A rack for carrying cargo on top of a vehicle is disclosed, comprising a pair of longitudinal support assemblies. Each support assembly is configured for mounting on a crossbar secured across the roof of a vehicle, and includes a stationary base and an extendable rail. Each rail is moveable between a first position in which the rail is substantially coextensive with the base, a second position in which the rail extends significantly beyond the end of the base, and a third position in which the rail pivots downward adjacent a side of the vehicle. Each rail also has a proximal end portion and a distal end portion, with a loading arm pivotably connected to the distal end portion. The loading arm is moveable between a collapsed position parallel the rail and an extended loading position.