· by Michael Blair, Nashville's Siding
By the end of a Middle Tennessee summer, the north wall of a lot of houses has turned faintly green, and the wall behind the big shade tree may be black in streaks. It is algae and mildew living on the film of pollen and dust that settles on siding, fed by months of humidity and slow drying. It is not usually a sign of a failing wall. The damage people do trying to remove it, though, is a common reason siding gets called in for repair, and nearly all of it comes from a pressure washer used the wrong way.
Why the pressure washer is the problem
Lap siding is built to shed water that runs down its face. Each course overlaps the one below, and the joint underneath is left open so anything that gets behind can drain out. A pressure washer aimed upward at a lap drives water straight into that joint, past the siding and onto the housewrap or, on older houses, onto bare sheathing that was never meant to get wet from below.
The pressure itself does damage too. It can etch the surface of cedar and engineered wood, lift paint off fiber cement at the edges, blow caulk out of the joints at windows and trim, and push water into the interior of a hardboard board that is already soft. Vinyl survives it better, but water forced behind vinyl sits against the wall with nowhere quick to go.
Soft washing, by material
The method that works is a cleaning solution applied at low pressure, given a few minutes to work, scrubbed where it needs it with a soft brush on a pole, and rinsed from the top of the wall downward with a garden hose. The cleaner does the work, so the rinse only has to carry it away. Wet the plants below first and rinse them again after.
What goes in the solution depends on the siding. Vinyl tolerates most household siding cleaners. Fiber cement and factory-finished engineered wood have care instructions from their manufacturers, and following them protects both the finish and the warranty. Painted wood and cedar want the mildest approach that works, because harsh cleaners and scrubbing can take the finish with the dirt.
- Vinyl: siding cleaner or mild detergent, soft brush, hose rinse
- Fiber cement: the manufacturer’s published cleaning guidance, low pressure only
- Engineered wood: gentle cleaner, no wand close to cut ends or seams
- Painted wood and cedar: the mildest cleaner that lifts the growth, no etching
Why it keeps coming back on the same wall
Growth follows moisture and shade. If a wall goes green within a season of being cleaned, look at what keeps it wet: a sprinkler head that sprays the siding every morning, a gutter that overflows onto that elevation, shrubs pressed against the wall so it never dries, or a tree that shades it from sunrise to sunset. Fixing one of those does more than any cleaner.
Mulch and soil piled up against the bottom course are worth checking at the same time. Siding should stop well clear of the ground, and a bed that has been topped up for years can bury the bottom edge, which keeps it wet and gives insects a path into the wall.
Fall is a good time, and what cleaning reveals
Late September and October suit the job. The worst of the heat is over, so cleaners do not dry on the wall before they can be rinsed, and the growth comes off before leaves and winter grime add another layer. Wait for a dry stretch so the wall can dry out behind any water that does get in, and finish well before nights drop toward freezing.
A clean wall also shows things the green film was hiding. Swollen or crumbling bottom edges on hardboard, rust streaks from fasteners, open joints at corners and cracked caulk around windows all become visible once the surface is clean. That is the moment to look closely, because those are the places where water gets behind siding for good, and they are what a proper inspection starts with.