CMAB Curb Mounted Aluminum Bases are an excellent on inch deflection tool for isolating the noise and vibration generated by rooftop equipment from the occupied space below.

Not too many years ago, cooling towers and curb mounted exhaust fans were the only pieces of mechanical equipment to be found on rooftops.  This has all been changed as curb mounted air conditioning equipment is commanding a steadily growing part of the industry.  While rooftop equipment manufacturers do what they can to isolate components within their packages, the effectiveness of this isolation is limited by shipping problems and available space as well as piping, electrical and fan connections.   Unfortunately many roofs are extremely sensitive to vibration and this makes the problem more difficult.

All of these things have been done by custom building bases in the field to solve existing problems.  This led to the development of the Curb Mounted Aluminum Base (Type CMAB).  the main members of these bases are made of extruded aluminum, welded in the corners for weather-tightness, and our weather seal is provided by a truly flexible EPDM connection rather than a sliding fit.  all of the steel springs are cadmium plated and whenever possible, bases are shipped in one welded piece to minimize assembly and installation time and caulking on the job site.  Where size does not permit one piece shipment we try to limit the number of joints to two and furnish a simple kit for fastening the two halves together.

Springs are selected from our standard A series.  while the A-45 spring is rated at 1.60" deflection it could only be used under very light equipment.  We would normally use the A-200 with a rated deflection of 1.15" and an additional travel to solid of 50% or 0.57".  Although the A-200 has 1.15" deflection the suggested specification calls for a minimum of 0.75" as the weight data we receive from the manufacturer may be inaccurate and we do not wish to overstate the CMAB capability.  While it would be desirable to provide more than 1" deflection, the design is limited by curb width and equipment contact problems.  The use of taller springs having approximately the same diameter could lead to better deflections on paper, but poorer horizontal stability and overall performance.

On installations where the manufacturer's sheet metal curbs are used as part of ducting, the gap caused by raising the unit when the curb mounted base is installed must be attended to separately.  An optional Return Air Closure Panel and/or an independent internal Supply duct CMAB can be used to maintain duct integrity.

CMAB curb mounted rooftop equipment bases are excellent 1" deflection tools and added insurance for this type of installation when the rooftop manufacture's curbs are being used.  For alternate complete curb information, see Bulletin RSC-22.

Completed Installation

SPECIFICATION

Curb mounted rooftop equipment shall be mounted on isolation bases that fit over the roof curb and under the isolated equipment.  The extruded aluminum top member shall overlap the bottom member to provide water run-off should the seal fail.  Bases without this protection are unacceptable.  the aluminum members shall house cadmium plated springs having 0.75" minimum deflection with 0.75" additional travel to solid.  Spring diameters shall be no less than 0.8 of the spring height at rated load.  Wind resistance shall be provided by means of resilient snubbers in the corners with a minimum clearance of 0.25" so as not to interfere with the spring action except in high winds.  The weather seal shall consist of continuous closed cell sponge materials bot above and below the base and a waterproof flexible duct-like EPDM connection joining the outside perimeter of the overlapping aluminum members.  Foam or other contact seals are unacceptable at the spring cavity closure.  Caulking shall be kept to a minimum.  Submittals shall include spring deflections, spring diameters, compressed spring height and solid spring height as well as seals and wind resistance details.  Curb mounted rooftop equipment shall be flexibly ducted; water piping provided with double arch EPDM expansion joints; freon lines with flexible metal connectors and electrical lines with flexible conduit.  Curb mounted bases shall be Type CMAB, double arch expansion joints Type MFTFU all as provided by Mason Industries, Inc. or approved equal.

Note:  Rooftop installations normally require deflection over 0.75".  For spring deflections up to 3 inches, see ROOFTOP SPRING CURBS Bulletin RSC-22.