sprinklernotes
Ceiling Pocket Volumes
You have asked the following questions: “NFPA 13D limits the total volume of all unprotected ceiling pockets in a compartment to no more than 100 ft3 whereas NFPA 13 is limited to 1,000 ft3. Typically, NFPA 13 is more stringent. Please confirm the above references are accurate and, if so, please explain the intent of the wide variation between the volumes.” We have reviewed NFPA 13, 2013 edition and NFPA 13D, 2013 edition as the applicable standards. Our informal interpretation is that these volumes are as intended. You’re right that NFPA 13 is typically a more stringent document but that primarily means that it provides a longer-lasting, more robust system along with the extent of protected areas to provide life safety and property protection. It doesn’t mean that all aspects are more stringent. Ceiling pockets are a good example. The allowance for ceiling pockets was first added to NFPA 13 back in the 2002 edition based on fire modeling performed by AFSA. It was shown that large ceiling pockets (bigger than 1,000 ft3 and much deeper than 3 ft) could be unprotected, and the adjacent quick response (QR) sprinklers in the lower ceiling would activate at least as fast as a standard response sprinkler in a flat ceiling. The requirement for QR sprinklers in a light hazard occupancy was only added in the 1996 edition, so the acceptance of a standard response activation time was not an issue (keeping in mind that the reduction of the remote area for using QR sprinklers is not allowed). On the other hand, NFPA 13D’s use of residential sprinklers (as well of NFPA 13 section 8.10) has always been based on a fast response element. As such, they can’t tolerate a significant delay in activation times. Considering that ceiling pockets have become a prevalent feature, it’s desirable that it’s addressed by the standards. The technical committee accepted a proposal allowing 100 ft3 as a small enough volume to have minimal impact on the activation time. Don’t forget that an indentation of no more than 12 inches is not considered a ceiling pocket. NFPA 13 usually remains a more stringent standard, even with larger ceiling pockets. As stated in section 1.2.1, the purpose of NFPA 13 is to provide a reasonable degree of fire protection for both life and property.
