Leases include mandatory obligations from the Residential Tenancies Act and obligations that fall outside of it. Everyone who lives in a rental property must follow the rules listed in the Residential Tenancies Act, even if they haven’t signed the lease.
Obligations that aren’t covered by the Residential Tenancies Act may include shoveling snow, not smoking in the property, and not having pets. If one roommate has not signed the lease, the question is whether he or she need to follow these rules. There are arguments that go both ways in this situation.
If a roommate has not signed the lease, it could be argued that he or she is not a party to the lease and is not bound by the extra obligations that go beyond the terms of the Residential Tenancies Act.
On the other hand, it could be argued that a roommate’s ongoing relationship with the landlord suggests they are a party to the agreement and therefore should assume those extra obligations.
It is most beneficial to assume that all roommates are bound by the terms of the lease. Even if a roommate is not going to sign the lease, he or she should ask to see it before moving in to understand the obligations their roommates may expect them to fulfill. Roommates may also want to consider creating a roommate agreement. A sample one can be downloaded here.