Can mildew grow on a metal roof?

U.S. Steel's guide to cleaning painted metal buildings names the condition mildew needs: a surface that stays moist for extended periods, such as a north wall, the underside of an eave or a sheltered area.

The same guide traces the start of it to something ordinary: dust carries a sufficient supply of organisms to begin growth anywhere, so on a metal roof the question is less whether organisms arrive than whether a spot stays wet long enough. It adds that a visible growth of mildew holds moisture easily, so a patch that gets started keeps some of its own damp.

Mildew sits inside a wider category. UF/IFAS Extension says mildew is mold, and the U.S. Environmental Protection Agency says molds reproduce by tiny spores, invisible to the naked eye, that float through outdoor and indoor air, and that no type of mold will grow without water or moisture. On tile, the Tile Roofing Industry Alliance makes a parallel point: mildew, algae and moss grow not on the tile itself but on the dust and organic material that settles on it. Metal is a different surface, and U.S. Steel's dust sentence is the one written for it.

The Metal Roofing Alliance draws the comparison itself: unlike asphalt roofs, which it says can be prone to moss, mildew and fungus, upkeep for a quality metal roof is simple, thanks to smoother surfaces and protective coatings.

Simple is a comparison, not an exemption. The Alliance does not say mildew cannot grow on metal, and the Asphalt Roofing Manufacturers Association, in its bulletin on algae discoloration, says almost all types of roofing systems are susceptible to algae discoloration, most readily visible on light-colored roofs and not so visible on darker shades. That line is about algae and about what shows, not a ranking of materials, but it leaves no roof type outside the question.

Neither document measures how much less growth a metal roof carries than a shingle roof does, and no such number appears on this page.

A dark or blotchy patch under an eave and a pale, dusty film across a sunny slope are two different conditions with two different mechanisms.

The first is the growth this page is about, and what the word covers is set out on what mildew is. The second has nothing living in it at all.

That second mechanism is chalk. U.S. Steel's guide describes it as a white by-product of ultraviolet degradation of the paint system, a film that builds over several years, makes colors appear lighter and is only partly washed off by rain. Chalking increases with sun exposure, the guide says, so the roof and the south side of a building may chalk more, and it is more noticeable on darker colors. Chalk across roof materials is answered on why a roof looks chalky.

The Metal Roofing Alliance's own page on coatings makes a claim for the paint itself, not for any cleaning done to it: painting adds a protective layer to the metal roof that can help prohibit rust, corrosion, UV damage and weather damage.

That sentence names four jobs for the coating, and mildew is not one of them. A paint credited with helping prohibit rust and corrosion is not thereby credited with stopping a biological growth, and the page that makes the claim is silent on it. Rust is the everyday example of oxidation: iron combining with oxygen, a chemical change in the metal rather than anything growing on the paint.

A mildewcide is a different subject again, an agent meant to kill or suppress mildew. Whether a given paint or product carries one, and for how long it works, is that product maker's claim to make, not something a metal roofing guide or a cleaner can state.

Which product and method take mildew off a given metal roof is a question for that roof's manufacturer, and a coating or finish question belongs with a licensed roofing contractor.

The Metal Roofing Alliance's own detergent and dwell instructions sit beside what the tile and shingle makers publish where each maker's roof cleaning instructions are compared.

Pressure is its own question. A figure one maker published for its tile or its membrane does not carry over to painted metal, and why pressure figures stay with the material they were written for is set out on the soft washing page.