"Phil L" wrote in message
.uk...

That's moss, not lichen...moss grows in balls, rolls down the roof into
the gutter etc etc.
Lichen is like a flat green blob (looks a bit like the droppings of a wood
pigeon) like this;
http://www.pixero.com/bilder/downloa...and_lichen.jpg

There are many kinds of lichen, from whitish scaly stuff through yellow,
green, brown, red ... and various shades of all of them. They are very slow
growing, unlike mosses, and are very difficult to scrape off. They grow a
lot on stones, I've tried scraping them off to use as dyes but gave up.

Mary