What's the difference, when to use each, and why the wrong one damages siding. From a 23-year CT contractor.
"Power washing" gets used to describe two completely different processes. The wrong one damages your siding; the right one is essential prep for any exterior repaint. Here's the distinction that matters.
High-pressure water — typically 2,500–4,000 PSI — used to physically blast contamination off the surface. Effective on concrete, brick, and masonry. Damaging on wood siding, cedar shake, and HardiPlank. Cedar shake hit with 3,000 PSI lifts grain, drives water into the substrate, and accelerates rot.
Low-pressure water (under 500 PSI) combined with a chemical solution — typically a mildicide and surfactant — that does the cleaning work that pressure would otherwise do. The pressure is barely above garden-hose level; the chemicals do the work. Effective on every exterior surface, including cedar shake.
| Surface | Right method | Why |
|---|---|---|
| Wood clapboard | Soft wash | High pressure lifts grain |
| Cedar shake or shingle | Soft wash only | High pressure damages cedar permanently |
| HardiPlank / fiber cement | Soft wash preferred | Painted finish can be damaged by high pressure |
| Stucco | Soft wash | Hairline cracks worsen under pressure |
| Vinyl siding | Either, but soft preferred | Mid-pressure is okay; high pressure can crack |
| Brick (not painted) | Power wash okay | Brick can take 1,500–2,000 PSI |
| Painted brick | Soft wash | High pressure strips paint |
| Concrete walkway | Power wash | Surface is hard enough |
| Wood deck | Soft wash + light scrub | Deck wood is exposed end-grain |
The single biggest reason cheap paint jobs peel is that they're painted directly over contamination — pollen, mildew spores, oxidized pigment from old paint, dirt, algae. Paint can't bond to those things. The bond is to the underlying surface. So pre-paint cleaning is about exposing the surface so the new paint can bond properly. This is non-negotiable on any exterior project.
Don't. Either you'll use the wrong pressure for your siding, or you'll do it the day before paint and the substrate won't be dry. Let your contractor handle it as part of prep.
No — soft washing is gentler than a garden hose. It's designed to clean without removing intact paint.
Stop the work. Ask them why. Power-washing cedar is a red flag for the contractor's understanding of the substrate.
Slightly more — the chemicals cost money. But it's typically a difference of $50–$100 on a full project, and it's mandatory on cedar.
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