Green Film on Vinyl Siding: What It Is and How It Comes Off

Why it shows up on one side of the house

Walk around a house with green siding and the pattern is usually obvious: the growth is heavy on the walls that get the least sun, and light or absent on the walls that bake all afternoon. That is not a coincidence.

Horticulture experts at the University of Maryland Extension explain that algae grows where an area is shady, damp, or holds standing water, and that it also needs nutrients on the surface it grows on. Dust and fine particles that settle on a wall supply those nutrients (Ask Extension, 2019). A shaded wall gives algae both things it needs: moisture that does not dry off quickly, and a thin layer of dirt to live on.

The same goes for the spots right around a problem: under a gutter that overflows, behind a shrub planted tight to the house, or where a sprinkler wets the bottom few feet of siding every morning.

Is it hurting the siding?

We found no trade or extension source that says the algae itself damages vinyl. The risk the siding industry does write about is how people remove it.

The Polymeric Exterior Products Association, the trade group for vinyl siding makers, says a power washer can be used on vinyl siding “with caution.” It warns not to aim the stream upward, because that can force water behind the panels, and it notes that some siding makers do not recommend power washing at all (PEPA cleaning and maintenance guide). The same guide lists undiluted chlorine bleach among products to avoid, because it can damage the surface.

So the two common do-it-yourself mistakes are a narrow, high pressure tip pointed up under the laps, and straight bleach out of the jug.

How we take it off

We do not blast siding. Our house wash is a soft wash: low pressure and a professional grade sodium hypochlorite (SH) cleaning solution, mixed for the surface and given time to work, then rinsed. The solution does the work that pressure would otherwise do, so water is not driven up behind the panels and paint, caulking and trim are left alone.

The same approach works on vinyl and aluminum siding, brick, stucco, wood and fiber cement. Each one is handled for what it can take.

Why it comes back, and how to slow it down

The Maryland Extension answer is blunt about this: a single cleaning will not solve it for good if the area stays damp and keeps collecting dust, and washing once or twice a year may be required (Ask Extension, 2019).

A few things make the gap between cleanings longer:

• Point sprinklers away from the walls.
• Fix gutters that overflow onto the siding.
• Trim shrubs and branches back so air can move along the wall.

When to book it

Any time the green bothers you is a fine time. If you want the house to look its best for spring, note that Kentucky trees often start pollinating as early as February and keep going until about mid June (Family Allergy and Asthma), so many people wash once the heaviest pollen has passed. There is more on that in pollen on siding in spring.