sprinklernotes
Calculating Sprinklers Above and Below Cloud Ceilings
You have asked the following question: “I have complete floor coverage using
standard coverage sprinkler protection at the roof deck of a light hazard
occupancy. There are ceiling clouds below that complete floor coverage as well
using extended coverage sprinklers with flex drops. I am not omitting any
sprinklers by using NFPA 13 Section 8.15.24. I have provided a calculation for
each of the areas (one for above sprinklers and one for below sprinklers). The
AHJ has demanded I include all of the sprinklers in a single calculation. Is
there any rule in NFPA 13 that allows me to only provide calculations to the
most demanding area, especially when I am not omitting any sprinklers above or
below, because each compliment of sprinklers would fully cover the floor area
by themselves?” In response to your question, we have reviewed NFPA 13, 2016
edition as the applicable standard. Our informal interpretation is the
calculation is required to include only the sprinklers at the actual ceiling.
The AHJ might be referencing Section 23.4.4.6.4 in this situation, but it isn’t
applicable. This section tells us when sprinklers are installed above and below
a ceiling from a common supply, the calculation shall be done for the largest
water demand. This is because each area is a separate compartment and it’s
confirming that the calculation only addresses one compartment (the most
demanding). But, this is not two ceilings, so Section 23.4.4.6.4 is not
applicable. This is a ceiling (at the upper roof deck) with multiple large
obstructions (the floating clouds). There is the case (your referenced Section
8.15.24 – Cloud Ceilings) where the collection of obstructions comply with
multiple restrictions and can be treated as a ceiling. But, as you already
stated, that only addresses whether or not sprinklers can be omitted from the
upper deck.
When we complete hydraulic calculations, we are looking for how many sprinklers
are likely to open in a single fire event with a safety factor. Research shows
us a single fire event is commonly controlled by three to four sprinklers
activating, not all the sprinklers within a design area. It is also known when
sprinklers are installed above and below an obstruction typically both
sprinklers will not activate dependent on different factors. This is explicitly
addressed by Section 23.4.4.7.3 which states: “The requirements of 23.4.4.7.1.1
to include every sprinkler in the design area shall not apply where sprinklers
are provided above and below obstructions such as wide ducts or tables.” This
is further clarified by Section 23.4.4.7.3.1 which states: “Sprinklers under
the obstruction shall not be required to be included in the hydraulic
calculation of the ceiling sprinklers.” There is a caveat to the preceding
guidance on not including sprinklers beneath an obstruction in the calculation.
Although very poorly written, the intent of Section 23.4.4.7.3.2 is readily
understood. It states: “Where the piping to sprinklers under obstructions
follows the same sizing pattern as the branch lines, no additional hydraulic
calculations shall be required for sprinklers under obstructions.” This is also
discussed in the NFPA 13 Handbook commentary for Section 23.4.4.7.3.1 where it
states: “When spray sprinklers or CMSA sprinklers, which are covered in
23.4.4.6.3, are being used and sprinklers are installed both above and below
obstructions, all sprinklers above and below the obstructions are unlikely to
open. Rather than force the user to calculate both sets of sprinklers
simultaneously, the user is directed to prove the most demanding situation with
calculations. In most cases, ceiling sprinklers will represent the greatest
demand.” Then the commentary for Section 23.4.4.7.3.2 furthers this
clarification by stating: “This requirement clarifies that as long as the pipe
sizing under the obstruction is the same as the branch lines, the sprinklers
under the obstruction will not be more demanding because of the increased
elevation head dropping down under the obstruction.” The intent of Section
23.4.4.7.3.2 is not to require the pipe size for the drop below the obstruction
to be the same size as the branch line immediately above the location. It is
meant to be at least as large as the smallest pipe size for the number
sprinklers on the drop (aka the end of the branch line). It would make no sense
to require larger pipe for the drop further upstream where the available
pressure is greater.
