What stucco actually is
Stucco is a cement-based coating applied in layers over a lath substrate, with a textured finish coat carrying the colour and the weather resistance. That finish coat is thin, porous and the only thing protecting the layers underneath.
What high pressure does to it
At concrete-appropriate pressure the finish coat erodes. The texture flattens, colour hardener comes off, and in bad cases the surface blows off entirely leaving exposed base coat. Worse, water gets driven behind the stucco through hairline cracks — and once it is behind the wall it has nowhere to go, which causes far more expensive problems than a dirty wall.
What stucco can tolerate
Well below what concrete takes. In practice, correctly done stucco cleaning uses pressure comparable to a strong garden hose and relies on cleaning solution and dwell time to do the actual work.
Existing cracks make it worse
Almost every Los Angeles stucco wall has hairline cracking from seismic movement and thermal cycling. Those cracks are usually harmless. Forcing pressurised water into them is how a cosmetic issue becomes a moisture problem inside the wall.
The question to ask before hiring
Ask what pressure they use on stucco. A company that knows the material will talk about soft washing, detergent and dwell time. A company that names a high number, or cannot answer at all, will damage your house.
Key points
What stucco actually is
Stucco is a cement-based coating applied in layers over a lath substrate, with a textured finish coat carrying the colour and the weather resistance. That finish coat is thin, porous and the only thing protecting the layers underneath.
The question to ask before hiring
Ask what pressure they use on stucco. A company that knows the material will talk about soft washing, detergent and dwell time. A company that names a high number, or cannot answer at all, will damage your house.