The two methods get confused constantly, partly because plenty of companies advertise “pressure washing” as a catch-all for exterior cleaning. On driveways the distinction barely matters. On a roof it is the whole ballgame.
What pressure washing does to a roof
An asphalt shingle is a mat saturated in asphalt and coated in mineral granules. Those granules are not decorative — they are the shingle’s sunscreen. High-pressure water dislodges them in quantity. You can often see the evidence in the driveway afterwards: a dark grit washed down with the runoff. That is roof life going down the storm drain.
- Strips protective granules, accelerating UV ageing of the shingle.
- Can drive water up and under shingle courses, past the point where the roof is designed to shed it.
- On tile, can crack or dislodge pieces and force water into the underlayment.
- Frequently voids shingle manufacturer warranties — check yours before anyone puts a wand on the roof.
What soft washing is
Soft washing applies a biodegradable cleaning solution at roughly the pressure of a garden hose. The chemistry kills the algae, mould, and mildew at the root; a gentle rinse carries the dead growth away. Nothing about the process relies on force, which is exactly why it is safe on surfaces that force would damage.
Shingle vs. tile — what changes
Both should be soft washed, but for slightly different reasons. On asphalt shingle the concern is granule loss and water intrusion. On concrete or clay tile the concern is breakage and the underlayment — tile is walkable only in specific places, and pressure can drive water past the tile into a layer that is much more expensive to repair than the tile itself. Barrel tile in particular hides a lot of organic growth in the valleys, which chemistry reaches and pressure simply blows around.
How to tell if a contractor is actually soft washing
Ask three questions. They are quick, and the answers are revealing:
- “What pressure will you use on the roof?” A soft wash answer is measured in low hundreds of PSI or “garden hose pressure.” If you hear a number in the thousands, that is a pressure wash.
- “Are you killing the algae or rinsing it off?” If the plan does not involve a cleaning solution dwelling on the surface, the growth is not being killed and it will return quickly.
- “How are you protecting the landscaping?” A crew that soft washes routinely will have a specific answer — pre-wetting plants, tarping, rinsing down afterwards.
What about the cost difference?
Soft washing is not meaningfully more expensive, and it lasts longer, so the cost per year of clean roof is lower. Where people get caught out is comparing a soft wash quote against a quick pressure-wash-and-go price. Those are not the same service, and the cheaper one is being paid for out of the roof’s remaining lifespan.