the fundamental problem is what i thought it would be — the roof beams (it's a lean-to roof) are about 120cm / 4' apart, which is way too much for a bitumen sheet to bridge. so they've started sagging, and leaking where they've sagged.
i'm gonna have to rebuild the roof, but it might be a while before i get around to it. back of the envelope says that's like a €1000 expense, maybe more if i wanna get fancy and e.g. get an engineer to run the numbers to make sure it will support the green roof i'd eventually like to add.
