Your roommate's unpaid rent is legally your problem. Here is what to put in writing.

Three people sign a lease for $2,400, and it is easy to think of that as $800 per person, but the lease usually does not work that way.

Most shared leases make the tenants jointly and severally liable. In practice, that means each tenant can be held responsible for the full rent, not just an individual share. If one roommate stops paying and moves out, the landlord can still demand the full $2,400 from the people who remain.

This is not some unusual clause hidden in the fine print. It is common in shared leases, and it can leave the roommate with the most reliable income or assets covering rent that someone else failed to pay.

What a roommate agreement can and can't do

A roommate agreement is a contract between the tenants. It does not bind the landlord or replace the lease, so it will not prevent the landlord from asking you to cover a roommate's unpaid share.

What it can do is make clear what each roommate owes the others. If you have to cover an extra $800 because someone did not pay, a written agreement gives you something to point to when asking for that money back. If the dispute ends up in small claims court, a signed agreement is also much more useful than two people giving different versions of what they had agreed to.

What to put in it

Rent split and due date. Spell out how much each person pays, who receives the money, and when it is due. If one roommate pays the landlord in full and collects from everyone else, put that in writing too.

Utilities and shared costs. List whose name is on each account, how the bills are divided, and what happens if someone moves out while an account is still in their name.

The deposit. Record how much each person contributed and how any refund will be divided after deductions. Security deposits can become a problem at move-out when one person receives the refund and everyone remembers the original split differently.

Leaving early. Decide who is responsible for finding a replacement, who keeps paying until the room is filled, and how much notice someone has to give before moving out. Without an agreement, an early departure can leave the remaining roommates covering the difference.

Guests and overnight stays. Set expectations for frequent guests, especially when a partner starts spending enough time in the home to affect utilities and other shared costs.

Damage. If one roommate, guest, or pet causes damage, say who is responsible for it. Otherwise, a deduction from the security deposit may end up costing everyone.

The fixes worth asking about

There are also lease arrangements that can reduce the risk, but the landlord has to agree to them. Individual leases, which are common in student housing and some coliving properties, make each tenant responsible for their own room or portion of the rent. If that option is available, it can provide much better protection when another roommate stops paying.

It is also worth asking whether the lease has a written process for replacing a roommate. The lease can spell out how someone who leaves is released and how a replacement tenant is added. Without that process, you may have to negotiate the change with the landlord after the problem has already happened.

Before signing, also check that everyone who plans to live in the unit is actually listed on the lease. An unauthorized roommate can create a separate problem if the lease limits who is allowed to occupy the property.

For the financial side of living with roommates, our data on what splitting rent saves by city shows why sharing a place can be worth it, while the coliving insurance gap looks at another risk renters often overlook.

Sources

• Nolo, Renters' Rights, on joint and several liability in shared tenancies