Every few weeks someone asks why we don't just pressure wash the roof. It's a fair question. Pressure washing works beautifully on driveways and patios, so it seems like it should work on a roof. On an asphalt shingle or tile roof, it doesn't.
What pressure does to a shingle
An asphalt shingle is a fiberglass mat coated in asphalt and covered in ceramic granules. The granules are the roof: they block UV, shed water and give the shingle its color. A pressure washer blasts those granules off. The roof looks clean for a season, then fades, cracks and fails years early. On tile, pressure forces water under the tiles and into the underlayment. On shake, it tears the wood fiber.
That's why GAF, Owens Corning, CertainTeed and the Asphalt Roofing Manufacturers Association all list pressure washing as a warranty violation.
What a soft wash does instead
A soft wash is a two-part process. First, a roof-safe cleaning solution is applied at low pressure, roughly what a garden hose produces. The solution kills algae, moss and lichen at the root rather than shaving off the top. Second, after the solution has dwelled, the roof is rinsed gently. Algae streaks typically disappear immediately. Heavy moss and lichen die, lose their grip and shed off over the following weeks of normal weather.
Because the growth is killed rather than trimmed, a soft wash holds for four to ten years depending on shade and moisture. A pressure wash on the same roof often shows regrowth within a year.
What we do to protect your property
- Pre-wet and cover plants near the drip line; rinse again afterward.
- Bag or divert downspouts so runoff doesn't reach beds or lawn.
- Protect painted surfaces, decks and outdoor furniture from overspray.
- Rinse and neutralize the roof surface before we leave.
Where pressure still makes sense
Concrete walkways, pavers, retaining walls and some unpainted metal surfaces. We use it there, with the right pressure and nozzle. Just not on your roof.
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Published by Fox Roof Cleaning, Millbrae, CA.