Can a shingle roof be cleaned without losing granules?

Read the asphalt shingle documents for what a cleaning has to avoid and three things turn up.

• Force. The Asphalt Roofing Manufacturers Association (ARMA) writes “Never use a pressure washer to clean an asphalt shingle roof,” and GAF writes that power washing “may dislodge granules, which can lead to premature shingle failure.”
• Friction. ARMA writes “Avoid scrubbing the surface, as this friction may loosen and remove granules.”
• Contact. In the very next sentence ARMA adds: “If possible, always work from a ladder and/or walkboards to avoid direct contact with the roof surface.”

What that loss means, as a term, is set out under granule loss, defined. The rest of this answer takes the three in turn and gives the step each document publishes against it. The prohibitions themselves, quoted in full beside the metal roofing guide's, are gathered where three roofing documents give one instruction.

The step ARMA puts where the pressure washer would have been is a sprayer. Its algae and moss bulletin says to “Apply with a sprayer,” let the solution dwell, “and then rinse thoroughly with low pressure water.” Its technical bulletin describes the rinse the same way: “rinse the solution from the roof by gently spraying the surface with water.”

GAF's bulletin on blue-green algae staining reaches the same place, and notes while it does that some roof cleaning companies offer the power washing it does not recommend.

ARMA's technical bulletin widens the rule past algae: high-pressure washing systems should not be used on asphalt roofing for any other purpose either, such as other stains or dirt. The reason the whole trade leans gentle, on shingle and elsewhere, is traced in why roof cleaners use low pressure; on this roof the short version is that pressure washing and granules do not mix.

Friction is easy to overlook next to a pressure washer, and ARMA does not overlook it. It names the two together: “DO NOT use a power washer or any type of brush or broom to clean algae from the roof surface.” Any type of brush means a soft one too.

What replaces the scrubbing is time. ARMA says the solution “should be gently dispersed on the roofing surface,” and its algae and moss bulletin gives it 15 to 20 minutes to dwell before the rinse, adding that extended dwell times may be necessary as long as the solution is not allowed to dry completely. The solution works while it sits, so nothing has to be rubbed into the shingle.

ARMA also tells the person applying it to use normal precautions for handling bleach, including eye protection and rubber gloves, and it leaves the mix to the roof's maker: directions for mixing “may vary among shingle manufacturers,” and other chemicals or methods need that manufacturer's approval. Whether the solution leaves a smell behind is covered in whether soft washing smells.

The third step keeps the person doing the work off the surface where possible. ARMA places its ladder and walkboards line straight after its warning about friction, and gives the purpose in the same sentence: to avoid direct contact with the roof surface.

There is a safety reason on top of the granule one. ARMA warns that the application and rinse “will make the roof slippery and potentially hazardous to walk on during treatment,” and GAF reminds owners that “the roof surface will become slippery when wet.” ARMA's standing warning is that “Because it is dangerous to walk, climb, or work on a roof, ARMA recommends that only trained professionals engage in such activity.”

Tile has its own, longer set of rules for anyone who has to step onto it, and those belong to whether anyone walks on the roof during cleaning.

Everything above is about not adding to granule loss during a cleaning. It says nothing about granules a roof may have lost before anyone washed it, and a cleaner is not the person to judge that. Shingle condition, and any loss that is already there, is a question for a roofing contractor.

The algae is a different matter from the granules, too. ARMA writes that “There is no scientific evidence that algae is damaging to asphalt shingles, although it certainly affects the aesthetics of a roof,” and the fuller argument is in whether roof algae damages shingles.