The green film on the side of your house is not dirt. It is alive, it is growing, and that is why rinsing it with a garden hose buys you about eight weeks before it looks exactly the same.
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What is growing on your siding
Usually a mix of algae, mildew and mold. Algae is the green; mildew and mold tend toward gray and black. All three want the same three things — warmth, moisture and shade — and a humid city in the bend of the James supplies all three from roughly May through September.
This is why the north and shaded elevations go first, and why the wall behind a large shrub is always worse than the wall in open sun. If one side of your house looks noticeably worse than the others, you are seeing the pattern rather than a defect.
Why it always starts on the north wall
A north-facing wall in Richmond can stay damp until midday for much of the year. Add a tree canopy — which most of Henrico, Chesterfield and the older city neighborhoods have — and it may not fully dry at all during a humid stretch. Growth needs surface moisture, and that wall provides it almost continuously.
Homeowners in twenty-year-old subdivisions often notice this suddenly. The trees planted when the neighborhood was built are now tall enough to shade the house, and a wall that was fine for fifteen years starts growing algae. Nothing changed about the house. The sun stopped reaching it.
Is it damaging the siding?
Slowly. Algae itself is not eating the vinyl, but it holds moisture against the surface and against the seams, and organic growth in the laps traps dirt and keeps the wall damp. On painted wood it is a genuine problem, because persistent moisture is what lifts paint.
The bigger practical issue is that it does not stop. Each season it establishes further into the surface texture, and a wall left for four or five years takes more work to bring back than one washed on a normal cycle.
Why washing it off does not work
Because the growth is rooted in the surface texture, not resting on it. Pressure removes the visible layer and leaves the roots, so it regrows from the same place within a couple of months. A soft wash uses a detergent that kills the organism, which is why one holds twelve to twenty-four months and the other holds a season.
High pressure on siding also drives water behind the laps and into the sheathing, and leaves lap marks that do not come out. Our full approach is on the house washing page.
What else that film might be
- Chalky and comes off on your hand as powder — oxidation, which is the vinyl or paint pigment breaking down. Washing will not reverse it.
- Tiny black specks that will not wipe off — artillery fungus, thrown from hardwood mulch. Different problem, harder to remove, worth catching early.
- Yellow-green dust in spring — pollen. It bonds into a film here and does need washing, but it is seasonal rather than biological growth.
- Dark vertical streaks under the gutter line — runoff staining carrying roof and gutter debris down the wall.
Common questions
How often should I wash my siding in Richmond?
Every twelve to twenty-four months for most homes. Heavily shaded north walls come back first, and if you can see the shaded side is worse than the sunny side, it is time.
Can I just use a hose and a brush?
On a reachable ground-floor section, sure. The problems are height, the fact that scrubbing does not kill the growth at the root, and that the streaking you leave behind is more visible than what you started with.
Does the green mean I have mold inside the wall?
Almost never. Surface growth on the exterior and moisture inside a wall assembly are unrelated in most cases. If you also have a musty smell indoors, that is worth investigating separately.
Will it damage my vinyl siding permanently?
Not quickly, but it holds moisture against the surface and gets harder to remove the longer it establishes. On painted wood it matters more, because persistent damp is what lifts paint.
Is bleach safe on siding?
A properly diluted and rinsed detergent mix is what the entire soft wash industry uses, and it is safe when the landscaping is soaked first and everything is rinsed after. Undiluted bleach straight from the bottle is not.
Why does my neighbor’s house stay clean?
Sun exposure, nearly always. Compare tree cover rather than siding brand.
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