What is the green fuzz on my roof?

Before any name, look at the texture of the green fuzz, because each of the three growths that turn part of a roof green is built differently.

• Fuzz you could pinch: a soft, raised cushion with a pile to it is the shape of moss, a plant rather than a stain.
• A crust you could not lift with a fingernail: a layer that looks painted onto the surface, or a leafy one with lobes, is the shape of a lichen.
• A patch that looks green only after rain: gray on a dry afternoon and green after a shower is how the U.S. Forest Service describes a lichen behaving.

Moss first. The Asphalt Roofing Manufacturers Association (ARMA) describes moss as a non-vascular plant that “obtains water through its leaves, unlike most plants that gather water through its root system,” and draws the consequence in its next sentence: “Moss must therefore have a moist environment to survive.” The glossary entry for moss carries the full definition.

Lichen next. UF/IFAS Extension describes lichens as “a fungus and an alga or cyanobacteria growing together in a mutually beneficial relationship,” in which the fungus gathers water and minerals and gives the alga structure and protection, and the alga supplies carbohydrates. The same publication says lichens “may be flat, leafy, crusty, powdery, or hair-like” and that their colors run from white and gray to red, green, yellow or black, so green is only one of their looks. A second UF/IFAS publication describes the crusty kind as existing “as if painted onto the surface they grow on.” The lichen glossary entry covers the partnership in more depth.

The third look is the one that fools people. The U.S. Forest Service explains that a lichen switches on and off with water: wet, it photosynthesizes and grows; dry, it becomes brittle and goes dormant.

Its color follows the same switch. “When a lichen is dry, its color is usually gray or colored like the fungal cells on the upper cortex,” the Forest Service writes, and when it is wet those cells turn transparent so the algal partner shows through. Green algae partners, in the Forest Service's words, “generally give the lichen a bright green color when wet,” while a cyanobacterium partner turns it dark green, brown or black.

So a lichen that is green one morning and gray by afternoon has not come and gone; it has dried. Long dark streaks down a whole roof plane are a different pattern: ARMA describes algae discoloration as usually brown to black, and the black streaks on a Naples roof are explained on the roof cleaning page. A whole roof slowly taking on a green cast is a separate question again, answered under why a roof turns green.

Location narrows it further. ARMA writes that in North America moss “tends to grow on north facing roof planes that receive less direct sunlight and stay damp longer than south facing planes,” and that overhanging branches add shade and drop debris that “further holds in moisture and acts as a food source for moss.” It adds that moss spores “may also be carried by wind or animals and can easily spread throughout neighborhoods.”

On tile, the Tile Roofing Industry Alliance (TRI Alliance) says mildew, algae and moss “do not grow directly on tile but on the accumulation of dust and other organic materials that settle on tile,” and that spores start to grow with enough cycles of heat, moisture and light. The settled layer, not the tile, is what that growth takes hold on.

Lichens take hold on a wide range of surfaces. The Forest Service counts houses among the things that hold still long enough for a lichen to attach, and UF/IFAS lists tree bark, rocks, soil, fence posts, fences and window screens. ARMA also separates growth that covers a roof from growth that marks one spot: algae discolorations “should not be confused with moss or tree droppings, which typically produce only localized discoloration.”

The three are not equal once they are on a roof. ARMA writes that “unlike algae, moss can be detrimental to asphalt shingle performance,” and what it says moss does to a shingle is set out under whether moss grows on a Florida roof.

ARMA sends questions about moss on asphalt shingles to the shingle manufacturer, not to a cleaner. A shingle whose edge has lifted or curled is a question for a licensed roofing contractor, who inspects a roof. A cleaning company does not.

One roofing maker names lichen and moss together in a milder sentence. GAF's homeowner education page says “dark stains on your shingles may be moss, lichen, or blue-green algae,” and that this may not require a new roof. How lichen differs from algae once cleaning is the subject is answered under whether lichen comes off a roof the way algae does.