A ladder rack assembly for a work vehicle comprises first and second tubular extrusions that are designed to extend across the width dimension of the vehicle above its roof line where the extrusion is designed to include a pair of parallel channels including a wedge-shaped flange whereby clamping plates may be coupled to the flanges for rigidly joining the extruded cross members to vertically extending support legs. The support legs themselves may have the same cross sectional profile as the extruded cross member when the ladder rack assembly is to be affixed to the box of a pickup truck. Alternatively, a special clamping mechanism is provided for coupling the cross members of the ladder rack assembly to the rain diverter rail of the motor vehicle when the ladder rack assembly is to be used with a van, SUV or panel truck.