Architects seem to relish creating difficulty for the sprinkler folks, particularly when it comes to ceilings. Not happy with a perfectly good smooth flat ceiling, they must transform it into the Sistine Chapel. Years ago, it was vaulted and soffited ceilings. After hours spent trying to get the sprinklers spaced correctly, tearing hair out over dealing with NFPA 13 obstruction rules, Michelangelo tells us he does not like the placement of the sprinklers. It is times like these that I wish someone would invent invisible paint (and no, not for the architect).

Compounding the issue is the tendency for architects to stampede towards the latest trend (are you old enough to remember the mauve explosion?). Soon, every ceiling had elevation changes and angles that drove sprinkler contractors to the brink. At the time, we could only hope that, like disco, this too would pass.

The recent trend in the effort to make the lives of sprinkler designers and contractors miserable is the cloud ceiling. Admittedly, a nicely designed cloud ceiling has a very pleasing aesthetic. Cloud ceilings can also be functional, assisting with improving the acoustics and lighting of a space.

The problem for the sprinkler designer is where to put sprinklers. Are they required at the upper ceiling level, at the level of the clouds, or in both locations? The decision by the engineer to require sprinklers above and below the cloud ceiling can spark disagreement from the sprinkler contractor.

In his June/July 2012 “Heads Up” column in the NFPA Journal, Matt Klaus, Principal FPE with NFPA, does an excellent job of describing the issues with cloud ceilings.

He notes that NFPA 13 currently has provisions for dealing with many cloud ceiling situations. This includes NFPA 13 rules requiring 18 inches clearance below sprinklers so the spray pattern can properly develop. In addition, NFPA 13 (2013) paragraph 8.5.5.3.1, which requires sprinklers be installed under fixed obstructions over 48 inches wide, and paragraph 8.15.14 addressing open-grid ceilings, can be used for guidance.

As with many rules, there are often situations where the strict application of the code provisions seems a bit heavy-handed. Take the case of a room in which the ceiling has a thin gap around the entire perimeter of the ceiling, which is open to the space above. NFPA 13 would appear to require the installation of sprinklers both above and below the ceiling. For situations like this, many feel there is a need for special rules for cloud ceilings, identifying the cases where sprinklers above the cloud ceilings are not necessary.

The Fire Protection Research Foundation (FPRF), with support from the sprinkler industry, initiated a project to examine this issue. The FPRF contracted with Hughes Associates, Inc. (HAI) of Baltimore, Maryland, to conduct a two-phase project. The lead researcher for HAI was Jason Floyd, PhD. The purpose of the project was to, “obtain an understanding of how cloud ceiling panels impact sprinkler actuation thresholds with an overall goal to provide the technical basis for sprinkler installation requirements.”

The second phase of the project was recently completed. Full reports for both phases can be found at www.nfpa.org/research.

Phase 1 Study

The goal of the first phase of the project was to determine, “sprinkler installation requirements for large contiguous clouds. For the purpose of this project, this was defined as a cloud whose size and cloud-to-cloud spacing would require at least one sprinkler to be installed below each cloud panel when using a normal flat ceiling sprinkler spacing. Specifically, the project was tasked with determining the maximum separation distance between clouds where structural ceiling sprinklers (sprinklers above the cloud ceiling) would not be necessary and/or effective.”

The work addressed cloud ceilings that are level and co-planar. In addition, cloud ceilings and the above ceiling spaces are noncombustible. Initial fire testing was performed and then modeled using the Fire Dynamics Simulator (FDS). Based on the results, an FDS model was developed to run the simulations that produced the results for this phase.

In both phases, the pass-fail performance criteria was to ensure life safety and property protection consistent with the goals of NFPA 13. A pass would mean that sprinklers could be installed below the cloud ceiling without providing sprinklers above. The specific pass-fail criteria is provided in the side bar.

The FDS simulation for Phase 1 used four cloud panels each 15 feet by 15 feet. Simulations were performed with ceiling heights of 8 feet, 14 feet, 20 feet, and 34 feet. Ceiling plenum heights used were 2 feet and 4 feet. Fire locations were varied and medium, and fast fire growth rates were modeled. The gap sizes modeled varied between 6.25 percent and 21.875 percent of the ceiling height.

The key conclusion from the Phase 1 work was a recommendation, consisting of two rules that could be used to determine if sprinklers above the cloud could be eliminated.

The first is a One-Part Rule stating:

The gap between a wall and any cloud or between two adjacent clouds is less than or equal to 1 inch of gap per foot of ceiling height.

The second is a Two-Part Rule stating:

• The gap between a wall and any cloud is less than or equal to 1 inch of gap per foot of ceiling height, or
• The gap between any two adjacent clouds is less than or equal to 1 ¼ inch of gap per foot of ceiling height.

Phase 2 Work

The Phase 2 project looked at panels in which, “clouds are small enough (or have a large enough aspect ratio) that at least one sprinkler per cloud is not required based upon the listed sprinkler spacing, then a ceiling jet might encounter additional gaps between clouds.”

The hypothesis being there are gap sizes for which the ceiling jet will have sufficient velocity to jump the gap to activate adjacent sprinklers at the cloud level. As with Phase I, the panels were level and co-planar and the panels and plenum space were noncombustible.

Several smaller cloud panel sizes were evaluated. Square panels with dimensions of 2, 2.5, 3.33, 5, and 10 feet were modeled. Ceiling heights of 8, 14 and 20 feet, and gap sizes of 4 to 20 inches, were evaluated. A fire with a medium growth rate was used.

The conclusions for the Phase 2 work also include two rule recommendations.

The first rule consists of the formula:
A = 0.076 x (1/RG)2
Where,
A = max sprinkler coverage permitted (sf)
RG = gap area fraction/ceiling height (1/ft)

For this rule, if the planned coverage area per sprinkler is less than A, then it is permissible to delete sprinklers above the cloud ceiling.

The second rule is in tabular form and presents gap area fraction to ceiling height ratio (RG) as a function of 1) cloud panel size and 2) whether a sprinkler is located in each panel, every other panel or every third panel.

The Phase 2 report limits the use of these rules to:
• Uniform gap sizes.
• Uniform cloud arrays.
• Flat, level clouds all mounted at the same elevation.
• Ceiling heights of 8 to 20 feet.
• Cloud sizes greater than 1.1 feet.

In my view, the Phase 2 rules will be difficult to apply. Due to the timing of the NFPA 13 code development cycle, it appears the Phase 1 and Phase 2 rules will not get the chance to be incorporated in the 2015 edition. Those wishing to take advantage of the report recommendations to eliminate sprinklers above the cloud ceilings will need to use the equivalency clause in NFPA 13 using the report as justification and hoping that the authority having jurisdiction agrees.

The research is valuable in that it gives us the answer to how to treat the cloud ceiling that has a gap around the room perimeter. It also, more or less, confirms that with few other exceptions we will need to keep our heads both above and below the clouds. (I would have liked to make reference to Joni Mitchell’s hit song, but Matt Klaus beat me to it.)

Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer with bachelor’s and master’s degrees from the University of Maryland Department of Fire Protection Engineering. He is past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates Inc., a 15-person fire protection engineering firm with offices in Honolulu and Guam. He can be reached via email at SDannaway@ssdafire.com.