Repairs to shared structures

Hi Everyone,.

I am looking at purchasing half of a semi detached house (each half is an up-down duplex). The building will require some work, including repairs to, if not replacement of, the shared flat roof. I am wondering if anyone has experience with this type of repair to common structure, and how these are agreed upon between the two owners. I have heard that it is sometimes spelled out in the titles, has anyone encounted this?

Thanks,
Josh

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