Han wrote:

Aaron Fude wrote in -
september.org:

Hi,

On the north side of my house I have moss on the slate roof. I'm
concerned that it's pushing the slates apart. My roof is 90 years old.

How do I combat this problem?

Thanks,

Aaron

I don't think that slate by itself would support moss growth. While it is
possible that the slate is deteriorating to the extent that it is
generating a growth medium, it seems more likely that debris is collecting
on the roof and shade is protecting the moss from drying out. SO the
question is whether there are increasingly large trees leaning over the
roof where there were none before.

There are mosses, algae, lichen, etc. that will happily grow on barren
rock if it gets adequate humidity and diffuse sunlight.