Will Pressure Washing Damage My Roof Shingles?

What the granules are actually for

The coloured grit on an asphalt shingle is not decoration. It is a ceramic coated mineral layer bonded into the asphalt, and its job is to absorb ultraviolet light so the asphalt underneath does not. Asphalt left exposed to sun dries, hardens and cracks.

That is the whole reason pressure is the wrong tool. A pressure washer does not distinguish between an algae colony sitting on the surface and the granules bonded into it. Both come off.

The damage is also slow to show. A roof that has been blasted often looks excellent for a season or two. The bill arrives years later, as a roof that aged faster than the identical one next door.

What the trade body says

The Asphalt Roofing Manufacturers Association is the trade association for the companies that make these shingles. Their guidance on cleaning algae off a roof is not ambiguous.

They instruct homeowners not to use a power washer, or any type of brush or broom, on the roof surface. They state that high pressure washing is likely to damage asphalt roofing, and that the friction of scrubbing may loosen and remove granules.

That is the body representing the manufacturers, not a cleaning company arguing for its own method.

What one manufacturer puts in writing

GAF, one of the largest shingle manufacturers in North America, publishes a technical bulletin on algae staining. It tells you not to power wash, in those words, and gives the reason: it may dislodge granules, which can lead to premature shingle failure.

The same bulletin sets out the method GAF does recommend. A mixture of four parts water to one part bleach with a cup of trisodium phosphate, or a phosphate free substitute, applied with a garden sprayer, left on the surface for up to twenty minutes, then rinsed with low pressure water. It also says to protect the shrubs below, and warns that a wet roof is slippery.

Read that recipe again and notice what is missing. There is no pressure anywhere in the process except a low pressure rinse at the end.

About the warranty question, honestly

You will read a lot of confident claims that pressure washing voids your shingle warranty. We went looking for the clause and could not find one that names pressure washing directly.

What we can tell you is this. Limited shingle warranties generally exclude damage caused by improper maintenance, and the manufacturers publish care guidance that rules out power washing. Whether a particular claim gets denied is between you, your installer and the manufacturer, and it turns on the exact document covering your roof.

So the honest advice is to read your own warranty, or ask the manufacturer, rather than taking our word or a competitor word for it. We would rather you check than repeat something we have not verified.

What soft washing actually is

Soft washing is the method above, done properly. A cleaning solution does the work of killing the organism, it is given time to work, and the roof is rinsed at low pressure. The water is a carrier, not a tool.

This is why a soft wash and a pressure wash are not two prices for the same job. They are different jobs. One is the method the manufacturers describe. The other is the one they tell you not to use.

It is worth knowing that cleaning of any kind is temporary. ARMA says so plainly: the effectiveness is only temporary and the discoloration may recur. Anyone promising a permanent result is promising something the manufacturers do not.

How to tell which one you are being sold

Ask what goes on the roof and how long it stays there. A correct answer involves a solution and a dwell time of roughly fifteen to twenty minutes. If the answer is only about equipment and water, that is a pressure wash.

Ask how the roof gets rinsed. Low pressure is the right answer.

Be wary of anyone quoting a pressure figure for an asphalt roof at all. On this surface the useful number is dwell time, not PSI, and a contractor who leads with pressure is telling you which job they intend to do.

And ask what happens to the plantings. The manufacturer guidance says to protect the shrubs below, which means somebody has to actually do it.