Does lichen come off a roof the way algae does?

The two growths do not sit in the same place. 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.” Growth living in a film like that fits what CDC's journal Emerging Infectious Diseases calls a biofilm, microbial cells attached to a surface and enclosed in a polymer matrix, though that is a bridge from general biology rather than a roofing document's word. A crustose lichen sits closer. The U.S. Forest Service says crustose lichens “form a crust over a surface, like a boulder, the soil, a car, or your roof shingles”, and that they “are pressed against their substrate.”

Lichens attach to their surfaces by different means, the Forest Service says, and one of them is rhizines, fungal filaments that do not move water or nutrients to the lichen; “they simply hold the lichen down to whatever it is sitting on.” That page does not tie rhizines to crustose lichens in particular. For those, UF/IFAS Extension is plain: crustose lichens “exist as if painted onto the surface they grow on. It is nearly impossible to pry the organism from the surface.”

A lichen is two organisms acting as one. UF/IFAS Extension describes lichens as “a fungus and an alga or cyanobacteria growing together in a mutually beneficial relationship.” In that arrangement, it says, the fungus obtains water and minerals and “provides structure and protection for the alga”, while the alga provides the carbohydrates the fungus needs.

That division of labor is the useful part for a roof owner. Roof algae is a photosynthesizing organism working alone, living in the film on the surface; EPA describes cyanobacteria as microscopic organisms that use sunlight to produce their own food. A lichen adds a fungal body whose job is structure, and the structure is what anchors it. The lichen entry covers the growth forms; UF/IFAS lists lichens as flat, leafy, crusty, powdery or hair-like, in colors from white to gray, red, green, yellow or black. The algae entry covers the other half of the comparison.

They also start new colonies differently. Roof algae travels as airborne spores in GAF's account, while a lichen mostly sends out tiny bundles holding all of its partners at once, which UF/IFAS calls soredia, and the Forest Service notes that its fungal partner makes spores as well. A soredium that lands somewhere suitable begins a new lichen in the same arrangement as the parent.

Lichen has an on and off switch, and it is water. The U.S. Forest Service: “When lichens are wet, they "turn on" and start photosynthesizing and growing. When lichens are dry, they "turn off", become brittle and go dormant.”

The color changes with it. A dry lichen, the Forest Service says, is usually gray or the color of its fungal layer, and when it is wet the algal cells underneath show through: green algae make the lichen bright green, and cyanobacteria make it dark green, brown or black. The same crust can look like two different things on a dry afternoon and after a shower.

A gray, brittle patch in a dry stretch is not, on its own, evidence the growth has died.

The roofing trade writes a great deal about algae and very little about lichen. The one maker sentence read that names lichen on a roof is GAF's homeowner page: “It may look like dirt, but dark stains on your shingles may be moss, lichen, or blue-green algae. This may not require a new roof”. GAF does not say what removes it.

TRI Alliance's line on removal is specific about what it covers: “Fortunately, mildew, algae and moss can be removed or treated. If the growth returns, treatment can be repeated as necessary.” Lichen is not on that list. The description of the algae itself, the streaks and the dust film under them, is on the page about the black streaks on a Naples roof. The same organisms, TRI Alliance says, grow on everything from stucco and siding to driveways, which is why roof algae on the pavers below is a question of its own.

So the honest answer stops at the difference. Nothing read promises that a lichen crust comes off a roof in the way a dust-film growth does, and ARMA says of its own bleach-and-water procedure for algae discoloration that its effectiveness “is only temporary, and the discoloration may recur.” What shows on the surface once a crust is gone is taken up in why lichen leaves marks after cleaning.