If you live in a condominium complex, including those that were converted from apartments, you and your neighbors have drains that join up at certain points before exiting the property. These are shared or common drains—and unfortunately, they can become clogged at a point far down the line. When one person uses their plumbing, everyone else whose drain connects above the clog gets water backing up into their tub. The solution is simple: You call a plumbing service to have the drain snaked and opened up. But unlike other plumbing repairs, this is difficult to prevent unless you actively call for plumbing repair before you actually need it.
Only by Having a Plumber Snake It Proactively
If you want to try to prevent the shared drain from backing up, all you can really do is call a plumbing repair service and ask them to send a plumber's snake down the tub drain in hopes of breaking up any partial clogs. There's no set schedule for this. Let's say you do this when you first buy the place and move in; after that, you'll want to call the plumbing repair service maybe every few years and ask them to do this work again.
You Really Can't Control What Your Neighbor Puts Down the Drain
A clog in a shared drain usually forms due to accumulated hair and gunk that gets caught at one point in the drain. More hair and more gunk get stuck there over the years until one day, one of your neighbors turns on their faucet, and you find water backing up into your tub. And you and your neighbors don't even have to be deliberately careless; this just happens to shared drains. Unfortunately, this means you will have to call a plumbing repair service to have the drain unclogged.
It's Unpredictable, and You Just Have to Keep Monitoring
You can do only so much to keep a shared drain clear. Even if every neighbor whose condo is connected to the drain keeps hair and other foreign objects out, there's no way to tell how soap and scum will form along the insides of the pipe. Maybe the water softener one of you uses starts to fail, and more scale starts to build up along the sides of the pipe. All you can do is monitor how well the water drains in tubs, and have a plumbing service come over to occasionally snake the drain just to be sure.
No one likes to see water backing up into the tub or to smell that awful smell, but if you live in housing where units share drains, it is a risk, especially in older buildings. The good news is that, once you have the drain cleared, it should take a while before you have to worry about this again (as in several years, assuming everyone is careful about what goes down the drain).
For more information about plumbing repair, contact a local plumber.