The short answer
The green film is algae, and it grows on the side of your house that stays damp longest — almost always the north face, which never receives direct sun in the northern hemisphere, plus any wall shaded by trees or a neighboring house. Moisture, not dirt, decides where it grows. Killing it requires root-level soft-wash chemistry; rinsing or blasting it off leaves the roots to regrow within a season.
The Compass Is the Culprit
Stand in the yard and check: the green wall faces north, or close to it. In Michigan the sun tracks across the southern sky all year, which means a house's north face lives in permanent indirect light — morning dew and rain that burn off a south wall by 10 a.m. linger on the north wall past lunch, and in the cool shoulder seasons, sometimes all day. Algae doesn't care about dirt, age, or siding brand; it cares about sustained moisture, and the north face is where a building serves it. Add local shade — a maple over the corner, a fence line, the neighbor's house across a narrow gap — and any wall can join the club, but the compass picks the default member.
What the Green Actually Is
The film is airborne algae that lands everywhere and survives only where the drying math works in its favor. It roots into siding texture — vinyl's grain, fiber cement's finish, brick's pores, painted wood's micro-cracks — and once established, it holds its own moisture film, which makes the wall damper, which grows more algae. That self-accelerating loop is why a faint green tint becomes a green wall in a couple of seasons, and why the problem always seems to 'suddenly' get bad. Mildew joins in the most sheltered zones; on chronically damp walls, lichen eventually follows, and it's far harder to remove than either.
Why Rinsing and Blasting Both Fail
A garden hose or pressure wand shears the visible algae off the surface and leaves the roots in the texture — the wall looks clean for a few months, then re-greens on the same wet-wall schedule, usually within the year. Pressure adds injury to futility: driven water behind siding laps, etched finishes, furred wood. The fix that lasts is chemical: soft-wash solution applied at low pressure, given dwell time to kill the organism at the root, then rinsed gently. Dead roots don't regrow — treated north walls typically hold two to four years even though the moisture math never changes, because the algae has to recolonize from zero instead of rebounding from roots.
Tilting the Moisture Math
You can't rotate the house, but you can shorten the wall's wet hours at the margins: trim back the trees and shrubs shading the face and crowding airflow, aim sprinkler heads so irrigation stops watering the siding on a schedule, extend downspouts that splash the wall's base, and keep the gutter above from overflowing down the face. None of it makes a north wall behave like a south wall — the treatment cycle is the honest answer — but each fix stretches the cycle, and together they're often the difference between a two-year wall and a four-year wall.
The Bottom Line
One-sided green is physics, not neglect, and it has a permanent tendency with a periodic fix: root-kill soft washing on the honest cycle, plus whatever shade and water adjustments your lot allows. We quote the green side's regrowth expectations straight — north walls green first, and anyone promising otherwise is selling the rinse.
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