Can old tile stains be removed completely?

An old stain starts out young and invisible. ARMA's prevention bulletin says “By the time the algae is visible, it likely has been present for several months or longer.”

The same bulletin describes how it shows itself: small spots first, then streaks running down the roof plane. On tile, the Tile Roofing Industry Alliance says the growth is not in the tile at all. In its words, “Mildew, algae and moss do not grow directly on tile but on the accumulation of dust and other organic materials that settle on tile.”

No document read for this answer says how a stain's age changes a cleaning, and ARMA's line that algae discoloration can be difficult to remove carries no age at all. What the dark streaks on a roof are covers the organism and the pattern it leaves. Lichen is its own case, with its own answer in whether lichen comes off a roof the way algae does.

ARMA's technical bulletin does not promise a clean slate. It says “Algae discoloration can be difficult to remove from roofing surfaces,” and when it describes its bleach-and-water solution for asphalt shingle, the word it uses for the discoloration is “lightened.”

TRI Alliance, writing about tile, says mildew, algae and moss “can be removed or treated.” Neither association says a single treatment returns a roof to the color it had before the growth. That is the honest ceiling on the word completely.

Moss gets its own warning, and it is written for shingle. ARMA's prevention bulletin says “In severe cases, it may take more than one bleach treatment to kill all of the moss.” That is ARMA's line about moss on asphalt roofing. It is not a figure for algae and it is not a figure for tile, and no tile document read for this answer gives a count of treatments for anything.

Whether a treatment kills the growth or only takes it off is a separate distinction, and whether a soft wash kills algae or only removes it handles it. This page names no chemistry and no dwell for tile, because those belong to the roof maker and to the job.

The day of the cleaning is not the last word in ARMA's account. Its prevention bulletin for asphalt roofing says treated algae will disappear and wash away with the rains that follow.

So on a shingle roof an old stain can look different after later rains than it did when the rinse water ran off. ARMA gives no count of rains and no number of weeks, and nothing here supplies one. What it describes is a process that keeps going after the wash, not a result that is fixed the moment the work ends.

Two other things can make a roof hard to judge in the days after a wash: a slope that has not finished drying, and on concrete tile the temporary light bloom that TRI Alliance and Eagle Roofing Products call efflorescence. Why some tiles look darker than others after cleaning sorts those reasons without reading any particular roof.

Not every old discoloration on a tile roof is a stain sitting on top of the tile. Some of it is the tile, and a cleaning does not reach that.

In TRI Alliance's account a slurry coat can oxidize, chalk and wear toward the concrete, and integral color can lighten at the surface; both are changes in the tile rather than deposits on it, and whether roof cleaning brightens the color of the tile follows them in detail.

Take the growth off a chalked slurry coat and what shows is the chalking itself. That can read as an old stain that did not come out, when it is the tile's own aged surface. Why a roof looks chalky covers that surface, and whether roof stains are permanent sorts every kind of mark by whether it lasts.

TRI Alliance pairs periodic cleaning and resealing with the surface finish of cement or clay tile, and resealing is a coating step. Florida Statutes 489.105(3)(e) places coating with the roofing contractor “except when coating is not represented to protect, repair, waterproof, stop leaks, or extend the life of the roof,” so a coat on a tile roof is a question for a roofing contractor and not part of a cleaning.

Some old marks on a tile roof are not growth and not the tile's color either: orange-brown streaks below metal, mineral deposits, a splash of paint. None of the roofing documents read for this answer names a treatment for them.

ARMA writes about algae and moss. TRI Alliance writes about growth, color and efflorescence. Eagle's guidance covers water, dilute cleaners and foot traffic. None of those documents addresses a rust, mineral or paint mark on roof tile, so this page names no method for one and does not suggest that a cleaning takes it off.

That kind of mark goes to a roofing contractor. Eagle's owner information says that if an inspection is wanted or repair work is needed, the owner should contact the original roofing contractor, or another qualified roofing contractor if that one is unavailable, and TRI Alliance sends a roof still under its installation warranty to that contractor for all inspections or repairs.