Can a roof be cleaned while it is raining?

Walk through ARMA's own shingle sequence and the problem shows itself. The Asphalt Roofing Manufacturers Association's prevention bulletin gives the method in one line: "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." Its technical bulletin states the window as at least 15 and no more than 20 minutes. Those figures are ARMA's for asphalt shingle, and they do not carry over to tile or metal.

Rain does not land on the first step or the last. It lands in the middle, during the dwell time, when the solution is supposed to sit on the surface and work. Water falling on a freshly applied solution dilutes it and starts rinsing it off before the minutes have run, which turns the middle of the sequence into an early, uncontrolled version of the end. No roofing maker publishes a rule for applying a solution in active rain, so this is the mechanism reasoned from the procedure, not a quoted instruction.

ARMA's prevention bulletin draws the opposite edge of the window, a solution left to dry completely, which may prevent complete rinsing. Drying out is one way to spoil that sequence; rain is another. The whole apply, dwell and rinse sequence, as each maker publishes it, is laid out under how soft washing applies, dwells and rinses, and the deliberate final rinse-down has its own glossary entry.

Rain that arrives after the job is finished is a different case. ARMA's prevention bulletin describes treated algae disappearing and being carried away by the rains that follow a treatment, and the roof cleaning page answers what rain right after a treatment means.

The second problem is not chemistry. ARMA's technical bulletin warns that "this solution application and rinse process will make the roof slippery and potentially hazardous to walk on during treatment". That warning describes the process itself, with the water put on deliberately.

Eagle Roofing Products, a concrete tile maker, says the same about tile in two places. Its cleaning article: "Walking on a roof can be difficult and dangerous and introducing water only heightens these concerns." Its Florida warranty owner information, as archived, is shorter: "Be careful, tiles can be slippery."

Rain adds water to the entire roof at once, including the parts nobody meant to wet, on top of a solution the makers already describe as a slipping hazard. Who should be on a roof in the first place, with the fall protection and warranty language the makers publish, is covered under who belongs on a roof and what the warranty says. Whether anyone walks the roof during a cleaning at all is answered at does anyone walk on the roof during cleaning.

Much of South Florida's rain does not come alone. NWS Miami describes showers and thunderstorms in the South Florida summer as "almost a daily occurrence during the summer".

Once there is thunder, the National Weather Service's guidance ends outdoor work of every kind: "If you hear thunder, lightning is close enough to strike you." Its rule is to move at once to a substantial building or an enclosed metal-topped vehicle, and to "Stay in safe shelter at least 30 minutes after you hear the last sound of thunder." A roof is not a place to wait out a storm cell.

What is left after thunder and dwell time are accounted for is the gray area, a light drizzle with no storm nearby, and that is exactly where the documents go quiet. No roofing maker or association publishes a rain threshold, a drizzle allowance or a minimum dry forecast for a roof wash, and this page does not supply one. The wet months as a calendar, with both weather offices' dates, are set out at roof cleaning in the rainy season, and the heat that comes with them at roof cleaning in the summer heat. Whether a roof needs a wash at all is a separate matter of what is growing on it, taken up even for a recent roof at do new roofs need cleaning.