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Roof Cleaning Guide
Why Your Roof Has Black StreaksAnd why pressure washing it is one of the worst things you can do. A no-nonsense guide for Bay Area homeowners who want a clean roof without trashing it. Get a Free Roof Inspection
5.0 Stars | 156+ Google Reviews | $2M Insured | Soft Wash Certified
Soft washing protects your roof warranty. High pressure voids it. Here is why that matters.
Drive through any Peninsula neighborhood in spring and you will see them - those ugly black streaks running down the north and shaded sides of asphalt shingle roofs. Most homeowners assume it is dirt, soot, or aging shingles. It is none of those. It is a living organism. And the way most people try to clean it off is exactly what guarantees it comes back, kills your shingles early, and voids your warranty in the process. What the Black Streaks Actually AreThe technical name is gloeocapsa magma. It is a cyanobacteria - a type of algae that travels by air and feeds on the limestone filler inside asphalt shingles. The Bay Area's marine layer, fog, and shaded north-facing roofs create the perfect breeding ground. Once it lands, it digs in. The black streaks you see are colonies of bacteria with a UV-protective dark pigment. As they eat the limestone, they hold moisture against your shingles, which accelerates granule loss and shortens roof lifespan by years. Translation: it is not just ugly. It is actively shortening the life of your roof. Why High Pressure Wrecks Your RoofIf you Google "roof pressure washing" you will find a thousand cheap operators willing to do it. Here is what they will not tell you. It Strips the GranulesAsphalt shingles are protected by a layer of mineral granules. High pressure blasts them off. You will see piles of them in your gutters within hours. Those granules will not grow back. It Voids the WarrantyEvery major shingle manufacturer (GAF, CertainTeed, Owens Corning) explicitly forbids high-pressure cleaning. They specify low-pressure soft washing only. Use the wrong method and your warranty is gone. It Drives Water Up Under ShinglesPressure washing typically goes from the bottom up, which forces water beneath the shingle overlap. That water leads straight to the decking, the underlayment, and eventually your ceiling. It Does Not Kill the AlgaeEven if you blast the streaks off, the bacteria is microscopic. The roots stay. Within months the streaks return - now on a damaged roof. Tile Roofs CrackConcrete and clay tile roofs are common in the Bay Area. They look bulletproof but are surprisingly brittle. Pressure washing them causes hairline cracks you will not see until the next leak. Walking on a Wet RoofMost pressure washing crews are not trained to walk a steep, wet, mossy roof safely. The injury risk is real. Hire someone with the right gear, training, and insurance. The Right Way: Soft WashingSoft washing is the manufacturer-approved method for cleaning a roof. Instead of blasting, we apply a low-pressure (under 100 PSI - basically garden hose pressure) sodium hypochlorite solution that kills the bacteria at its roots. The cleaner does the work. Within 15 to 30 minutes the streaks are visibly fading. We rinse, neutralize the runoff to protect your plants, and walk away with a roof that looks new - and stays clean for three to five years. The short version: chemistry kills algae. Pressure does not. Pressure damages the surface. Use the right tool for the job. How Often Do Bay Area Roofs Need Cleaning?Most Peninsula and South Bay homes need a soft wash every three to five years. Houses near the coast, under heavy tree cover, or with significant north-facing exposure are on the shorter end. If you can see streaks from the street, it is time. If you cannot remember the last time it was cleaned, it is time. A single soft wash can add five to ten years of useful life back to a shingle roof - that is real money against a $20,000 to $40,000 reroof. Get a Free Roof InspectionWe will walk your property, photograph the trouble spots, and give you an honest read - even if the answer is "your roof does not need cleaning yet." We have been doing this in the Bay Area since 2017 and have a 5.0 star rating across 156+ Google reviews to prove the approach works. Call (650) 248-0727 Book OnlineServing San Mateo, Palo Alto, Atherton, Hillsborough, Burlingame, Belmont, San Carlos, Foster City, Redwood City, Menlo Park, and the entire Peninsula and South Bay.
Miller's Pressure Washing | Licensed and Insured | $2M General Liability | 9 Years Serving the Bay Area | 156+ Five-Star Google Reviews
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Bay Area Homeowner Guide
The Spring Pressure Washing Checklist for Bay Area HomesThe winter rains are gone. Now is the moment to clean off the moss, mildew, and dirt before peak entertaining season. Here is exactly what to tackle, in what order, and why. Get a Free Onsite Quote
5.0 Stars | 156+ Google Reviews | $2M Insured | Serving the Peninsula and South Bay since 2017
A field guide written by the Miller's crew - not generic internet advice
If you live anywhere from San Francisco to San Jose, your home spent the last six months getting battered by Pacific moisture, salt air, pollen, and tree debris. By May, the damage is visible: green streaks down the siding, black roof stains, slick decks, and driveways that have gone two shades darker than they should be. The fix is straightforward. The trick is doing it in the right order so you do not undo your own work. Here is the checklist we walk every Bay Area homeowner through this time of year. The Six Surfaces That Need Attention in MayTackle them top-down so debris and runoff flow toward what gets cleaned last.
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The RoofThose black streaks on north-facing roofs are not dirt. They are gloeocapsa magma, a cyanobacteria that feeds on the limestone in asphalt shingles and shortens roof lifespan. Bay Area marine layer creates ideal conditions for it. What to do: Soft washing only. Never high pressure on shingles or tile. A licensed contractor uses a low-pressure sodium hypochlorite solution that kills the bacteria at the root and rinses safely off the roof. Skip this step and the streaks return within months.
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The GuttersEucalyptus, oak, and redwood debris turns into a soggy mat that holds water against fascia boards. Left alone through one more rainy season, that water rots wood and stains the gutter exterior with permanent black tiger stripes. What to do: Hand-clean the troughs first, then exterior gutter brightening with a specialty cleaner. May is the right window because the trees have finished their spring drop and the first foggy mornings will not refill them for weeks.
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The SidingStucco, fiber cement, wood, and vinyl all collect what we call the green tax - mildew on the shaded side and dust on the sunny side. Look closely at the north and west elevations of your home. If you see anything that looks like green chalk, that is mildew, and pressure alone will not kill it. What to do: Soft wash with a mild biocide, then a low-pressure rinse. High pressure on stucco blasts off the texture coat. High pressure on wood siding drives water behind the boards. Always go gentle.
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The Deck or PatioBay Area decks pick up a slick black film every winter. It is not just dirt - it is a mix of mildew, pollen, and tannins from overhead trees. By May this layer is genuinely slippery and dangerous, especially around hot tubs and pool surrounds. What to do: Wood decks need a wood-safe brightener and the gentlest pressure that still cleans. Composite decks need a manufacturer-approved cleaner - most warranties forbid bleach. Stone, paver, and concrete patios can handle hot water and surface cleaning. Get this done before Memorial Day if you plan to entertain.
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The Driveway and WalkwaysConcrete is porous. Every oil drip, leaf tannin stain, and rubber tire mark from the past year is sitting in the surface, waiting for warm sun to bake it permanent. Pavers and stamped concrete add a second problem: organic growth in the joints. What to do: Hot water surface cleaning with a rotating attachment. This delivers even, streak-free results and removes oil that cold water cannot touch. After cleaning, sealing extends the result by two to three years. We can do both in a single visit.
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The Windows and Solar PanelsSave these for last. They will get hit by overspray no matter how careful the crew is, so cleaning them after the rest of the work is the only way to guarantee a streak-free finish. Solar panels are worth special attention - dust and pollen can drop output by 15 to 25 percent through summer. What to do: Pure-water window cleaning leaves no streaks and no residue. For solar, soft brush plus deionized water - never harsh chemicals or hard water spots that can etch the glass. Why the Order MattersMost DIY homeowners start with the driveway because it feels like the biggest visible win. Then they clean the gutters, and the gutter runoff streaks the freshly cleaned driveway. Then they hit the siding, and that runoff streaks the driveway again. Then they remember the roof and have to do everything a third time. Top-down is the only way. Roof, then gutters, then siding, then deck, then hardscape, then glass. That is the sequence professional crews use, and it is the sequence we use on every full-property service we run. DIY or Hire a Pro - The Honest AnswerWe will not pretend every job needs a professional. Here is the straight take. DIY is Fine For
Hire a Pro For
Ready to Tackle Your Spring Checklist?Most Bay Area homes need a half-day to a full day for a complete top-down service. We will walk your property, build a clear quote, and schedule around your calendar - not ours. Call (650) 248-0727 Book a Free Onsite QuoteServing San Mateo, Palo Alto, Atherton, Burlingame, Hillsborough, San Carlos, Belmont, Foster City, Redwood City, Menlo Park, and the entire Peninsula and South Bay.
Miller's Pressure Washing | Licensed and Insured | $2M General Liability | 9 Years Serving the Bay Area | 156+ Five-Star Google Reviews
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AuthorCasey Miller | Miller’s Pressure Washing Archives
August 2026
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Miller’s Pressure Washing
Serving San Mateo County, the Peninsula, Coastside, and select South Bay communities including Saratoga.
Licensed & Insured • Locally Owned Since 2017
Residential Services
Roof Soft Washing House Washing Driveway Cleaning Gutter Cleaning Window Cleaning Solar Panel Cleaning Deck Refinishing Patio Cleaning Bird Guard InstallationSpecialty Services
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