Although your roommate might be legally responsible for providing the landlord notice and paying rent before leaving, things don't always work out as planned. Depending on your lease, it's likely you may owe the rent if your roommate moves out before the end of lease, and while some landlords may be flexible and work with you, many landlords simply won't care.
Protect Yourself from a Departing Roommate
If your roommate gives you any kind of notice that he or she is leaving in mid-lease, then you may want to get him or her to sign an agreement stating that the departing roommate will:
Unfortunately, sometimes a roommate just bails and shows no sign of paying what he or she owes. If your roommate leaves the state, you may be out of luck, but if you know that the roommate is still local, consider taking the matter to small claims court. You don't need a lawyer to file in small claims court, and it's a fairly simply process whereby you show up with lease in hand and explain that your roommate left without paying rent. If the roommate fails to show up, then you will get an automatic judgment in your favor. Be aware, however, that collecting that judgment may be more difficult. Still, it's usually worth it, so set aside an afternoon and take your claim to small claims court.
If You Want to Stay
If you want to stay in the lease, you may be tempted just to sneak in a new tenant without talking to the landlord. Don't do this, because that violates your rental agreement and is grounds for eviction if the landlord finds out. Chances are the landlord will be amenable to your proposed new roommate anyway, so make sure you do it the legal way and have the proposed tenant fill out a rental application.
Do be careful, however, since the departure of the existing roommate can give a problematic landlord an excuse to evict you, even if you cover the missing roommate's rent. This is because your lease likely has a term of stay, which would be violated by your old roommate moving out. However, most landlords just want to get paid and not be hassled with having to find new occupants, so it is unlikely that a landlord will evict you unless your landlord already wants you out for some reason.
If You Want to Move Out
If you decide that also want to leave the rental arrangement, you may want to move out yourself. If you decide that you want to follow your roommate out of the rental, then there are a couple of things you should do to minimize your exposure:
Have Legal Questions About a Roommate Moving Out? Contact a Lawyer
It can be quite stressful when a roommate who's helping pay the rent moves out, particularly if it's sudden and unexpected. While landlords still have to cover their own costs and make a living, sometimes negotiating for a little wiggle room can help. If you need help dealing with a roommate moving out, consider speaking with a local real estate lawyer to find out how they can help.