Why does a white tile roof turn black?

The change on a white tile roof starts with something too thin to notice. The Tile Roofing Industry Alliance (TRI Alliance) puts it plainly: “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.” The film comes first. The growth comes to the film.

In general biology that is how a biofilm gets its start. CDC's journal Emerging Infectious Diseases, writing about biofilms on surfaces of every kind, says attachment occurs most readily on surfaces that are rougher, more hydrophobic and coated by what it calls conditioning films. Treating a layer of settled dust on a roof as that kind of film is a bridge from a general finding, not a sentence TRI Alliance wrote, but the two point the same way.

No source read publishes how long that film takes to build or how soon growth follows. TRI Alliance lists what affects the potential for growth instead: “location of the structure, the direction the roof is oriented to the sun and the quantity of trees, canals and lakes in the area.” Its guide covers concrete and clay roofing tiles, barrel tile among them, and nothing on that list depends on what color the tile is.

Once growth takes hold, the color follows. The Asphalt Roofing Manufacturers Association (ARMA) writes in its June 2024 bulletin: “Natural pigments produced by these algae may cause white or light-colored roofs to gradually turn dark brown or black. Discoloration may appear uniform or as streaks.” Gradually is ARMA's word, and neither of its bulletins puts a date on the change.

Put in order, TRI Alliance's tile guide and ARMA's two bulletins, one of them written for asphalt roofing, describe a sequence without a timetable:

• Dust and organic material settle on the tile (TRI Alliance).
• Cycles of heat, moisture and light let spores start to grow on that layer (TRI Alliance).
• The discoloration begins as small spots and spreads into streaks on the roof plane (ARMA's algae and moss bulletin).
• By the time it is visible, ARMA says, the growth has likely been present for several months or longer.
• The roof reads dark brown or black, either evenly or in streaks (ARMA's June 2024 bulletin).

The black itself is a pigmented sheath the organism makes against ultraviolet light, which is explained in why a roof has black streaks. ARMA names Gloeocapsa magma as probably the most prevalent of several algae species that contribute to that discoloration, and the field description of the streaks on tile and shingle is on the roof cleaning page, under the black streaks.

White tile is where the pigment has the most contrast to work against. ARMA's point that the discoloration shows most readily on light-colored roofs concerns how soon the dark is seen, not how quickly the growth arrives, a difference worked through in whether white roofs need cleaning more often.

The last question is whether the tile changed. Eagle Roofing Products, a concrete tile manufacturer, answers for its own tile: “Throughout the years, however, homeowners may notice a buildup of algae, mold and mildew. This is typical of all natural materials and, while these types of biological pollutants are not harmful to the integrity of the tile, some homeowners may consider their appearance less than desirable.”

Read that sentence for exactly what it covers. It is Eagle speaking about concrete tile and about biological growth: algae, mold and mildew. It says the growth is not harmful to the integrity of the tile. It is not a finding that any particular white roof is in good condition, and nothing seen from a driveway or a ladder turns it into one. The black is a layer on the tile; what the tile under that layer is like is a separate question.

Not every change in a light tile is growth, either. TRI Alliance says slurry-coated concrete tile “can oxidize and turn chalky before gradually wearing down to the concrete base,” a change in the tile's own coating, and toward chalky rather than dark. Which change is on one roof is not something this answer can name.