You may have to use rat traps or glue traps. The rat traps are much like the mouse traps only larger. There are ones that are disposable and ones that you can reuse. The glue traps won't kill them them right away and the rats may move the boards as they try to escape. I used the trap that you can reuse as it is easier to set than the disposable ones are, however they are mopre expensive. Also so if you have a big problem with them then the disposable ones may be best.
glue boards http://www.doyourownpestcontrol.com/ratgluetray.htm
snap traps http://www.doyourownpestcontrol.com/ratsnap.htm
This wasn't the answer I expected. I am a breeder so I know the obvious answer...traps. Please cancel my payment.