Why is my roof turning green?

A patch that looks gray and dry through a rainless stretch can be the same patch that reads green after a wet week, with nothing new added to the roof in between.

The U.S. Forest Service describes the switch in one line: when lichens are wet, they turn on and start photosynthesizing and growing, and when dry, they turn off, become brittle and go dormant. A dry lichen is usually gray, or the color of the fungal cells on its upper surface, the fungus being one of the two partners in a lichen.

So a roof carrying lichen can look gray and dull for weeks at a stretch and still be carrying the colony the next rain shows up green. The color change needs water, not necessarily a new arrival.

Once a lichen is wet, the fungal cells on its upper surface become transparent, the Forest Service writes, and the algal cells underneath get a chance to show their color.

Which color depends on the partner. Green algae generally give a lichen a bright green color when wet, the Forest Service writes, while cyanobacteria give it a dark green, brown or black color, so not every wet lichen reads green. UF/IFAS Extension describes a lichen as a fungus and an alga or cyanobacterium growing together, the fungus providing structure and protection and the alga providing carbohydrates; it is the photosynthetic partner, not the fungus, that shows through when the lichen is wet.

Cyanobacteria are the organisms the roofing trade calls blue-green algae. The EPA describes them as microscopic organisms that use direct sunlight to produce their own food, and UF/IFAS Extension says they are true bacteria, neither plant nor fungus.

A lichen switches on and off with the weather. Moss asks for something steadier.

ARMA's bulletin on asphalt roofing describes moss as a non-vascular plant that takes in water through its leaves rather than through roots, so it must have a moist environment to survive.

That is a different requirement from a lichen's. A lichen can go dormant through a dry spell and show green again after rain; moss, by ARMA's description, needs the moist environment itself. Raised, fuzzy green growth, and how it differs from a flat crust or a stain, is answered on the green fuzz on a roof.

Green growth and black streaks are described differently in the roofing documents, and ARMA says not to confuse them.

ARMA's bulletin describes roof algae discoloration as usually brown to black, and says algae discolorations should not be confused with moss or tree droppings, which typically produce only localized discoloration. What the black streaks are, and the dark pigment behind them, is answered on what the black streaks on a Naples roof are.

On tile, the Tile Roofing Industry Alliance says mildew, algae and moss do not grow directly on the tile but on the accumulation of dust and other organic material that settles on it, and that spores start to grow after enough cycles of heat, moisture and light. What is actually growing on a given roof, and whether any shingle under it has lifted, is a question for a licensed roofing contractor. Why a roof can look worse after a wet dry season is set out on a roof that looks worse after a wet winter, and what overhanging trees add is covered on whether palm trees make roofs dirtier.