Your parents have a lease--just not a written one. When there is no written lease, the rental is pursuant to an oral (often called verbal) lease.Anyone who rents, whether by an oral or written lease, has rights. For example, the landlord has the obligation to provide premises that are fit to be lived in (this is called the "implied warranty of habitability"). If the landlord does not, and major roof leaks could easily be a violation, the tenants could sue the landlord for compensation for the time they have been living with the problem; they could also sue for a court order forcing the landlord to make the repairs.That's the good news. The bad news is that as tenants under an oral lease, your parents are month-to-month tenants. That means that the landlord could terminate their tenancy--make them move--on 30 days notice if the  landlord chose.