Cats will eliminate inappropriately usually for two reasons. One being
behavioral and the other being medical. It's important to rule out a medical
problem first, so I suggest you consult with your vet.

Judy

Close the door to your room mate's room, for a start.

Do you have more then one cat? Is the roommate a new one? These are what
needs to be know. If you have more then one cat get another litter box.
We had this problem and as soon as we got two boxes it stopped they both
use each box but the threat from either cat is not there now. As for
your room mate your cat is trying to let you know that it feels you have
are neglecting him.