Dwell time

For asphalt shingle, ARMA publishes the window in two documents. Its technical bulletin says “The solution should be left on the roof for at least 15 minutes but no more than 20 minutes,” and its algae and moss page tells the applicator to “allow the solution to dwell on the roof surface for 15 to 20 minutes” before a low-pressure rinse.

The two documents do not read identically at the far end. The bulletin sets 20 minutes as the most, while the algae and moss page adds that “Extended dwell times may be necessary,” with one condition: “avoid letting the solution dry completely as this may prevent complete rinsing.” Neither window is printed here as a rule for any roof but asphalt shingle.

GAF's shingle bulletin gives a window of its own for its bleach, water and TSP mixture, applied with a garden sprayer: “let sit on the roof surface for up to 20 minutes, and rinse with low-pressure water.” GAF states a ceiling and no minimum, and the instruction is GAF's for its own shingles.

Metal runs on a shorter clock and a different chemistry. The Metal Roofing Alliance lets its mild detergent solution “stand for 5 to 10 minutes, before thoroughly rinsing the surface with plain water.” A detergent window on metal is not a bleach window on shingle, and neither carries over to the other roof. No tile dwell window appears in the documents read for this page; a tile dwell is the tile maker's instruction. None of these documents names a water temperature either, and whether a soft wash uses hot water has its own answer.

Dwell exists because of how the method is built. In a soft wash the cleaning solution, not the force of the water, acts on algae, mold or moss, and it is given minutes on the surface to do that; the low-pressure part of the method is defined at soft washing.

Every published window here ends in a rinse, and ARMA warns that the application and rinse process “will make the roof slippery and potentially hazardous to walk on during treatment.” How the publishers word that last step is collected at rinse-down, whether a roof gets rinsed after soft washing has its own answer, and the full sequence is at how soft washing works.

Dwell time also says nothing about the plants below the roof edge, which is where pre-wetting comes in, or about whether the growth is gone for good, which is answered at whether soft washing kills or removes algae.