When are rafter ties required, and how far apart can they be?

Where ceiling joists do not connect rafters to resist outward thrust, rafter ties are required, spaced no more than 4 feet on center; otherwise the ridge must be a structural ridge supported by a wall or beam (IRC R802.3.1).

In a conventionally framed roof, the rafters push outward at the walls under load. Ceiling joists that run parallel to and are connected to the rafters act as ties that resist this thrust. Where the ceiling joists are not connected to the rafters at the top wall plate, rafter ties must be provided instead.

Rafter ties are installed near the top of the wall plates and must be spaced no more than 4 feet on center.

Where neither ceiling joists nor rafter ties are provided to resist the outward thrust, the ridge must be designed as a structural ridge beam supported by a wall or girder.