Beam and Joists
NFPA 72 defines “ceilings” in Chapter 3. A “level” ceiling has a slope of less than or equal to 1 in 8. A “smooth” ceiling is uninterrupted by continuous projections extending more than 4 inches below the ceiling.
When a ceiling surface has solid projection more than 4 inches, the distance between the projections center to center determines whether the projection is a joist or beam. Projections more than 4 inches that are spaced 36 inches or less center to center are classified as joists. Projections projecting down from the ceiling more than 4 inches that are spaced more than 36 inches center to center are classified as beams. Both types of projections can impact detector spacing and location.
Joists and beams have different effects on automatic detection requirements, and also differ in how they effect requirements based on the use of heat detectors or smoke detectors.
Heat detectors on a solid joist ceiling require a 50% reduction (one-half) in spacing perpendicular to the run of the joists. There is no adjustment required in spacing parallel to the joists, but detectors are required to be mounted to the bottom on the joists.
Heat detectors on beam ceilings require a 2/3 reduction in spacing perpendicular to the run of the beams. There is no adjustment required in spacing parallel to the beams, but the location of the detectors further depends on the depth and spacing of the beams.
Where the beam project more than 18 inches below the ceiling and are spaced more than 8 feet on center, each bay formed by the beams must be treated as a separate area. Detectors would be required to be mounted on the ceiling.
Where the beams are less than 12 inches below the ceiling and spaced less than 8 feet on center, detectors are permitted to be mounted to the bottom of the beams. In this case, the only adjust would only be perpendicular to the beams (2/3 reduction in spacing.
The adjustment rules for smoke detectors are completely different that the rules for heat detectors. NFPA 72 considers joist to be equivalent to beams for smoke detector spacing guidelines. The spacing adjustments for smoke detectors on beam ceilings are based on three factors:
1. Ceiling height
2. Beam depth
3. Beam spacing
The relationship between these factors determines what effect (if any) beams have on smoke detector spacing.
Beam depths less than 10% of the ceiling height have no effect on smoke detector spacing. Smoke detectors may use smooth ceiling spacing (30’) without any required adjustments. For example, a 10’ ceiling with 11” beam depth may use smooth ceiling spacing. Beam spacing is not even a consideration factor.
Smoke detectors may be located on the bottoms of the beams or in beam pockets.
When the beam depth is equal to or greater than 10% of the ceiling height the beam spacing must then be considered. If the beam spacing is less than 40% of the ceiling height a 50% reduction of spacing (one-half) perpendicular to the run of the beams is required.
For example, consider a 10’ ceiling with 12” beam depth (=10%) and beam spacing of 47” (<40%). There is no adjust of spacing parallel to the beams, and detectors may be located on the ceiling or the bottom of the beams.
When the beam depth is equal to or greater than 10% of the ceiling height and the beam spacing is equal to or greater than 40% of the ceiling height smoke detectors must be located in each beam pocket. Each beam pocket is treated as a separate area for smoke detection.
For example, consider a 10’ ceiling with 12” beam depth (=10%) and beam spacing of 48” (=40%). Smoke detectors would be required in each beam pocket, and each beam pocket would be treated as a separate area for smoke detector spacing.