Q: I have a project where a fire rated gypsum panel ceiling must be installed at a different elevation than what is listed in a UL design. What can I do?
A: Before a complete answer can be given, it must be noted that a gypsum panel ceiling does not have a fire rating by itself. Fire resistive assembly ratings are given to the total system, not to a single component. This is true for both walls and ceilings. In the case of a ceiling, the rating applies to the ceiling, the floor or roof structural framing and the building materials on top of the framing such as plywood or steel deck and concrete. There have been some “membrane” tests run on ceiling systems that are isolated from the supporting structural frame, but those are unique and designed to meet specific building code requirements.
In general, when fire tests are conducted on an assembly, they are purposely run at what was considered to be a worst-case scenario. That means the ceiling plenum is at a minimum depth. For a wall assembly, the steel framing is not only the thinnest steel thickness allowable but the framing is also at the maximum spacing. Acknowledging that specific project designs have the potential of varying from a physical fire resistive test, this conservative approach gives the designer some leeway on meeting both project needs and the intent of the building code’s mandated fire-resistive rating.
This is routinely interpreted to mean that if an assembly was tested with a 12-inch-deep plenum, which is determined by the elevation of the ceiling, the fire performance will not be negatively impacted by an actual plenum that is 18 inches. This implies that all other aspects, components and details are the same as what was originally tested. The converse of this is not true. If the test was run with an 18-inch plenum, then an installation with a 12-inch plenum may cause excessive heat build-up and put the assembly at risk for non-performance. That is why, in many assemblies, the ceiling is directly attached to the framing structure to minimize the ceiling plenum.
When the tested assembly has the ceiling attached to the framing system, a ceiling grillage can be used to lower the ceiling to the desired elevation. To document this, Underwriters Laboratories has written the following: “Where a ceiling is supported directly from structural members, it may be lowered. If necessary, intermediate supports may be used to support the ceiling, provided they produce an in place stiffness equivalent to that of the originally tested elements. A suggested method for providing an equivalent in place stiffness is by use of 1 1/2 in. cold-rolled channels made of 16 gauge or heavier painted or galvanized steel, with the web oriented vertically and suspended from the structural members by 12 SWG or heavier galvanized steel wire at a maximum spacing of 48 in. OC. The channels may be oriented parallel or perpendicular to the structural members but should be spaced not more than the spacing of the members.”
The above can be found on the UL LLC website under Product IQ™. The title of the document is “Guide Information for Fire Resistance Ratings.” The specified 16-gauge channel using the descriptor system supported by the Steel Framing Industry Association is a 150U50-54. The 150 is the 1 1/2-inch depth, U provides the shape of the member, and the 50 denotes the 1/2-inch flanges. The 54 is the mil thickness of the steel. To finish the grillage system, it is suggested to wire-tie 7/8” deep drywall furring channels to the 1 1/2 inch cold-rolled channels. Using the same nomenclature as the channel, this would be a 087F125-17. Any alternatives to this should be accompanied with applicable data and documentation.
An immediate question that may come up is whether a drywall grid system can be substituted for the framing that is called out in the UL document. The answer to that question lies in whether or not the proprietary grid system can deliver “in place stiffness equivalent to that of the originally tested elements” as required. The individual manufacturers of the grid systems should be able to provide technical support to the contractor who desires that substitution.
UL LLC does publish a guide that allows for a ceiling that will be at a lower elevation from that which was tested. Ultimately the authority having jurisdiction over any given project must give their approval for any deviations from what was tested. The initial design calling for the ceiling at a lower elevation should have been discussed at plan review. A contractor who desires any substitution from the approved plans should get formal approval from the AHJ prior to installation.
Robert Grupe is AWCI’s director of technical services. Send your questions to grupe@awci.org, or call him directly at (703) 538.1611.
