New Rules for Smoke Detectors

At first glance, it appears that there are many changes to the spot-type smoke detection section of Chapter Five in NFPA 72, 2007 Edition. After a quick check of “Smoke-Sensing Fire Detectors,” section 5.7, you will notice there are no dots indicating...


5.7.3.2.4.2 For level ceilings the following shall apply:

(1) For ceilings with beam depths [ of

(2) For ceilings with beam depths [ >1 foot and ceiling heights of >12 feet] equal to or greater than 10 percent of the ceiling height (0.1 H) and beam spacing equal to or greater than 40 percent of the ceiling height (0.4 H), spot-type detectors shall be located on the ceiling in each beam pocket.

These changes remove the specific, but arbitrary, measurements of 12 inches and 12 feet, and replace it with the geometric concept of the inverted cone of smoke/heat above a fire. As the distance above the fire increases, so does the diameter of the smoke (cone) impinging upon the ceiling. This cone concept is not new for 2007. These ratios can be found in the appendix of earlier editions of NFPA 72. They were intended to be used as a guide for heat detector spacing. While these ratios were never a code requirement for heat detectors, they are suddenly necessary for the proper spacing of smoke detectors.

The next change in the 2007 edition section 5.7.3.2.4.2 addresses “waffle” ceilings that are typically made of cast concrete and are used to span larger areas. Some designers/inspectors were confounded with beams that ran both across the room and along the length. This scenario is now addressed with the following language, (unless they are >24 inches deep or >12 feet apart, in which case we are still confounded):

(3) For waffle or pan-type ceilings with beams or solid joists no greater than 600 mm (24 inches) deep and no greater than 3.66 m (12 feet) center-to-center spacing, the following shall be permitted:

(a) Smooth ceiling spacing including those provisions permitted for irregular areas in 5.6.5.1.2, substituting “selected spacing” for “listed spacing”

(b) Location of spot-type smoke detectors on ceilings or on the bottom of (waffle) beams

The reference to “selected spacing for listed spacing” used above and in the next section, refer to the fact that heat detectors are given a specific UL listed spacing to use, as opposed to the rules for spot-type smoke detectors which are permitted to have “other spacing” “selected” by the designer. Refer back to rule 5.7.3.2.3.3 shown above. (So a smoke detector layout can be greater than the above-mentioned 30-foot spacing!)

Continuing in section 5.7.3.2.4.2 we find that if a hallway is

(4) For corridors 4.5 m (15 feet) in width or less having ceiling beams or solid joists perpendicular to the corridor length, the following shall be permitted:

(a) Smooth ceiling spacing including those provisions permitted for irregular areas in 5.6.5.1.2, substituting “selected spacing” for “listed spacing”

(b) Location of spot-type smoke detectors on ceilings, sidewalls, or the bottom of beams or solid joists

The next new rule in this same section will have a bigger impact than you might first expect.

(5) For rooms of 84 m 2 (900 feet 2 ) area or less, only one smoke detector shall be required.

Picture a long, winding hallway in an apartment building leading to an elevator lobby. If the area is

In conclusion, it will take much scrutiny to uncover the new smoke detector spacing changes when reviewing your 2007 edition of NFPA 72. The sections you should pay particular attention to include the reference to only one smoke detector being needed in a 900 square foot space or less, the fact that joists or beams in hallways fifteen feet wide or less will not mandate the addition of additional smoke detectors, the clarifications in the section on waffle-type ceilings, and the changes for requiring detectors in each beam pocket when there are joist/beams that are deeper than 10% of the ceiling height when beam spacing is greater than 40% of the ceiling height. I hope this helped, because I need an aspirin.