If you do not have a post office box , this is the time to go get one or a NON USPS box with say a shipping firm that operates in a mall. Reason being is that you want to have your mail separated from room mate so when you do issue proper notice or if you must use the courts in the end , then roommate cannot just destroy your mail, steal it or take your copies of things you would need for court . SO you have established a month to month tenancy with this person, issue to this person a simple proper notice that meets your states laws for a month to month tenant to move out, such as EG date tenant name & mailing address , this is your notice to vacate (address) by ( date that meets your states statutes signed ( your name and new PO box address) and any other information your state requires . Make say 4 copies , keep two for your records (one stored off site ) and mail the other two say via certificate of mailing and second one confirmed mail delivery, WHY (unless your state requires certified mail ) you will have sent it two ways , neither would have to be signed for but each would give to you a method of mailing that the courts would likely not reject since even if tenant tried lying & claiming they did not get notice (staggering the notices say by 2 days apart would let a Judge decide easily that you did indeed send notice and dismiss any claim by tenant they didn't get notice and proceed to evict tenant (and if the judge is annoyed enough the judge might surprise the tenant with as little as 24 hours to get out )
