Actually depending upon the wood the 2x12's are pretty close to being rated as a floor for a 20' span. It would be a bouncy floor. Once a beam is installed to cut the span in 1/2 the 2x12's will be fine as a floor - plenty of support.
The only concern (maybe?) is you do not want to ruin the roof - IE cause leaks ?
A greenhouse would be fairly heavy - depending upon how much dirt/plants you want to put up there, Unless you are just using it to grow out seedlings
Possibly you could use one area as a greenhouse near a corner, then the rest as a deck. My only though is - why do this on the roof?
My greenhouse is on the ground, facing south - up against the house. It can get up over 100 in the Spring & fall & can help heat the house
I can see a deck on the garage if there are decent views
If not then you are better off building it on the ground