Algae & Mildew Removal
The flat dark growth that reads as dirt until someone slips on it. What it is, why a hose does nothing, and the method that actually clears it.
It Looks Like Staining. It Is a Living Organism.
Algae is the most commonly misdiagnosed thing on a Pacific Northwest property. It is flat, it is dark, and it spreads evenly, so it looks like the surface is simply dirty. It is not. It is a colony, it is anchored, and rinsing it moves almost none of it.
On a roof it shows as the black streaks running down from the ridge, usually worst on the north slope. On concrete and pavers it is the dark film that makes a shaded walkway genuinely treacherous once it is wet, because it holds a layer of water against a surface whose texture is the only thing giving you grip. On siding it is the green-grey cast that makes a house look tired years before the paint has actually failed.
Mildew is the same problem in the sheltered spots: under eaves and soffits, on north-facing trim, in the corners that never get rained on hard enough to be washed. It needs the same treatment, applied somewhere a pressure washer should never be pointed.
It turns up on almost everything, so the destination depends on the surface: siding and roofs go through soft washing, while walkways and patios are treated and then cleaned with a surface cleaner.
How We Remove It
Algae is killed, not blasted. On every surface it grows on, the chemistry does the work and the water only rinses away what the chemistry has already finished.
Match the Method to the Surface
Roofs and siding are soft washed, always. Hardscape gets a surface cleaner at a pressure set for the material. Nothing gets a narrow tip held close, which is how concrete ends up permanently striped and siding ends up with water behind it.
Pre-Wet the Surroundings
Plants and beds adjacent to the work are wetted down first so that any incidental overspray is diluted on contact rather than absorbed.
Apply and Let It Dwell
The cleaning solution goes on at low pressure and is given the contact time it needs to kill the organism rather than just lighten it. Skipping dwell time is why algae reappears within weeks of a rushed job.
Rinse Down and Check
Surfaces, siding, walkways and plantings all get rinsed. On slip-prone walkways we check the cleaned surface underfoot before leaving, because that is the whole point of the job.
Where We Find It
Algae and mildew follow shade and retained moisture, which makes them wildly uneven across a single property.
North-Facing Roof Slopes
The black streaking that darkens over time and traps heat and moisture against the shingles.
Shaded Walkways & Steps
The highest-risk surfaces on most properties. Thin, dark, and far slicker wet than it looks dry.
Siding & Trim
The green-grey cast on north elevations, and mildew in the sheltered corners that rain never reaches hard.
Soffits & Eaves
Protected from weathering, which means nothing washes it off naturally. Soft wash only, never pressure.
Patios & Stone
Covered and tree-shaded patios can grow a slick film through the wet season while the rest of the hardscape looks fine.
Commercial Entries & Breezeways
Shared walkways and entryways, where the slip exposure belongs to the property owner rather than the resident.
Frequently Asked Questions
What are the black streaks running down my roof?
That is almost always Gloeocapsa magma, a blue-green algae. The black colour is a protective sheath the organism produces against UV, which is why the streaks darken over time. It spreads by airborne spores, so it tends to appear across a whole neighbourhood rather than on one house, and it is worst on north-facing and shaded slopes where the surface stays damp longest.
Why is the walkway so slippery when it does not look that dirty?
Because algae is flat and thin, so it reads as discolouration rather than as growth. What it does is hold a film of water against the surface, which removes most of the effective grip from a textured broom-finish concrete. It is at full strength here from roughly October through May, which is exactly when it is most likely to be wet underfoot.
Will a pressure washer get rid of algae for good?
It will take the visible layer off and it will come back, usually quicker than before, because pressure roughens the surface and a rougher surface holds more moisture and debris. Algae has to be killed with a cleaning solution and given dwell time. On siding and roofs, pressure is also actively damaging, which is why soft washing exists.
Is what I am seeing algae or mildew?
In practice it rarely changes the treatment, and both respond to the same approach. Algae is usually green to black and sits in streaks or sheets on exterior surfaces exposed to rain. Mildew tends to be grey to black, patchier, and shows up in the more protected spots: under eaves, on soffits, on north-facing trim and in corners that never get direct weather.
Have a question we didn't answer? Call (425) 298-7041or submit a quote request — we respond within 24 hours.
Soft Washing Across Seattle & the Eastside
Algae tracks shade and moisture, so the worst of it is concentrated in predictable places. Tree-canopied lots in Woodinville, Issaquah and Sammamish and the damp valley ground around Snohomish carry the heaviest load. Waterfront exposure in Kenmore, Medina and Mercer Island keeps surfaces damp well past sunrise. Even in more open Renton, Everett and Lynnwood, the north side of the house and the shaded run of path behave nothing like the rest of the property.
Pick your city for what we see locally and how we approach it there.
Don't see your city? We work across the greater Seattle area, roughly an hour out from Kenmore. See the full service area, or call (425) 298-7041 and we'll tell you straight away whether you're in range.
Related Services
Algae removal is usually part of one of these:
Is Your Walkway Slick, or Just Dirty?
If it is darker and more slippery wet than dry, it is algae and it will not rinse off. Send a photo for a free assessment across Seattle, Bellevue, Kirkland, Redmond, Sammamish, Bothell, Kenmore, Woodinville and the wider Eastside.
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