Bath tub over drain
 

The overdrain is not connected to the drainage system. Bath tub was already installed when we bought the house. I began to notice when my wife would bath and water got in the overdrain it was just running straight under the house. How do I get to the drainage system to connect it?

From under the house you may be able to see whats happening there. If there isn,t a joist in they, you could cut out some floor and correct the connection. If a joist is in the way it will get tricky. A photo from under would help.