Streaks and Staining on Stucco

Why stucco stains differently

Stucco is a cementitious surface with an open texture, so water and whatever it carries soak in rather than beading off. That means staining is genuinely in the surface rather than on it, and it also means the wall holds moisture longer than siding does, which supports organic growth in shaded areas.

Where the streaks come from

Vertical streaking below windows, sills and roof edges is runoff carrying dirt, frame oxidation and roof material down the wall. Dark patches near grade are splash-back. Rust-coloured stains near fasteners are metal corroding behind or within the surface, and that one is a repair issue rather than a cleaning one.

Why pressure washing stucco is a mistake

High pressure erodes the surface texture, opens the pores further and drives water deep into the wall assembly. On modern stucco over a drainage plane this can compromise how the wall sheds water. The damage is not visible on the day, which is exactly why it keeps being done.

What works instead

Soft washing at $0.18 per square foot cleans chemically at low pressure, which suits a porous surface. Realistic expectations matter: absorbed staining lightens considerably rather than vanishing, and a wall that has been stained for a decade will look much better rather than new.