I'm afraid I can't see it working - what about your guttering? Apart from the ivy blocking any guttering the bags will impede the flow of rain off the roof, causing all sorts of problems
The plants will prefer to grow upwards up the pitch of the roof (supposing it's a pitched roof).
Ivy can be absolutely rampant and when wet with rain it'll be really heavy and as has been said, may pull the bags off the roof. The bags will also be heavy when full of compost and water - is the structure of the roof sound enough to support it? Also putting something like that on the roof may cause damp penetration and rot.
I'm sorry, I don't want to be a wet blanket, but I'm sure there are better ways to get some greenery along the wall of that garage .... if you post a photo of it we might be able to make some helpful suggestions