I've just had a sefer torah passed to me to fix that has a serious rip through two amudim in the last yeriah (they added sellotape to try and prevent it getting worse, rather than just leaving it alone).
Fortunately the rip is very clean and when joined together and patched the ink should create a bridge to make the letters guf echad and mukaf g'vil.
The first part of the rip in the last amud goes straight through one line and so technically is fixable. However the second part in the penultimate amud goes through many lines of k'tav. Keset Hasofer 18 says that if it goes through 3 or more lines then the sheet needs replacing and stitching does not suffice. We don't stitch nowadays but Mishnat Hasofer says this applies also to gluing patches from the back. Though if one came across such a rip repaired this way done by someone else, then it doesn't make the Torah pasul and it is kasher b'dieved because it is a hiddur.
The synagogue obviously wants to keep costs of repair down (small community with some major building debt).
Two questions: 1) Since the rip is clean and could be fixed mechanically without the need to rewrite the section is there a kulah that can be relied on? Source? If one does something that one knows is okay only b'dieved knowingly how does that stack up?
2) Since one is supposed to rewrite the whole yeriah (ie 3 amudim) but the TaZ also says that b'dieved a single amud is also kasher, would one have to rewrite only this one amud, the two last or the whole yeriah. Again are there any other sources.