Roof Cleaning glossary

Soft wash - applying a cleaning solution at low pressure so chemistry, not force, removes the growth. Dwell time - how long the solution sits on the roof before rinsing. ARMA describes allowing the solution to dwell 15 to 20 minutes, then rinsing thoroughly with low-pressure water. Downstreaming - the industry term for drawing cleaning solution into a low-pressure stream at the pump so it is diluted and applied without high force.

Granules - the mineral coating on asphalt shingles that shields the asphalt from UV. Granule loss - when those come off. ARMA warns that pressure washing causes granule loss and very likely premature failure of the roof system, and that scrubbing with a brush or broom loosens and removes them through friction. This is the damage that matters.

Gloeocapsa magma - the blue-green algae ARMA identifies as the most common cause of roof discoloration. It produces a dark pigmented sheath to protect itself from UV, which is the black streak you see. Lichen - a fungus-and-algae organism that bonds to the shingle surface and is much harder to remove than algae. Moss - a true plant that retains moisture against the roof and can lift shingle edges.

Algae-resistant (AR) shingles - shingles made with copper-containing granules that inhibit growth. Malarkey describes premium lines using at least a 10% copper granule blend, and ARMA notes such products are designed to inhibit algae growth for extended periods. Zinc/copper strips - metal strips installed near the ridge that release ions in rainwater to suppress regrowth; per Malarkey, ideally installed at new roof installation rather than retrofitted.