sprinklernotes
Closets Located Under Stairs
You asked the following questions: “We are installing a sprinkler system in a residence per NFPA 13D. We have a closet under the stairs that is less than 400 cubic feet in size. The AHJ is requiring a sprinkler to be located at the low point in the closet, which is under the intermediate landing of the stairs. His justification was that the stairs are a means of egress in a fire event. Is a sprinkler required at the low point of the closet?” In response to your question, we have reviewed NFPA 13D, 2016 edition as the applicable standard. Our informal interpretation is that with a closet not larger 400 cu. ft. size, a single sprinkler at the highest ceiling elevation shall be sufficient without regard to obstructions or minimum distance to the wall. The small areas in closets under the intermediate landing of stairs are usually filled with storage and will prevent the activation of the sprinkler in a fire event. With that being said, this section does mention minimum distances and does not mention maximum distances. The minimum distance is a problem since the sprinkler has to be kept out of the 4-in dead air space pocket created where the wall, ducts, or equipment intersects the ceiling. The maximum distance was not mentioned since this is not a concern for such a small space. This is supported by the commentary in the Sprinkler Systems for Residential Occupancies Handbook. This commentary indicates that in a small closet, the possibility of heat from a fire setting off a sprinkler is not a concern, regardless of where that sprinkler is in relation to the other objects that might be near the ceiling. The possibility of water spray absorbing the heat and helping to control the fire is also not an issue since the whole space will be flooded with water. Water will be bouncing off the walls and getting to the fire from many directions. A closet with 400 cu. ft. in volume will typically be about 50 sq. ft. in area, assuming an 8- ft high ceiling. At a minimum pressure of 7 psi, a sprinkler with a K-factor of 4.1 will discharge 10.8 gpm, creating a density of 0.22 gpm/sq. ft. in the closet, which is more than four times the density required for residential sprinklers. This excess density in a small space will more than make up for the obstructions caused by light fixtures or other items near the sprinkler. There is additional information in Supplement 2 of the Sprinkler Systems for Residential Occupancies Handbook that says the same thing. It also indicates that with a ceiling height of 8 ft to 10 ft, this would have a floor area of approximately 40 sq. ft. to 50 sq. ft. While this size room was never tested in full-scale fire tests, the committee was sure that this did not represent a significant problem as long as the sprinkler would open. Lastly, there is not a concern of getting water to the farthest distance under the stairs or getting the entire floor area wet. While this annex material is not referenced in NFPA 13D it does give us some guidance and the same issue is directly addressed in the 2016 edition of NFPA 13R in annex material A.6.4.6.3.2, that indicates in such small closets, a single sprinkler installed at the highest point will help contain a fire by cooling the hot gases that will collect at the uppermost point. There can be many different obstructions that will prevent water from reaching all portions of the closet or compartment. The annex gives three common examples: (1) Heating and air conditioning closets where the plenum totally blocks the spray pattern and in some cases only 4 inches or less exists between the wall and the plenum for the sprinkler; (2) Globe or fluorescent lights in closets that hang down obstructing the sprinkler; and (3) Closets under stairs that switchback where the spray pattern will not reach the low side. Number 3 acknowledges water will not reach the far spaces under the stairs where sprinklers do not have to spray to the lower portion of the closet area.
