sprinklernotes
Narrow Ceiling Pockets
You asked the following question: “We have a ceiling pocket at the edge of the
room that contains the drapes for the exterior windows. It very narrow (only 1
ft) and well below the 1,000 ft3, but the depth is 3 ft 5 in. Are sprinklers
required within this pocket?”
We have reviewed NFPA 13 Standard for the Installation of Sprinkler Systems,
2016 edition as the applicable standard. Our informal interpretation is that
sprinkler should not be required.
While a literal interpretation of the standard would require sprinklers to be
installed because the depth exceeds 3 ft, that is not the intent of the
standard. A standard ceiling pocket captures the entire fire plume and delays
the activation of sprinklers installed below. AFSA did the fire modeling that
the criteria are based on and that study told us that a much deeper pocket will
still provide acceptable activation times, but the committee was uncomfortable
with accepting pockets with larger depths. An arbitrarily maximum depth of 3 ft
was assigned. Slightly exceeding 3 ft will not affect the performance. The
pocket will actually have the same impact on the heat flow as a skylight. We
ignore skylights because the air within them is heated by the sun, and the
buoyancy of the pre-heated air will limit the amount of smoke that will
actually make it into the skylight – this is why deflector distances are
measured to the imaginary plane at the bottom of the skylight and not up to the
glass or plastic cover at the top.
Since this allowance is not explicitly identified in the standard, it should be
discussed with the local AHJ and a written agreement with this interpretation
should be obtained before proceeding further with the project.
