Posted: 06/09/2013 at 15:57
That first garage roof is sagging already. I just copied your photograph into GIMP and drew a straight line along its ridge, and there's a gap under the line, as big as the ridge structure where it's in line with the garage door.
If you do rebuild that roof, you could put a solid roof there and put a trellis over it, but I suspect the supports for the trellis would be ingress points for rainwater. An alternative would be posts mounted on the end wall to support wires stretched to the barn wall, but then you're putting a lot of force on the posts and bolts, increasing as plants on the wires get heavier. Also, you'd be waiting a while for plants to grow from ground level to the wall top and then along the trellis to its peak, and I reckon most climbers would find their way under a tile somewhere eventually and up behind the barge boards almost immediately.
I'm not sure how the succulent roof was done, but I assume it could be achieved by putting wooden rails around the outside of the roof, leaving slots for drainage, and placing potted plants on the roof. As long as it's not delicate and they can't slide around, they shouldn't make holes in it.
A variation of that would be a stepped planter. You could have a series of low walls along the roof, filled with soil between them to make beds, and leave a couple of spaces to walk up there to check on them, feed them or whatever. Bury a perforated hose or length of 50mm drainpipe in each one with an open end by the steps so you can just pour diluted feed into it and reasonably expect it to be spread along the full length. Two down-sides: one is that it's one heck of an engineering project, to the extent that I'd want to get the walls' max load checked before I put that much stuff on them, and the other is that when it needs replacing it'll be another huge project (and a real mess if it comes down on a car because you put it off too long).
For a lightweight option, replace the existing roof with big 4mm acrylic sheets and seal the edges with gutter sealant. It'll save you using electric lights in the garage during the day. You may need to get slightly clever with the woodwork underneath them to get the overlaps rights, but it's not that big a deal. I was going to do that for a covered washing line here, but the company went for exposed rotary ones instead.