Does roof algae damage tile?

Eagle Roofing Products, a concrete roof tile manufacturer, takes up whether algae damage tile in a single sentence of its own cleaning advice.

Its 2019 article on cleaning concrete tile says that over the years owners may notice a buildup of algae, mold and mildew, and calls that “typical of all natural materials.” Then it draws the line this page is about: “these types of biological pollutants are not harmful to the integrity of the tile,” though some owners, Eagle adds, may consider their appearance less than desirable. Integrity on one side, appearance on the other. The growth is a problem of how the roof looks, in Eagle's framing, and not of what the tile is.

Two limits come with that sentence, and they matter as much as the sentence does. It is a maker's statement about its own concrete tile, not a laboratory measurement, and it says nothing about clay, shingle or metal. It is also not a reading of any particular roof. What the dark color on a roof actually is, and why it forms in streaks, is set out on the roof cleaning page's section on the black streaks on a roof.

The Tile Roofing Industry Alliance, the association for concrete and clay tile, describes where the growth actually sits, and its answer is a layer of settled dust rather than the tile itself.

TRI Alliance's Roof Maintenance Guide says mildew, algae and moss “do not grow directly on tile” but on the dust and other organic material that settles on it, and that spores start to grow with enough cycles of heat, moisture and light. Call the resulting layer a biofilm or simply a stain; TRI's point is its address. It sits on top. The guide's word for the result is a “dull surface appearance,” and it adds that the growth “can be treated to restore the original look.”

TRI writes about tile in general here, concrete and clay, and does not split the point by profile, so it reads the same for a barrel tile as for a flat tile with no rolls at all. What TRI will not say is that anything keeps the growth away for good: it knows of no job-site coating with a lasting preventative effect, a line read in full in whether a tile roof needs sealing after cleaning.

Read the same tile documents for the word damage and it turns up beside other things: a foot, a broken edge, a cleaner that does not suit concrete.

• Broken tiles. TRI Alliance says tiles damaged by foot traffic or otherwise should be replaced, because broken tiles may cause problems by “allowing water and UV rays to penetrate through to the underlayment and damage it.” That is the tile problem that reaches past the surface, and it is where the roof leak paths the sources name begin.
• Foot traffic. TRI's walking guidance tells anyone who must walk a tile roof to use caution to prevent damage to roof tiles “that may void your warranty,” and to keep off hips and valleys, where the specially cut tiles are harder to replace.
• The wrong cleaner. Eagle says a cleaner used on its concrete tile should be compatible with concrete, because “there are products that can affect the PH of the tile or even etch the surface.” Eagle also publishes a pressure ceiling for its concrete tile, and like every such figure it belongs to one material; roof pressure numbers do not travel from one surface to another.
• Surface changes TRI does not call damage. TRI says slurry-coated concrete tile can oxidize and turn chalky as the coating wears, a kind of chalking of the finish, and that later erosion will never affect the physical properties of the tile. It calls efflorescence “superficial” and says it in no way affects the tile's quality or function.

Algae is not on that list as a cause of harm to the tile body. That absence is the honest core of the answer: the sources that talk about tile damage tie it to force, breakage and chemistry, and the tile maker that speaks to the growth directly says it is not harmful to the integrity of the tile, only that some owners may find its look less than desirable.

It is tempting to hear Eagle's sentence as a promise that any dark tile roof is sound underneath. No source says that, and this page does not either.

The growth and the condition of the tile are separate questions. A roof can carry algae and a cracked tile at the same time, and nothing about the color tells you which tiles are whole. Eagle's Owner Information sends inspection and repair to the original roofing contractor and, if that contractor is unavailable, tells the owner to “select another qualified licensed roofing contractor.” Whether a cleaning at least makes a cracked tile easier to spot is taken up in whether a clean roof makes damage easier to see.

Asphalt shingle is a different material with a different documented position, and ARMA's two-sided reading of it is set out on the page about whether algae damages asphalt shingles. Other roof questions, from pressure to walking, are gathered in the roof and exterior cleaning answers.