Both of mine are on breeze block bases with soil borders & I don't have any problems. My old greednhouse was on slabs and i found that to be a real problem as it was constantly damp in there as it couldn't drain away, thats why I went for soil this time the soil wicks it away.

Also I top my blocks off with some 4"x2" tanalised and fix this with frame fixings about 6" long and then just fasten the greenhouse doen to the wood. This acts as a thermal break and gices me a bit ogf extra headroom but it means I have a step in to the house.

__________________
ntg

Never be afraid to try something new.
Remember that a lone amateur built the Ark.
A large group of professionals built the Titanic

http://grief-encounters.blogspot.com/