I Really Do Know Clouds
Cloud ceilings and the provisions of NFPA 13 (with apologies to Joni Mitchell)
NFPA Journal®, July/August 2012
Many rooms and spaces conceived by interior designers and architects today do not fit neatly into the prescriptive requirements of some sprinkler system design standards. Among these are cloud ceilings.
A cloud ceiling is simply a suspended ceiling that covers only a portion of a room or space below, typically to hide mechanical equipment. NFPA’s technical advisory program, which gives NFPA members an opportunity to ask questions relating to code requirements, receives multiple questions each week on how to locate sprinklers when cloud ceilings are present. The questions typically go like this: Where in NFPA 13, Installation of Sprinkler Systems, do I find the design criteria for cloud ceilings? Do I put sprinklers above and below the cloud? When can I omit sprinklers above a cloud ceiling? When can I omit sprinklers below a cloud ceiling?
Like Joni Mitchell once sang, I’ve looked at clouds from both sides now. Unlike Joni, however, I really do know clouds.
Although NFPA 13 doesn’t define cloud ceilings or provide guidance on them as a specific obstruction, the standard does provide the answers to these questions through the use of obstructions rules and deflector position requirements. NFPA 13 requires sprinklers to be located within 12 inches (30 centimeters) of the ceiling to ensure that sprinklers will operate once the gas layer develops at the ceiling. The standard also requires sprinklers to be located in such a way that objects within 18 inches (45 centimeters) vertically of the sprinkler do not prevent the spray pattern from developing and that obstructions more than 18 inches (45 centimeters) below the sprinkler are not so large that water spray cannot reach large areas of the floor. When followed correctly, these rules will help the designer determine how to handle the cloud ceilings.
In most instances, sprinklers will be required both above and below the cloud. As noted above, sprinklers must be within 12 inches (30 centimeters) of the upper ceiling. Most clouds are located more than 12 inches (30 centimeters) below the upper ceiling to allow mechanical equipment to be located between the upper and lower ceiling, which necessitates sprinklers above the cloud ceiling. In most cases, the clouds are large enough to create obstructions that prevent the spray pattern from reaching the floor. The presence of these obstructions requires that sprinklers be placed below the cloud as well, so that there is complete coverage.
There are some applications in which sprinklers can be provided above the cloud and omitted below it. This requires the clouds to be less than 48 inches (121 centimeters) wide and the area covered by the cloud to be at least 70 percent open (unobstructed). Similarly, sprinklers can be omitted above a cloud ceiling where the deflector of the sprinklers below the clouds is within 12 inches (30 centimeters) of the upper ceiling. In these instances, the sprinklers meet the deflector location requirement and no additional protection is necessary.
Although NFPA 13 provides direction that can help designers figure out where to place sprinklers when cloud ceilings are present, many people want more explicit language to address these architectural features. The Fire Protection Research Foundation recently began a research project to study and potentially test cloud ceiling configurations, meaning the NFPA 13 technical committees could have data for the next revision cycle to help them provide explicit cloud ceiling requirements, if necessary. Watch this column for further developments.
Matt Klaus is a senior fire protection engineer at NFPA and staff liaison on NFPA 13, 13R, 13D.