The north side of an Ohio Valley home turns green first because it gets the least direct sun and stays damp the longest after rain and dew. That combination is exactly what algae and mildew need to grow on siding.
Sun, shade, and moisture
In the northern hemisphere, north-facing walls receive the least direct sunlight, so they dry slowly. Ohio Valley humidity keeps them damp, and any tree cover makes it worse. Algae and mildew take hold and turn the wall green or black within a season.
It is alive, not just dirty
The green and black discoloration is living growth, not soil. Pressure washing removes the visible film but leaves the root, so it returns. Soft washing kills the organism at the source, which is why it lasts longer on shaded walls.
Check the other shaded spots
East walls under overhangs, areas behind shrubs, and any spot that stays damp and shaded will show the same pattern. Wherever water sits and sun does not reach, expect growth.
Five-minute self-check
- Walk around the home and compare the north wall to the south.
- Look under overhangs, behind shrubs, and near downspout splash zones.
- Touch a discolored area — slimy means live growth.
- Note any tree branches shading the wall.
- Check the roof’s north slopes for black streaks too.
Related reading
See our soft washing page and our post on black streaks on the roof.