Does roof cleaning hurt plants?

The clearest evidence that a roof cleaning can hurt plants comes from the people who publish the cleaning instructions. They do not treat it as a rumor. They write it into the procedure.

The Asphalt Roofing Manufacturers Association (ARMA), in its technical bulletin on algae discoloration, tells the person applying its bleach and water solution: “Be sure to apply it carefully to avoid damage to other parts of the building and the surrounding landscape.” ARMA's separate page on algae and moss says the same thing in different words, calling for proper precautions to protect landscaping and surrounding areas from the chlorine bleach solution.

GAF, a shingle maker, puts it in the shortest line of all, right after the instruction to rinse its mixture off with low-pressure water: “Be sure to protect any shrubs below.”

The reason is gravity and drift. Whatever grows under a roof line sits below the slope the solution runs down and the edge it drips from, and within reach of spray that drifts, which ICPI's paver guidance calls overspray. Where the rinse water finally ends up, and the rules about drains, is a separate question answered under what happens to the runoff from roof cleaning.

The precautions come from two trades, and it matters which is which.

• From the roofing side, ARMA asks for the solution to be gently dispersed on the roof surface and applied carefully, and GAF asks for the shrubs below to be protected. Both lines are written for bleach solutions on asphalt shingle.
• From the paving side, the Interlocking Concrete Pavement Institute (ICPI) gives the mechanism behind pre-wetting: where strong acidic cleaners might drain onto vegetation, saturating the vegetation with water first minimizes its absorption of cleaner rinse water. ICPI wrote that for acidic cleaners on pavers, not for a roof wash.
• ICPI's same Tech Spec 5 says low branches, shrubs and vegetation beside the work should be tied back or covered against overspray of cleaning solutions, and that automatic sprinkler systems should be turned off during cleaning, sealing and drying.

Put together, the roofing lines say protect the plants, and the paving lines describe how one neighboring trade does that for its own work. The full sequence, step by step, is set out under what is under the roof while the work happens. None of the documents presents a step as removing the risk.

Which precautions a particular house needs depends on what grows where. A free estimate is the place to raise it, and the project details field on the form can carry the useful facts: beds or hedges tight against the house, a sprinkler zone on a timer, or plants an owner especially values.

The published steps all happen before or during the work: precaution and protection in ARMA and GAF, pre-wetting and covering in ICPI, and a rinse-down at the end of the roof procedure. After that, the documents stop.

No trade association or roofing maker document read for this answer publishes a step that neutralizes plants or soil after a roof wash, and the word neutralize does not appear in any of them. That absence is part of the answer. A claim that solution on a plant can simply be reversed afterward has no published procedure behind it, and neither does a claim that a roof wash is harmless to what grows beneath it.

What the documents support is narrower and more honest: the risk is real, it is known well enough to be written into the instructions, and care before and during the work is what the documents prescribe to lower it. Plants are not the only living things in the yard, and animals get their own answer under whether pets should stay inside during roof cleaning, while people get theirs under whether you can stay home during roof cleaning.