Why does lichen leave marks after cleaning?

The question usually starts with a shape. After a roof is washed, a ring, a blotch or a lighter patch shows in the spot a lichen covered, and the owner wants to know why the growth is gone and its outline is not. This page does not say what that shape is on any roof. Nothing seen from the ground settles it, and a cleaner is not the person to settle it.

What the sources do offer is a short list of mechanisms that can each leave something visible where a lichen grew. None of them is a diagnosis. Each is a question worth carrying to a roofing contractor who can look closely.

• The surface under the crust. The U.S. Forest Service says lichens can cause some damage to buildings and man-made structures, very slowly and without endangering them, and a covered patch spent that time differently from the open surface beside it.
• The lichen itself, still there. A dry lichen goes dormant and gray, so part of a colony can remain and read as a mark rather than as a growth.
• On asphalt shingle, the granule layer. ARMA and GAF both name what loosens granules, and a patch without them could read as a mark.

A crustose lichen does not rest on a roof the way dust does. The U.S. Forest Service says it forms a crust over a surface and names roof shingles as one of the places it does so, and it says crustose lichens are pressed against their substrate. UF/IFAS Extension writes that these lichens grow as if painted onto the surface and that prying one off is close to impossible.

That close, fixed contact is what separates a lichen from the streaks it is often grouped with, and the practical difference between the two is set out in how lichen compares with algae on a roof. Here the point is narrower: the crust sat in one place, pressed against one patch of surface, for as long as it took to grow.

The Forest Service is measured about what that contact does. “Just about anything that holds still long enough for a lichen to attach to and grow is a suitable substrate,” it says, and houses are on its list. And: “Although lichens can cause some damage to buildings and man-made structures, it is a very slow process and does not endanger those substrates.”

The outline follows from that in a general way. A patch that spent a long stretch covered did not see the same sun, rain and air as the uncovered surface beside it, so the two need not look alike once the cover is gone. That is reasoning, not a finding any source publishes about roofs, and it explains why an outline is possible. It says nothing about what any one outline is. Nothing read says a lichen eats through tile or shingle.

Some marks may be the organism itself. The U.S. Forest Service says that when lichens are dry they “become brittle and go dormant,” and that a dry lichen's color is “usually gray or colored like the fungal cells on the upper cortex.” A thin gray remnant is easy to read as a stain.

Water changes the look. When a lichen is wet, the Forest Service says, the upper cells turn transparent and the algal cells underneath show: green algae give a bright green, and “Cyanobacteria will give the lichen a dark green, brown, or black color.” A mark that looks pale and flat on a dry afternoon and darker after rain is behaving the way the Forest Service describes a lichen behaving, which is a reason for a closer look, not a conclusion.

Roofing makers say little about lichen, and what they do say is cautious. GAF's homeowner page notes that dark stains on shingles “may be moss, lichen, or blue-green algae”, and adds “This may not require a new roof”. The lichen entry covers the organism and its growth forms, including the microscopic bundles, called soredia, that UF/IFAS says it releases to start new colonies.

An asphalt shingle has a surface layer that tile does not, and it changes what a mark could mean. CertainTeed describes shingle granules as fine grain crushed rock pressed into the asphalt: “They block ultraviolet (UV) light from reaching the asphalt in the shingle.”

The aging of unprotected asphalt that follows is what the UV degradation entry describes, and what the granules do is set out in the granule loss entry.

The roofing bodies name what loosens granules. ARMA's bulletin warns “Avoid scrubbing the surface, as this friction may loosen and remove granules,” and warns against high-pressure washing on asphalt roofing. GAF's bulletin says power washing shingles is not recommended because “it may dislodge granules, which can lead to premature shingle failure.” Those are statements about methods, and none of them says any particular outline is granule loss.

The honest end of this answer is a routing, and the line between washing a surface and judging it is drawn in what an exterior cleaner is not.

On a roof, that line is set out in the boundary between cleaning and roofing work. A lichen outline sits right on it: the cleaning is what revealed it, and deciding whether it matters is a roofing contractor's call.

When you ask, the useful details are the ones this page cannot supply: the roof material, where the marks are, whether they change color when wet, and how long the lichen had been growing before it was noticed. A roofing contractor can weigh those against what is actually on the roof.