Does soft washing kill the algae or just remove it?

The first thing that happens is chemistry. A soft wash puts a cleaning solution on the growth and gives it time, and the roofing documents describe what that does in careful verbs. The Asphalt Roofing Manufacturers Association (ARMA) writes that “Discoloration may be lightened by applying a solution of liquid household bleach (sodium hypochlorite) and water.” Lightened is ARMA's word for what the solution does, and whether a wash can be said to kill the algae depends on whose words are read next.

The Tile Roofing Industry Alliance (TRI Alliance) uses two verbs side by side. It writes that mildew, algae and moss “can be removed or treated,” and it explains why tile holds the growth at all: they “do not grow directly on tile but on the accumulation of dust and other organic materials that settle on tile.” Removed and treated are both on the table in one sentence, which is the honest starting point for this question.

A membrane maker spells the two stages out for its own product. Duro-Last describes its roof wash as “formulated to kill the biological growth so it can be cleared off” with a hose, a push broom or a power washer, all on its single-ply membrane. That line is Duro-Last's, for its roof, and it is quoted here only because it names both steps plainly: first the growth is killed, then it is cleared.

The word kill also has a regulatory use: the US Environmental Protection Agency names pesticide classes by what they kill, and the class named for algae, with the trade's spelling, is explained under algaecide, defined. No roofing body quoted here calls its cleaning solution by any of those class names, and whether a particular product is registered as one is on that product's label.

ARMA's “Algae & Moss Prevention and Cleaning for Asphalt Roofing Systems” describes the next stage for asphalt shingle. After its bleach treatment, it writes, “Algae will disappear and” wash away “with subsequent rains.” That is a sentence about time. What was treated on the day keeps leaving the roof as later rain falls on it, and ARMA names no number of days or storms for it.

This is why the question of kill or remove does not resolve on the day of the wash. On shingle, ARMA does not want the clearing hurried by force either: its technical bulletin says “DO NOT use a power washer or any type of brush or broom to clean algae from the roof surface.”

Moss runs on a slower clock in the same ARMA document. “Moss will loosen over time and may be removed with a leaf blower. In severe cases, it may take more than one bleach treatment to kill all of the moss.” The document uses kill here, for moss, and it pairs the word with a caution that one treatment may not finish the job.

So for moss the sequence is stretched out further than for algae: a treatment, a period while the moss loosens, and in heavy growth possibly another treatment before all of it is dead. In ARMA's text, loosened moss may be removed with a leaf blower, not with pressure.

ARMA's technical bulletin also tells algae discoloration apart from moss or tree droppings, which it says typically produce only localized discoloration, and it sends questions about moss on asphalt shingle to the shingle manufacturer. How moss behaves on roofs in this climate is its own answer: moss on Florida roofs.

The last stage is the one the roof cleaning page already sets out as an honest expectation about roof streaks: ARMA calls such cleaning only temporary, and TRI Alliance writes that treatment “can be repeated as necessary” if the growth returns.

Put that beside the first three stages and the answer holds together. On the day the solution treats the growth; over the following rains the treated algae clears; moss loosens more slowly and may need a second treatment to kill. None of that makes a roof immune. How long a roof tends to look clean afterward is a separate question with its own answer on how long a roof stays clean.

Whether a particular roof solution counts as a biocide under EPA's terms is, again, a question for its label, and no roofing document quoted here turns any of these stages into a promise about how long a roof stays free of mildew, algae or moss.