I was once roped into replacing a flat roof on the end two garages in a run of eight, they were all owned independently and I could foresee 'awkwardness' from the owner of the third garage, should we provoke future issues for him.
I laboured the point about this and the fact that we would have to do a really neat join, where the new roof met the old, over the partition brick wall. I was trying to get out of the whole job, by raising this risk, but I ended up doing it.
As I removed the failed roofing felt, it started to become clear that it had been done before, and badly, including replacing the rotten roof sheets, which had rotted again.
I was now even more nervous about the joining issue when we reached the existing good roof at the partition.
As I carefully peeled back the felt, I could see where the original roof had been cut and the new roof fitted to it.
The boards were chipboard and had been rather roughly cut - with a hammer.
You could see the individual 'bite' marks as the 'craftsman' had worked his way along.
Luckily, our roofing material top sheet was not the usual felt, but stuff that was liberated from the nearby Evode/Bostik factory, basically metre-wide rolls of Flashband, which covered the dodgy joint up very well.
No issues were ever reported.