[kurt]

You can buy plenty of sizes of greenhouses on the web.The suggestion of a wind load above is a serious consideration when ordering said greenhouse.Local codes and the neighbors approval should be taken into consideration.Soil,water source, equipment,debri,containers will add up.A visit of your existing roof structure building plans might to a qualified engineer/architect might be wise.Heavy rain run off,roof/slope/gutters/drainage should be looked at.Some roof warrantys require "walk protection"for service access to thwart roof membrane puncture.A neighbor of my brother in law in PA did something similar.He put in a roof hatch from his access to the roof with a nice steel angled stepped stairs.He did have some home insurance upgrades.He also put a water source,slopsink and a rudimentary drain for cleaning.Have fun and good luck.