Double Curbs are used with larger roof top units when either the purlin point load exceeds the typical 300 pounds per point load, or when the purlins must be spliced to allow the duct work to drop through the bottom of the unit.

Remember this one important detail when using the TRAC Rail System with the outer curb of a double curb system: position your equipment frame or inter-curb support framing 2” below the top of the purlins or bar joist elevation.