tree coverage or north-facing roof sections may see growth return faster and benefit from more frequent cleaning. Some homeowners opt for annual treatments to keep their roof looking pristine year-round.
The Bottom Line
Never pressure wash a roof. Never let anyone else pressure wash your roof. If a company shows up with a pressure wand pointed at your shingles, send them home.
Soft washing is the only manufacturer-approved, ARMA-recommended method for cleaning asphalt, metal, tile, and cedar shake roofing. It's what we use on every roof we clean, and it's the only method that delivers lasting results without causing damage.
If your roof has black streaks, green moss, or lichen growth, get a quote for professional soft wash roof cleaning. Your roof — and your wallet — will thank you.
