Pressure washing on Vashon — and the one surface we won't do it to
A pressure washer is the correct tool for concrete, siding, decks and docks. It is the wrong tool for an asphalt shingle roof, and we'd rather explain why up front than take the job.
What grows on island hardscape, and why
The grey-green film on a Vashon walkway, deck or dock is mostly algae, with moss joining in wherever the surface is shaded. Both want the same things this island reliably provides: moisture, mild temperatures, and shade. Roughly 40 inches of rain across about 150 wet days, under tree cover, with winters too mild to kill anything back.
Two consequences worth separating:
It's a safety issue before it's a looks issue
Wet algae on a dock ramp or a north-side walkway is genuinely slick. On an island where a lot of homes have wooden dock access, steps down a bank, or a shaded path to the door, this is the reason most people finally book — not appearance.
Organic growth holds water in the surface
On wood, a living layer keeps the boards damp for longer after rain, and prolonged dampness is what drives rot and accelerates weathering. Cleaning it off isn't only cosmetic — it lets the wood actually dry between rains.
Matching pressure to the surface is the whole job
"Pressure washing" describes a tool, not a single setting, and using one setting everywhere is how surfaces get damaged:
Different surfaces, different approach
- Concrete driveways and walkways — tolerate the most pressure. Usually the most dramatic before/after on a property.
- Wood decks and docks — need meaningfully less. Too much pressure raises the grain and leaves permanent wand stripes in the boards.
- Siding — lower pressure again, and directed carefully. High pressure aimed upward under lap siding drives water behind it, which is the opposite of helpful.
- Asphalt shingle roofs — no pressure washing at all. See below.
Why the roof is different
This is the part most homeowners are never told, so it's worth being direct about it. The Asphalt Roofing Manufacturers Association — the trade body for the companies that make the shingles — states:
"Never use a pressure washer to clean an asphalt shingle roof as this will cause granule loss and very likely premature failure of the roof system."
— Asphalt Roofing Manufacturers Association
The mineral granules on a shingle are its sunscreen — they're what keeps UV from breaking down the asphalt underneath. A pressure washer removes moss and granules with equal enthusiasm. The roof looks terrific the same afternoon; the cost arrives over the next few summers as accelerated wear, long after the invoice cleared.
So roofs get the method the manufacturers do endorse: a treatment applied with a sprayer, left to dwell, then rinsed at low pressure. Slower, and it leaves your granules where they belong. That's covered in full on the roof cleaning and moss removal page.
If another company is willing to pressure wash your shingles, that's now a decision you can make with the manufacturers' own guidance in hand.
What we'll pressure wash
- Driveways, walkways and patios
- Decks, docks and stairs
- House siding (at appropriate pressure)
- Fences and retaining walls
As Russell puts it on the original site: from siding to driveways and all the spaces in between.
Common questions
Why won't you pressure wash my roof?
Because the asphalt shingle manufacturers' association explicitly says not to — it causes granule loss and very likely premature roof failure. Roofs get low-pressure treatment and rinsing instead.
Will pressure washing damage my deck?
Not at the right pressure. Wood needs far less than concrete; too much raises the grain and leaves wand marks. Matching pressure to surface is most of the craft.
What's the grey layer on my dock?
Mostly algae and weathering rather than the wood's actual colour — which is why a cleaned dock often looks startlingly new.
How often should hardscape be washed here?
Every one to two years for most Vashon properties; annually for shaded walkways and docks where algae regrows fastest and the slip risk matters.
Related
Roof cleaning and moss removal (the low-pressure method) · Gutter cleaning · Windows and solar panels