How long does a roof stay clean after a soft wash?

Start on the day after the wash. The Asphalt Roofing Manufacturers Association (ARMA) describes what happens next on a shingle roof treated with its bleach-and-water procedure: the treated algae disappears and is carried off by the rains that follow. The bulletin does not say how many rains, or how many days, and neither does any other document read for this page.

ARMA is just as careful about what the treatment does in the first place. Its technical bulletin on algae discoloration reads "Discoloration may be lightened by applying a solution of liquid household bleach (sodium hypochlorite) and water." The verb is lightened, not removed, and the same bulletin states the limit plainly: "The effectiveness of such cleaning is only temporary, and the discoloration may recur."

So the first stretch after a cleaning is the roof shedding what was treated, not the start of a countdown. What those black streaks on a roof are made of, and why they sit where they do, is set out on the roof cleaning page. Whether a treatment kills the growth or only takes it off the surface is a separate question, answered at does soft washing kill the algae or just remove it.

A clean roof is not a sealed one. ARMA writes that the organisms behind the stains share one habit: "All species are transported through the air and tend to collect and grow on roofing structures and other outdoor surfaces." Nothing about a cleaning stops the air from delivering more of them.

GAF's current technical bulletin on blue-green algae says the same thing from the shingle maker's side: "Blue-green algae spreads from one infected roof to another via airborne spores." ARMA's prevention bulletin adds that the spores are carried by the wind or by animals, and that this is why growth across an affected neighborhood is common. A washed roof sits in the same air as every roof around it.

On tile, the Tile Roofing Industry Alliance (TRI Alliance) explains where new growth takes hold: "Mildew, algae and moss do not grow directly on tile but on the accumulation of dust and other organic materials that settle on tile." Dust keeps settling after a cleaning, and TRI Alliance says what starts the next round: "With enough cycles of heat, moisture and light, spores start to grow." ARMA notes that algae growth occurs “to varying degrees” in all regions of the U.S. and southern Canada, “especially those subjected to warm, humid conditions.” Why one particular roof darkens faster than another, with shade, dew and trees in the picture, is its own page: why a roof is dirty again after a year.

The step that makes a duration impossible to promise is the one nobody can watch. ARMA's prevention bulletin puts it in two sentences: "The algae discoloration typically begins as small spots then quickly transforms into streaks on the roof plane. By the time the algae is visible, it likely has been present for several months or longer."

That lag cuts both ways. A roof can look spotless while new growth is already underway, and the day the first spots appear is not the day the growth began. Any figure for how long a roof will stay clean would have to predict something that, by ARMA's own account, is invisible for months. ARMA does not tie that lag to a roof's age, a point that matters for whether new roofs need cleaning.

The one roofing maker read for this page that names a trigger names a sign rather than a date: Eagle Roofing Products' spring article on concrete tile points to significant discoloration on the roof. It names what the roof looks like, not how much time has passed, and how that sign sits against the Naples calendar is weighed under the right time of year for roof cleaning in Naples.

No roofing manufacturer or trade association read for this page publishes how long a cleaning lasts. The only sourced statements are ARMA's "only temporary" and "may recur", and one sentence from TRI Alliance: "If the growth returns, treatment can be repeated as necessary." That sentence describes recurrence. It does not set a cleaning interval, and TRI Alliance does not name one anywhere in its guide.

A number in years, then, is a claim the documents do not support, whoever prints it. TRI Alliance also states that there is currently no known manufacturing procedure or job-site coating with a long-term preventive effect on the growth. What a cleaning does not do for a roof is set out under where a soft washing job stops. How often a Naples roof gets cleaned is answered on the roof cleaning page without an interval, and whether a second wash can follow soon after the first is taken up under repeat roof cleaning in the same year.