Most professional house washing uses diluted sodium hypochlorite — bleach — with a surfactant to help it cling. It is faster and more effective on heavy mildew than anything else discussed on this site, and it is also the thing most likely to cause real damage if used carelessly.
This page is neither an endorsement nor a warning-off. It is what it actually involves, so you can decide.
Why it works so well
Mildew and algae are living organisms. Sodium hypochlorite kills them outright rather than loosening them, so the black streaking on a shaded wall goes and stays gone far longer than after a mechanical clean. Oxygenated cleaners do the same job more slowly and less completely.
It also works at genuinely low pressure, which is exactly what you want on siding — the approach described in soft wash vs pressure wash.
What it will damage
Plants. This is the big one. Hypochlorite kills foliage on contact and will burn a lawn. Run-off collects at the base of exactly the wall your planting is against.
Metal. It corrodes aluminum, galvanized steel and bare fasteners. Window frames, light fittings, downpipes, air conditioning units, outdoor furniture.
Fabric and anything colored. Awnings, cushions, outdoor rugs, car paint where it dries, and your own clothing.
Your pump, if it goes through it. On a consumer machine the downstream injector means it bypasses the pump, which is what makes this mechanically possible at all. An upstream injector or an onboard tank not rated for it is a different matter.
Your lungs and eyes. It is a respiratory irritant, and mixed with anything acidic or ammonia-based it produces toxic gas. Never combine it with another cleaner.
The rules, if you use it
- Downstream only, never through the pump. Confirm your machine’s injector is downstream, which on consumer machines it almost always is.
- Pre-wet everything — every plant, every bed, the lawn, the patio below. Keep wetting them during the job. Dilution is your protection.
- Cover what you cannot wet — light fittings, furniture, the car.
- Eye protection and old clothes. Not optional. Splash-back off a wall is constant.
- Never mix it with anything. No acidic cleaners, no ammonia, no “boosting” with a second product.
- Do not let it dry on the surface. Keep the wall wet during dwell and rinse thoroughly.
- Flush the injector afterwards. Clean water through the pickup tube with the soap tip fitted, thirty seconds minimum. Residue destroys check valves.
- Rinse the planting again at the end, generously.
- Mind the wind. Drift carries it onto cars, neighbors’ planting and you.
Dilution
Hypochlorite is sold at different strengths — household bleach is weak, pool chlorine is much stronger — so a ratio means nothing without knowing the starting concentration. Work from the instructions on the product you have bought, and remember the injector dilutes it again (the arithmetic).
Start weaker than you think and test on an inconspicuous area. Two gentle applications are better than one strong one, and considerably safer.
When not to bother
For a modest amount of growth on one wall, an oxygenated house wash will do the job with none of the care above, and the time difference is twenty minutes. The case for hypochlorite is heavy growth, large areas, or a wall that keeps coming back.
For a roof, do not use a pressure washer at all — here is why — and consider a no-rinse treatment applied from the ground instead.
