Will roof cleaning stain my pavers?

Follow the water first. A cleaning solution goes on at the top of the slope, sits for its dwell, and is rinsed. The rinse water carries what is left of the solution, and the growth it loosened, down the slope and off the edge of the roof. From there it lands on whatever is below the eave: a planting bed, a walkway, a patio or a paver driveway.

That path is a mechanism, not a finding. No document read for this page measures how much of a roof wash reaches the ground, how diluted it is by the time it lands, or what it does to a paver when it gets there. The runoff from the rinse and any overspray that drifts on the wind both end up below the roof, and the documents stop short of saying what either does to the surface they land on.

The roofing documents do acknowledge that the path exists. ARMA's technical bulletin on algae discoloration, right after it describes a solution of household bleach and water, tells anyone applying it: "Be sure to apply it carefully to avoid damage to other parts of the building and the surrounding landscape." The Asphalt Roofing Manufacturers Association's prevention bulletin adds a call for proper precautions to protect landscaping and surrounding areas. Neither bulletin names pavers, and neither measures anything.

The chemistry in ARMA's shingle procedure is not foreign to pavers. The Interlocking Concrete Pavement Institute's Tech Spec 5, in its February 2018 revision, lists household bleach, with a stiff bristled brush, as the treatment for leaf, wood rot and tobacco stains on concrete pavers. In that document bleach is a paver cleaner, used on purpose.

Natural stone has a separate line from the Natural Stone Institute, a mild bleach solution for algae or moss in outdoor stone areas, and it is written about stone such as travertine, not about concrete pavers.

The color warning ICPI does print is about a different chemistry: "Cleaners with strong acids will change the color of the pavers slightly." None of that means a bleach solution is harmless to every paver or every sealer. ICPI's list says what the institute uses bleach for, not what diluted roof runoff does, and whether a particular sealer tolerates a bleach solution is a question for that sealer's maker. Whether bleach itself is hard on concrete pavers, stone or a sealer is argued in full under will bleach hurt my pavers.

The staining risk ICPI does describe is about where a cleaner ends up and how long it stays there. Tech Spec 5 tells the person cleaning pavers to anticipate where the cleaning fluids will drain, across the pavement and not onto grass or vegetation, and then warns: "Sediment or cleaners allowed to pond in low spots may stain the pavers."

The same document asks for a test before the whole surface sees a cleaner: "In order to achieve proper results, cleaners should be tried on a small area to test results and any color changes." ICPI writes both lines about a cleaner applied to pavers on purpose, not about water coming off a roof, and no source read for this page measures how diluted roof runoff behaves once it collects in a low spot. What carries over is only the pair of conditions ICPI names: a cleaner left standing in a low spot, and a surface whose reaction to a cleaner has not been tested.

ICPI adds site steps for any cleaning or sealing job: turn off all automatic sprinkler systems during cleaning, sealing and drying, cover and protect the surfaces and vegetation around the work, and tie back or cover low branches and shrubs against overspray. How pre-wetting, covering and the sprinklers fit around a roof job is set out under what is around the roof while the work happens. Where the wash water is allowed to drain is a separate matter of county and district rules, covered at what happens to the runoff from roof cleaning.

A paver surface can look uneven for reasons that have nothing to do with a roof. ICPI explains that if pavers must be replaced, there may be a difference in color from the surrounding pavers, and that this variation should eventually disappear. Its other two color lines, strong acid cleaners and cleaners left to pond, are both about cleaning the pavers themselves.

A cleaner is not the right person to decide which of those explains a given mark, and this page does not try. What the documents support is narrower: the conditions that stain pavers are named, the roof-runoff case is not measured, and whether a particular sealer tolerates a bleach solution belongs to that sealer's maker. How the common paver stains come off, and what a paver cleaning removes, is set out under what comes off in a paver cleaning.

• Both jobs on one property: the order that follows from the documents is reasoned at should the roof be cleaned before or after the pavers.
• Pavers sealed only days earlier: a curing sealer raises its own question, answered at a roof cleaned right after the pavers were sealed.