Leaders that Bring Rainwater Through House Pipes/Sewer

[Rosalie DiPietro]

I  manage two properties (2 units in each property, flat roof) where the leaders feed into a drain pipe that takes rainwater through the building sewer system.  Recently, on more than one occasion, one of those properties, during heavy rain/flash flood, the water overflows in the basement shower and causes the floor/back half of the basement to flood.

I contacted my plumber about putting in a back flow valve in the main trap in the front of the building to prevent the City sewer from backing into the house.  I thought the rainwater/sewer water was backing up from the street into the house.  However, he suggested I first try re-routing the leaders to the outside/street so that the rainwater doesn't enter the building.

Has anyone had this problem?  How did you resolve it?

Thanks for your input!