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 in the Residential Tenancies Act, even if they didn’t sign the lease.
Obligations that are not under the Residential Tenancies Act may include shoveling snow, not smoking in the property, and not having pets. If one roommate did not sign the lease, the question is whether they need to follow these rules. There are arguments that go both ways in this situation.
If a roommate did not sign the lease, they may not be a party to the lease and bound by the obligations beyond the terms of the Residential Tenancies Act.
On the other hand, 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, they 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.