House soft washing

The green or black film on siding is mildew and algae bonding to a slightly porous surface, feeding on airborne pollen and dirt that collects in the texture of vinyl, HardiePlank, or paint. It grows fastest in shaded, humid spots that never fully dry between rains — which is why the north side of a house is almost always the worst, and why the same house can look clean on one side and dirty on the other.

Soft washing works by combining a surfactant-based cleaning solution — water, a mild bleach-based agent, and a surfactant that helps it cling to a vertical surface instead of running straight off — with low pressure, generally under 500 psi, closer to a garden hose with a pump than a pressure washer. The solution dwells long enough to break down the mildew and algae, and a low-pressure rinse carries it away. A pressure washer, by contrast, runs 1,500 to 3,000-plus psi directly at the surface — more than enough to force water past seams behind siding panels, warp vinyl, or strip paint.

Different materials need different handling within that same soft-wash approach. Vinyl is fairly forgiving but can still crack or blow inward under real pressure. HardiePlank is more rigid, but its factory finish can dull or streak if handled too aggressively. Brick and stucco are porous — they need a longer dwell time at a gentler concentration to avoid staining rather than a stronger mix. Painted wood is the most delicate of all and gets the lowest pressure and shortest, most careful treatment.