In 1982 we showed the following method for increasing deflection by suspending units from springs on top of a steel frame work and using the CMAB curb without springs as the seal.

There were 3 or 4 very successful installations, but the method was too cumbersome and expensive. We found the only answer was to build a complete spring curb instead of just a cap on top of a standard curb. In our new complete curb the springs are adjustable from the outside so they can be changed if need be and the base can be leveled. Our standard deflections are 0.75”(19mm), 11/2”(38mm) or 21/2”(65mm) and occasionally as much as 41/2”(114mm). They can be built to any height when access doors are needed, and configured to provide level unit support, regardless of the slope of the roof. We started working with this design in 1988, and have had great success. It is OSHPD approved for seismic zones. The following sketch gives you a broad idea of the construction. Please refer to the “Base” section of the catalog for specific bulletins.


Curb mounted rooftop equipment shall be mounted on spring isolation curbs. The lower member shall consist of a sheet metal Z section containing adjustable and removable steel springs that support the upper floating section. The upper frame must provide continuous support for the equipment and must be captive so as to resiliently resist wind forces. All directional neoprene snubber bushings shall be a minimum of 1/4”(6mm) thick. Steel springs shall be laterally stable and rest on 1/4”(6mm) thick neoprene acoustical pads. Hardware must be plated and the springs provided with a rust resistant finish. The curbs waterproofing shall consist of a continuous galvanized flexible counter flashing nailed over the lower curbs waterproofing and joined at the corners by EPDM bellows. All spring locations shall have access ports with removable waterproof covers. Lower curbs shall have provision for 2”(50mm) of insulation. Curb shall be type RSC as manufactured by Mason Industries, Inc.