Lichens formation on building roofs: why do lichens grow on roof surfaces? Do lichens harm the roof surface? How should we remove or prevent lichens growth on building surfaces? This article describes and provide photographs of lichens growth that occurs on buildings and in nature and we provide advice about handling lichens growth on roofing surfaces.
We include links to references useful in the identification of algae, moss, lichens, and mold. Our photo at page top shows moss on an asphalt shingle roof.
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What does lichens growth look like on an asphalt shingle roof and how do we distinguish this material and its effects from algae, moss, or other roof problems?
Lichens on an asphalt shingle roof in New York (left).
Our photo (left) shows blue-green lichens covering a significant portion of the roof surface, and below we provide closeup photographs showing just what lichens does to the roof surface if you try to remove it
Lichens, one of the most hardy growth organisms found in nature, can grow in harsh conditions. Lichens is often found growing on roof shingles, especially asphalt and wood shingles as we show here. Lichens may even grow on some window glass.
What is Lichens?
Lichens is an admirably tough symbiotic organism made up of two partners, typically an algae (usually a green algae or Chlorophyta, less often a cyanobacteria) and a fungus (usually an Ascomycete, occasionally a Basidiomycete). 
The fungus (the microbiont half of the lichens duet) obtains nutrients out of the surface on which the lichens is growing (quite often stone) as well as primarily out of the organism's contact with air.
The algae (the phycobiont) provides photosynthesis to the duet.
Lichens on hot dry rock in Mexico (left)
Our photo (left) shows lichens growing on sun-baked rock in Boca de la Canada in Guanajuato - central Mexico.
Lichens not only grow in almost every environment on earth, they have some useful functions as a source of perfume, dye, or medical compounds.
Nevertheless, most people don't enjoy seeing lichens on their building, and as we describe here, lichens may shorten the life of some roofing materials.
Don't be mistaken by our beautiful lichens photos of this symbiant growing on rocks and roofs.
Lichens on cold damp rock in Iceland (left).
Lichens also is widely found growing on organic surfaces such as trees, leaves, and on occasion wood trim on buildings, and perhaps surprising to some, even on plastic or PVC building siding and trim as well as on aluminum siding and windows.
Unlike a pure fungus or mold, however, lichens does require sunlight in addition to nutrients. LIchens needs small amounts of moisture but can tolerate extended periods of extreme dryness or "dehydration". That may help explain why unlike moss, sun exposure and dry conditions won't stop or prevent its growth on a building roof or wall while moss (MOSS on ROOFS) would be less successful in such locations.
Is lichens on a roof as much of a problem as moss?
Yes and no, as we discuss below.
Because lichens growing on a roof surface does not have as much thickness of body as moss, it will hold less water on the roof surface and is less of a wear factor than moss.
Our photographs (above) show lichens growing on an asphalt roof shingle in situ (above left). In the photo where our pen is inserted for scale (above right) we had "picked" the spot of lichens gently and flipped it over. Our pen points to the hole left in the asphalt shingle as the lichens had such a good grip on the mineral granules that when removing the lichens it brought some of the roof protective coating along with itself. If you click to enlarge the photo at above right you'll see the actual mineral granules from the shingle adhered in the "roots" of the lichens.
Moss Damage Compared with Lichens Damage to Roofs
You can see that comparing the loss of mineral granules where we have gently lifted off moss (below left) and where we used a knife blade to gently lift off lichens from the shingle surface (below right), both growths have loosened the mineral granule coating and exposed the organic shingle substrate in a similar fashion.
Notice that both roof photos show that both moss and lichens are happy to co-exist on the same shingle surface.
Our photo at above left shows testing where moss was lifted off of the shingle, and at right, cup lichens was pried off of the roof using the tip of a knife blade.
Because moss is a thicker growth on roof surfaces, we suspect that its ability to hold water and moisture on the roof surface is greater than that of lichens.
So in some circumstances and climates, moss damage may be as severe or even more severe than lichens damage to a building roof, and we suspect that the degree of moss or lichens damage also varies by roofing material, with still more severe moss and water damage on wood shingle roofing.
If we have only lichens growing on a roof surface we would be less quick to try to clean it off since lichens not only has a tighter "grip" on the roof surface but the cleaning process for lichens risks doing more harm than good to the roof surface.
Removing the protective granules from an asphalt shingle or mineral-granule coated roll roofing surface is going to reduce the future life of that roof covering.
Moss and lichens are more than a cosmetic issue on many kinds of roofing materials - asphalt shingles, roll roofing, wood shingle roofs, wood shake roofs. By holding moisture against the roof surface lichens but more so moss speed the wear of the asphalt shingle surface in freezing climates by increasing frost damage to the mineral granule coating on the shingles.
How to Remove & Prevent Lichens Growth on Building Roofs, Siding, & Trim
This looks very unsightly as its dark grey against white PVC. I was trying to identify what it is from your site info., but I’m not sure. It adheres quite firmly to the PVC in places, but can be scraped away.
I’ve included a picture for you, taken through the (opened) window.
What do you think it is and how do I get rid of it (from the windows)? I would be grateful for any help/advice. - P.W. 8/17/2013
The roof tile and PVC trim deposits about which you inquire and show in your photo are undoubtedly lichens.
Although personally I'm enchanted by the toughness and beauty of lichens, I have to agree that agree lichens growing on your home's roof fascia and trim is unsightly. I don't usually see it on PVC - but lichens is the most hardy stuff imaginable - it's found worldwide, very often on stone as well as on many asphalt shingle roos, worldwide on stone, and sometimes on plastic, PVC, and aluminum siding, wood, and trim.
Unlike its presence on asphalt roofing, lichens on clay or plastic roof tiles, even on slate, is not likely to cause rapid damage nor have a large effect on roof life. But as your photo illustrates, it can be unsightly.
Options for Removing Lichens from Building Surfaces
You could physically remove or clean the lichens-infected building surfaces using scrubbing and a TSP substitute outdoor cleaner such as deck cleaner, taking care not to let runoff drain down and stain building siding. But the lichens will probably return as apparently it likes the chemistry of your PVC trim.
In some circumstances a power washer is used to remove lichens, but I'd be careful of that approach too as blowing high pressure water into building materials or openings can create costly, hidden damage, even mold contamination.
I am reluctant to advise painting the trim with a fungicidal-treated paint as you'd convert a no-maintenance surface to a recurrent need for repainting.
How to Clean Off & Prevent Future Lichens Re-growth on Building Surfaces
Lichens Removal/Prevention on Stone, Slate, Similar Surfaces: Cleaning & Using Biocides
Richardson and also Young et als. have written about the control of lichens growth on stone (presumably for the benefit of art, artifacts, and grave sites) and performed tests using Quaternary ammonium formulations and Tri-n-butyltinoxyde. 
Richardson points out that keeping the surface dry can reduce or prevent growth through use of an "external water repellant treatment" such as silicone resin water repellant coatings. He warns however that a toxic biocidal treatment is needed to eradicate existing lichens growth. The biocide chosen should not be one that itself harms the surface or material to which it is applied.
Examples of other problems (besides those listed above) that may arise using a biocide include
For practical help in lichens prevention on buildings we looked to experts in cemetery cleaning & maintenance. Cemetery experts recognize that the water-trapping effects of lichens on stone surfaces can lead to frost and other deterioration of gravestones and monuments. 
Try coppe or zinc strips or flashing to prevent lichens and moss growth
Some other measures that resist or retard moss growth also work for lichens - changing the chemistry of the host surface so as to create an imbalance between the algae and the fungus. For example you'll notice in some of my roof lichens photos that no lichens grown on the roof slope below areas where there is copper flashing; it appears that the copper oxides washing down over surfaces below retard both moss and lichens growth. So cleaning followed by some strategic placement of copper strips might work for you.
Lichens Removal/Prevention on Plaster, Wood, & Similar Surfaces
Lichens growth diagnostic questions
Is there a correlation between the lichens growth and sun or shade or moisture levels? Often I see more lichens growth on roof surface areas that are moist and shaded, though indeed lichens is tough stuff - and is found all over the world even under very hostile conditions like rock surfaces in northern Norway or sun-baked rock surfaces in central Mexico. Certainly lichens tolerates sunlight and hot dry conditions where moss will not grow.
Is there a correlation between this growth and different plastic products on the building exterior. I suspect that your PVC components are of a particular chemistry that's part of the problem. For that reason, a call to the manufacturer, getting a technical person who's interested rather than a stonewalling "customer service person" might be instructive.
Thanks for your response. I managed to remove the deposits using soap and water and a piece of scouring pad (that you use to clean pans etc). It was a bit difficult reaching the outside window frame, but I managed it in the end.
I’ve attached a photo taken through the open window. There’s a section of flashing (probably lead) just below the sill, and you can see more deposits on the tiles below it. The rest of the roof (away from the dormer) is not affected.
Hope that helps - P.
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