Can I sue my roommate who isnt on the lease for not paying?
You would use small claims court and even without a formal agreement the judge might side with you. Or not. And how much will filing cost for small claims court. How much will the cost be to serve (It cannot be you directly- as far as I know from several jurisdictions) Then how long before the case comes up By then your roommate will be long gone to perhaps out of county/state. So it better be an amount of money before you might break even. Then you have the time involved and anxiety and frustration. Might be the best thing just to get rid of the roommate- or you just move.