Ive just went to a similar situation last month. I have a Duplex that I paid about $150K. 3 weeks after purchasing I have a tenant complaining of toilet water from upstairs leaking and flooding her apartment. I took immediate action and hire a plumber to tear down the walls, kitchen cabinets and replace the leaking pipe. A week after the tenant replied the flood started again and got worse. I sent a plumber again and we found out that another pipe under ground was busted probably with the pressure coming from new pipes. We had to excavate a tunnel underneath her aparment and change the whole pipe. Luckily this tenant did not called health department or anything. She just moved for a week and moved back when we fixed all the pipes. In compensation for all the problems caused I installed a new kitchen cabinets since the old one was contaminated with toilet water. Also painted her whole apartment and paid her cleaning fees. It cost me roughly $4500 to change the pipes and cabinets. This is what happens when you purchase old buildings. You never know when the pipes are going to bust so the best think if one could do is to change the pipes when the unit is vacant to prevent these headaches. I think it is crucial that then tenant feels that you took all actions necessary to fix the problem. If not they will call code enforcement or any other departments to complain and when these people get involved then things will turn much more complex and expensive.