The front of the house looks fine from the driveway. Walk around to the side that faces the woods and the siding is green from the foundation to the soffit. Every homeowner in the Hudson Valley has seen it, and most of them have tried the garden hose, the deck brush and the rented pressure washer before they call. None of those work for long, because what is on the north wall is not dirt. It is alive, and it has to be killed, not rinsed. Here is what it is, why it picks that wall, what it is doing to the siding, and the one method that keeps it gone.
What The Green Actually Is
Algae, mostly, with mildew mixed in. The spores land on every wall of the house, but they only take hold where the surface stays wet long enough for them to feed, and that is the wall the sun reaches last. The north side dries hours after the south side every morning, sometimes not at all in a damp week, and in a humid Hudson Valley summer that is all the algae needs. The east wall is usually next. Once the film is established it holds its own moisture, which is why a wall that started going green two summers ago is solid green now. It is a close relative of the growth that puts the black streaks on the roof, and it behaves the same way: it feeds, it spreads, and it does not rinse off.
Why Your Wall, And Not Your Neighbor’s
Shade is most of it. A house under oaks and maples on a wooded lot in Hopewell Junction or Fishkill can go green on the north side in eighteen months while the same model on an open lot stays clean for three years. Humidity is the rest. Homes near the river or a pond in Wappingers Falls or Rhinebeck start every day damp. Painted wood and fiber cement, common on the older homes around Millbrook, hold the growth in the texture of the paint. Vinyl shows it sooner on light colors and hides it longer on dark ones; the growth is the same either way.
What It Is Doing To The Siding
On vinyl, the film dulls the color and holds dirt, and a wall that stays green for years oxidizes underneath and never looks new again. On painted wood and fiber cement it feeds mildew, and mildew under paint is the most common reason a new coat fails early. On brick and stucco it holds water in the surface through the freeze-thaw cycle. And behind all of it, the caulk around windows and trim stays wet, softens and lets water into the wall. None of this happens in a season. All of it happens in five years of leaving the north side alone.
Why The Hose And The Pressure Washer Do Not Work
Water does not kill algae. A hose moves the loose layer and leaves the rest. A pressure washer strips the visible film and leaves the organism alive in the texture of the siding, so the wall is green again within the year, and along the way the pressure has lifted paint, forced water behind the vinyl laps and left the wand marks you can see at an angle in the afternoon. The annual wash that pressure-washing companies recommend is not a maintenance schedule. It is the interval at which their method stops working. Our page on soft washing versus pressure washing explains the difference for every surface on the house.
What A Soft Wash Does Instead
A house soft wash applies a cleaning solution at low pressure that kills the algae and mildew at the root, then rinses at about the force of a garden hose. The green is gone the same day, the siding, paint and caulk are untouched, and because the growth was killed rather than moved, the wall stays clean for two to three seasons. The north side usually needs the next wash before the rest of the house, and when it does, the whole house is due. That is the two-to-three-year schedule most Dutchess County homes end up on.
What It Costs, And What Waiting Costs
A house wash starts at $495 in Dutchess County, and a typical colonial runs a few hundred more. A repaint on the same house runs into five figures, and oxidized vinyl cannot be brought back at any price. The wash is the cheap end of the decision. Every job is quoted at a firm price in writing before we start, and the work is backed by a satisfaction guarantee.
Get The North Side Handled
Send us your address and a photo of the green wall and you will have a firm number, usually the same day. If the roof has the streaks too, the same visit handles both at a package rate. Request a quote or call (845) 709-9443.
Green Algae On Siding FAQ
Because it dries last. The north wall gets the least direct sun, stays damp after rain and dew, and green algae grows wherever a surface stays wet in the shade. The east wall is usually next. It is growth, not dirt, which is why hosing it off does not work.
To the siding, yes, slowly. It holds moisture against the surface, which shortens the life of paint and caulk, dulls vinyl, and on wood and fiber cement feeds the mildew that rots the finish. It is also what a home inspector photographs first.
You can strip the visible layer, but the algae survives and is back within the year, and the pressure lifts paint, drives water behind vinyl and leaves wand marks. The fix that lasts is a soft wash: a cleaning solution at low pressure that kills the growth at the root.
Kills it. The solution treats the algae and mildew rather than rinsing it, which is why a soft-washed wall stays clean for two to three seasons instead of one. The rinse is at garden-hose pressure, so the siding, paint and caulk are untouched.
Two to three years on most homes, closer to two on a north wall under heavy tree cover. The sunny sides go longer. When the shaded side needs it again, the whole house is usually due.
House washing starts at $495 in Dutchess County and a typical colonial runs a few hundred more, with the number set by the size, the stories and the condition of the siding. Every job is quoted at a firm price in writing. Our house washing cost guide lists the starting prices by home type.
