Smoke Detector Spacing on Level Ceilings with Beams and Joists
Following NFPA 72 2002 requirements could result in higher installation costs.
Ceilings with beams and joists affect detector spacing.
Executive Overview
NFPA 72, The National Fire Alarm Code, recommends using 900 square feet (30 feet on center) as a guideline for spacing spot type smoke detectors on smooth ceilings. While this is only a recommendation, virtually all manufacturers of smoke detectors follow this guideline. There has always been a question of how detector spacing needs to change when the ceiling is not smooth, but is interrupted by beams or joists. The language in the 2002 edition of NFPA 72 code was based on the fire dynamics research reported in
Industry Challenges
The 2002 edition of NFPA 72 requires that when beams project more than 12 inches below the ceiling and when ceiling heights exceed 12 feet, each bay (beam pocket) formed by the beams should/must be treated as a separate area. That is, each bay would need one or more detectors depending upon its physical size. When this requirement was first incorporated into the code, it was backed by the best CFD data available at the time. Subsequent to this code change, many fire alarm system designers discovered that following the literal code text could mean using a large number of detectors and incurring exorbitant installation costs in many common building designs. Historically, this part of the code has been a subject of contention in the fire protection industry and among NFPA 72 Initiating Devices Committee members.
Current Recommendations for Smoke Detector Spacing:
NFPA 72 requires different detector spacing depending upon whether the structural or ornamental members on the ceiling fall into the definition of a beam or a joist and the depth of those members. The code defines solid joist construction as members that are greater than 4 inches in depth and spaced 3 feet or less apart. Beam construction, on the other hand, is defined as members that are greater than 4 inches in depth that are spaced more than 3 feet apart. For the purposes of spot- type smoke detectors, the spacing rules are the same for ceilings with beams and solid joist construction.
For level ceilings, NFPA 72 currently requires spacing smoke detectors at one half their listed spacing, perpendicular to the run of the beams on a ceiling less than or equal to 12 feet in height, when the beams are less than or equal to 12 inches deep. When parallel to the beams, NFPA requires standard spacing for smoke detectors. In this case, the detectors may be mounted on the ceiling or on the bottom of the beams. If the beams exceed 12 inches in depth or the ceiling height of the space exceeds 12 feet, the detectors are required in every beam pocket on the ceiling.
Research
In April 2005, the Fire Protection Research Foundation funded new research using CFD computer modeling to reexamine smoke detector spacing on ceilings with beams and/or joists. The CFD analysis was performed by Schirmer Engineering Corporation
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