One common situation for commercial or multi-unit buildings still connected to the sanitary sewer system is to have a roof drain that runs down through the building and into the sanitary sewage lines.
The two disconnect routes
There are two routes for disconnection in this case.
1. Redirect the rainwater
One is to redirect the rainwater onto grade. This method is appropriate if there is sufficient drainage at grade to handle the flow during rainfall.
2. Install plumbing to direct rainwater
The second route is to install plumbing that directs rainwater to a storm drain underneath the street.