Soft Wash vs. Pressure Wash - What The Difference Actually Is

Pressure washing

Water, at pressures measured in thousands of PSI, directed at a surface to physically blast material off it. It is the right tool for a concrete driveway. It is the wrong tool for anything with a surface layer that matters, and every roof has one: the granules on shingle, the colour layer on concrete tile, the coating on metal, the cap sheet on a flat roof. Pressure removes growth and the surface together, and it drives water sideways and upward under laps that were designed for water running downhill.

Soft washing

A cleaning solution - typically a diluted sodium hypochlorite mix with a surfactant - applied at roughly garden-hose pressure, left to work on the growth for a period, then rinsed at the same low pressure. The chemistry kills the algae and lichen; the rinse carries it off; the surface is not abraded and nothing is forced under the laps.

Why the distinction gets lost

Because some companies own one machine and describe whatever it does as "washing", and because the pressure-washed result looks dramatic on the day. The damage does not show on the day. It shows as faster re-growth, premature granule loss, and - on tile - cracked tiles that were walked on and laps that were flooded.

Questions that sort one from the other

• What pressure do you work at on a roof? A soft-wash answer is in the hundreds of PSI or lower. A number in the thousands is a pressure wash whatever it is called.
• What is in the solution, and how do you protect the plants below?
• What do you do when you find a broken tile up there?

Our position

KLR does not wash roofs in-house. When a member wants a roof cleaned, we arrange it through a third-party cleaner we know uses the correct low-pressure process - the same arrangement we use for wind mitigation reports through a licensed inspector. And whoever does it, the roof gets looked at afterwards.

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