In article ,
"Mary Fisher" writes:

A build-up of mosses and deposited dirt can be a problem on a flat roof
where it can't get washed down by rain. We had such a one on the school
which I (with others!) governed, it cost us a lot of money.

What harm did it do?
My parents have a wonderful moss growth on a flat roof, underneath a
tall TV aerial where it gets fed with copious quantities of pigeon poo.

--
Andrew Gabriel