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.

How to choose

"Soft wash" and "low pressure" are not synonyms

Soft wash includes the cleaning solution and dwell time. Low pressure alone can just mean gentle water, which will not kill the algae.

Ask for dwell time and rinse to be described

ARMA's method is dwell 15-20 minutes then rinse thoroughly at low pressure. A contractor who cannot describe their dwell and rinse is improvising.

Learn to spot granule loss

It looks like coarse sand in the gutters or at the downspout outlet, and like bare, shiny patches on shingles. It is the clearest evidence of a bad method.

Know which growth you have before you shop

Algae, lichen, and moss need different amounts of work and carry different urgency. Using the right word gets you a more honest quote.