Why do roof cleaners use low pressure?

The reason for the tool starts with the stain. The Asphalt Roofing Manufacturers Association (ARMA) writes that the black discoloration “Often mistaken as mold or mildew” is “actually algae.” That makes the black streak a living growth, not loose dirt waiting to be knocked off.

Loose dirt answers to force. A growth that has settled into a surface answers to chemistry, and that is what the bulletins reach for. ARMA's technical bulletin on algae discoloration writes that “Discoloration may be lightened by applying a solution of liquid household bleach (sodium hypochlorite) and water.” The verb is ARMA's, and it is lightened, not blasted off.

Once the solution is the thing doing the cleaning, the question for the water changes. It no longer has to cut the stain away. It has to put the solution on and take it off again.

ARMA's “Algae & Moss Prevention and Cleaning for Asphalt Roofing Systems” describes the shingle method this way: “Apply with a sprayer and allow the solution to dwell on the roof surface for 15 to 20 minutes, and then rinse thoroughly with low pressure water.” A sprayer to apply, time to work, a gentle rinse to finish. None of the three steps needs force.

ARMA adds that “Extended dwell times may be necessary,” and in the same sentence warns against letting the solution dry completely, because that may prevent complete rinsing. The room that document leaves is more time on the surface, not more pressure. That window is the dwell time, defined for shingle, and it belongs to ARMA's solution on asphalt shingle only.

Metal follows the same pattern with different chemistry. The Metal Roofing Alliance's detergent solution stands 5 to 10 minutes, then the surface is rinsed thoroughly with plain water. Different roof, different recipe, different window, and the same shape: the solution sits, the water carries it away.

The bulletins do not treat force as a shortcut. ARMA writes “Never use a pressure washer to clean an asphalt shingle roof,” and in that sentence ties it to granule loss and very likely premature failure of the roof system. Its technical bulletin goes further, saying high-pressure washing systems “are likely to damage asphalt roofing” and should not be used for algae or for any other purpose.

That is the core of the answer. On shingle, the extra force a pressure washer brings is exactly the thing ARMA connects to lost granules. ARMA also writes that there is no scientific evidence algae damages asphalt shingles, though it affects how a roof looks, so the stain is the cosmetic problem and the pressure is the one the documents warn about.

How a shingle cleaning avoids each way of losing granules, force being only one of them, is traced step by step in cleaning a shingle roof without losing granules. The shingle and metal documents that rule the machine out are quoted together where three roofing documents give one instruction.

The Metal Roofing Alliance puts the low end of the scale first. Its guide starts by telling owners to “Try a garden hose attachment to spray away common dirt and debris (NEVER use a power washer)”.

When water is not enough, the next step it names is chemistry, not force: 1/4 cup of mild detergent per gallon of water, applied with a washcloth, sponge, soft bristle brush or non-abrasive pad. The guide's stated goal is “to use the lightest touch that will effectively do the job,” and it rules out steel wool, wire brushes and scouring powders. Each step up adds a solution or a light touch, never more pressure.

Put those pieces together and the low pressure explains itself. The solution does the cleaning. The dwell gives it time. What is left for the water is to carry the solution off the roof, and that job is a rinse, which ARMA's procedure describes as low pressure water.

Low pressure on its own is not the method either, since water with no solution only rinses; what makes a soft wash a soft wash is argued in full on the method page. Whether and why a roof gets rinsed after soft washing is its own answer.

People often want to know how low is low. No trade association or roofing maker read for this answer publishes a psi figure that defines soft washing; the figures in circulation come from vendors and blogs, not from the documents. Where a maker does print a pressure number, it prints one for a single material, and a pressure figure for any given roof comes from that roof's maker.

Low pressure is also not a promise about the result. ARMA's bulletin writes that “The effectiveness of such cleaning is only temporary, and the discoloration may recur.” The twice-yearly roof cleaning question has its own answer, and none of the documents quoted here sets a schedule.