Why is my roof dirty again after a year?

ARMA's prevention bulletin puts the start of the story outside the property line: “Algae spores are carried by the wind or by animals and can quickly spread from one rooftop to another. That is why it is common to see algae growth on rooftops throughout an affected neighborhood.”

GAF's current technical bulletin describes the same traffic from the shingle maker's side: “Blue-green algae spreads from one infected roof to another via airborne spores. These spores land randomly on a roof and take root.” And ARMA's technical bulletin adds that all the species involved are transported through the air and tend to collect and grow on roofing structures and other outdoor surfaces.

A cleaning changes one roof. It does nothing to the roofs, trees and other outdoor surfaces around it, and those are where the next spores come from. A washed roof in a street of streaked ones is back in the path of the same airborne supply the day the rinse water dries. How that spread works, roof to roof, is set out in how roof algae spreads from house to house.

Spores need somewhere to grow, and the Tile Roofing Industry Alliance says that on tile it is not the tile: “Mildew, algae and moss do not grow directly on tile but on the accumulation of dust and other organic materials that settle on tile. With enough cycles of heat, moisture and light, spores start to grow.”

A cleaning takes that film off. It does not stop the next film from settling. Dust keeps arriving on the wind, leaves and pollen keep landing, and the surface starts collecting again the day after it is rinsed.

Water does not wait for the rain. GAF's bulletin says blue-green algae requires moisture and proper temperatures to grow and names dew as the primary moisture source, and ARMA's technical bulletin says algae growth occurs “to varying degrees” in all regions of the U.S. and southern Canada, “especially those subjected to warm, humid conditions.” Why Naples roofs stain so quickly sets those conditions against the Naples climate normals.

The year a reader counts is the time until the return could be seen, and ARMA says the growth runs well ahead of that: by the time roof algae is visible, it has likely been there for several months or longer.

ARMA also describes the order it appears in. The discoloration typically begins as small spots before it turns into streaks down the roof plane. So a roof that looked clean for months and then streaked over a few weeks was likely regrowing through the months that looked clean. The streaks were the late stage, not the start.

That is also why “a year” is the reader's measurement and not a published one. No roofing maker or trade association read for this answer says how long a cleaned roof stays clean. Writing about its bleach-and-water method for asphalt shingle, ARMA's words are that “The effectiveness of such cleaning is only temporary, and the discoloration may recur,” and it attaches no time to either half. The duration question has its own answer in how long a roof stays clean after a soft wash, and that answer does not supply a number either.

TRI Alliance lists the environmental variables that affect the potential for growth on a tile roof: where the structure sits, which way the roof faces the sun, and how many trees, canals and lakes are in the area. A cleaning changes none of them.

Orientation is the plainest case. GAF's bulletin says blue-green algae generally grows on the northern exposures of a roof because that exposure receives less sunlight, and a cleaning does not change which way a slope faces, so the conditions that favored one side before the wash still favor it afterward. Why one exposure wins is taken up in why roof algae grows on the north side.

Trees add shade and the debris that settles into the film, and a lot backing onto a canal or a lake sits beside water all year. Why a roof gets dirty under trees covers the overhanging branch in detail. Moss is a separate matter on shingle: ARMA says that, unlike algae, moss can be detrimental to asphalt shingle performance, and what moss has done to a particular shingle roof is a question for a roofing contractor.

TRI Alliance says it directly: “While research continues, currently there is no known manufacturing procedure or coating applied on the job site that would have a long-term preventative effect on this condition.”

The same guide describes what happens instead. TRI Alliance writes that mildew, algae and moss can be removed or treated, and “If the growth returns, treatment can be repeated as necessary.” That is a statement that growth comes back and can be dealt with again. It is not a schedule, and TRI attaches no count or calendar to it.

Eagle's trigger for a cleaning is the roof's appearance: in Eagle Roofing Products' 2024 spring article the cue is significant discoloration, which is a sign and not a date. The same logic runs from the first day a roof goes on, which is why whether new roofs need cleaning is answered by signs rather than by age.