Considering curbs

New seismic- and wind-load requirements are changing the way roof curbs are designed and installed

Before the development and implementation of comprehensive building codes, it was common practice for rooftop equipment to be installed without being firmly attached to a building's structure. The thinking was the weight of an HVAC rooftop unit would be sufficient to keep a curb and the rooftop unit on top of a building. However, recent research into the effects of natural disasters has shown that to resist significant seismic and wind loads, rooftop units must be firmly restrained from moving off their curbs, and curbs must be attached to a building's structure. This research is changing the way curbs are designed, built and installed.

Many U.S. jurisdictions require a licensed engineer to certify that curbs and restraints meet the requirements of applicable building codes. The process by which curbs are now designed can be summarized in three steps.

First, a roof curb designer must determine the governing building code for a particular project. This information is typically provided in the mechanical or structural sections of project specifications. The designer should also be aware of any project requirements that are more stringent than those listed in the building code.

If the project specifications do not include an applicable building code, the designer can determine it by contacting the building department that has jurisdiction in the area where the building is located.