What's the difference between soft washing and pressure washing?
Pressure washing is the most damaging mistake a Portland homeowner can make with their roof, and unfortunately it's also a common one because pressure washing 'looks' more vigorous. The pressure that strips moss also strips the granules protecting your shingles. Here's the actual difference.
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What pressure washing actually does to asphalt shingles
Asphalt shingles are coated with ceramic granules that provide UV resistance, fire rating, and aesthetic color. The granules are bonded into the asphalt mat with adhesive that's designed to hold them through normal rain, wind, and foot traffic.
The PSI required to dislodge established moss from shingles is roughly 1,500-2,000 PSI at the surface. The PSI required to dislodge ceramic granules from the asphalt is roughly the same range. There's no spray pattern, nozzle angle, or operator skill that meaningfully separates the two; if you're stripping moss with water pressure, you're stripping granules at the same time.
Result: a pressure-washed roof looks clean immediately but typically loses 20-40% of its remaining useful life in a single cleaning. On a roof with 12 years of life left, that's 3-5 years of life destroyed in one weekend.
Soft wash: how it works without damaging the roof
Soft wash flips the model: low pressure (under 500 PSI), high chemistry.
- 1Sodium hypochlorite or zinc sulphate solution is applied to the roof surface at low pressure through wide-angle tips.
- 2The chemistry dwells on the surface for 10-20 minutes, killing the biological growth (moss, algae, lichen) at the cellular level.
- 3Dead biological matter is rinsed off at low pressure (500 PSI is well below the granule-strip threshold of 1,500 PSI).
- 4Optional zinc suppression pass extends residual moss/algae kill for 18-30 months after the cleaning.
Result: cleaner surface than pressure washing achieves (because chemistry penetrates moss roots while pressure only dislodges the visible mat), without shingle damage, with longer-lasting effect, and at similar cost.
How to tell which one a contractor is going to do
Ask directly. Legitimate Portland cleaning contractors are happy to walk through the protocol with you because the soft-wash-only message is part of how the industry educates customers. If a contractor:
- Pitches power washing or pressure washing your shingles: the wrong contractor.
- Talks about 'gentle pressure wash': probably means actual pressure wash at lower PSI, which is still damaging. Soft wash should be the term.
- Mentions PSI numbers: 500 PSI or under is correct; anything above 1,000 PSI is too high.
- Talks about chemistry, dwell time, and rinse pressure: correct soft-wash protocol.
- Has equipment that looks like a standard pressure washer: probably is one. Soft wash equipment uses wider nozzles and chemical injection at the pump.
The phrase 'low-pressure power wash' is marketing speak that often means 'still high-pressure'. Insist on the soft-wash term and ask what PSI they're using.
When pressure washing is actually appropriate
Pressure washing has legitimate uses; just not on asphalt shingles.
- Concrete patios, driveways, and walkways: pressure washing is correct and appropriate.
- House siding (vinyl, fiber cement, painted wood): soft wash is preferred but careful pressure wash at moderate PSI is acceptable.
- Metal roofs: pressure washing is acceptable because metal panels don't have granules to strip.
- Wood decks: soft wash with brightener is the modern standard; pressure washing damages soft wood fibers.
A contractor that does pressure washing on appropriate surfaces (concrete, metal) and soft wash on inappropriate ones (asphalt shingles) is a legitimate operator. A contractor that pressure washes everything is the wrong one for your asphalt roof.
Frequently Asked Questions
Can pressure washing be done safely at lower PSI?
Even at 1,000 PSI (low for a pressure washer), granule loss is meaningful on asphalt shingles. Soft wash uses 500 PSI maximum and relies on chemistry, not pressure, for the cleaning work. The pressure threshold isn't about technique, it's about physics.
Will I see the damage from pressure washing right away?
Sometimes. Heavy granule loss shows as bare patches and excessive granules in gutters within weeks. More commonly, the damage shows up 1-3 years later as accelerated weathering and the roof failing earlier than expected.
If pressure washing is so bad, why do contractors still offer it?
Three reasons: it's faster (more jobs per day), it looks more vigorous (sells better to homeowners who haven't researched), and it requires less skill than soft wash chemistry. The contractor's incentives don't align with your roof's interests.