A subframe for retrofitting an existing roof with a new roof is disclosed. In some embodiments, the subframe comprises a base, a first and second longitudinal flanges, a first wall extending between the base and the first longitudinal flange, a second wall spaced apart from the first wall and extending between the base and the second longitudinal flange, and a traversely-oriented punch out passing through the base, the first wall and the second wall. The punch out is configured to receive a rib of the existing roof. Once installed on the existing roof and coupled to the new roof, the subframe and new roof form an enclosed, self-supporting structure with increased structural capacity and stability in comparison to conventional retrofitted roof systems.