What will happen to the fifth floor?

The sewer is blocked, which is why this happens.

Generally speaking, there will be no backwater on the fifth floor, because the backwater of the main drainage pipeline starts from the lower floor. You can't get to the fifth floor, but if you don't pay attention to the water on the fifth floor and get the sundries into the drainage sewer, the drainage pipeline on the fifth floor will be blocked, and the wastewater can't be discharged into the main sewer, which will form the anti-water phenomenon on the fifth floor, which has nothing to do with other floors.