Why Seattle concrete turns green
Concrete is porous. Through the rainy season it stays damp for weeks at a time, and spots shaded by trees or the north side of the house never fully dry out. Algae shows up first as a dark green film, then moss fills the expansion joints and cracks. Fir needles and leaves left lying on the surface hold moisture and leave tannin stains. The result is a driveway or a set of steps that's slippery right when it rains the most.
What concrete cleaning includes
- Clearing loose debris and pulling moss out of joints and edges
- Cleaning flat areas with a rotary surface cleaner for an even, stripe-free finish
- Detailing along walls, steps and landscaping with a wand
- Spot treatment for oil, rust or tire marks, which may lighten rather than vanish completely
- An optional post-treatment to slow algae regrowth
- Rinsing the area so walkways are safe to use
Surfaces you can request
- Driveways and parking pads
- Public sidewalks along the front of the property
- Front walks, patios and pool decks
- Steps and stoops, often the slipperiest spot
- Exposed aggregate, which needs lower pressure so stones aren't knocked loose
- Pavers, where joint sand may need topping up afterwards
Washing the house at the same time? See pressure washing for siding, decks and fences.
Sealing after cleaning
Sealing is optional. A penetrating sealer can slow water absorption and make the next cleaning easier, but it needs dry weather to cure, so in Seattle it's usually a summer job. Ask the provider whether sealing makes sense for your surface and what kind of finish it leaves.
How often to clean driveways and walkways
Most homes clean driveways and walkways once a year, often in late summer or early fall before the rain returns. Heavily shaded concrete tends to benefit from a fall cleaning plus a preventive treatment. Steps and walkways that get slippery mid-winter can be cleaned any time it's above freezing.
What affects the cost
- Total square footage
- How heavy the moss, algae and staining are
- Surface type: broom finish, exposed aggregate or pavers
- Steps, edges and detail work around landscaping
- Stain treatment and sealing add-ons
- Outdoor water access