I want to build a greenhouse over a 12'x12' flat roof
which is part of my house. The current roof is tar and
gravel and needs to be replaced.

I'm going to build a wood floor for the greenhouse, so
the membrane underneath will not bear traffic. Since
greenhouses are damp/wet I need a waterproof membrane.
I'd like a long-term solution, since I'd have to tear
up the floor to replace the membrance underneath.

My first thought was copper sheet, but that sounds
expensive and difficult to install. Anyone got a good
idea for a waterproof membrane under my greenhouse
floor?

Thanx!