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 text was moved/removed, but you will notice a dozen or so vertical lines alongside the rules in this important section.

For those not familiar with the layout of the codes and NFPA standards, a line is used to denote changes from the previous edition. In this case, the previous edition is from 2002. The line is made vertically, in the margin alongside the passage in which the changes were made. (The hardbound, illustrated Handbook version uses an icon next to the paragraph stating “New,” but unlike the standard softbound edition, they do nothing to indicate something is missing.)

In the section being discussed here, two of these change lines are rather long, and along with the other ten lines, seem to indicate substantial changes have been made to the 2007 edition. Or have they? NFPA doesn't make it easy when they don't say what the change is, exactly. The changes can only be found by carefully comparing the text from the previous edition (or reading a comprehensive technical article in your favorite trade magazine by yours truly). Most of the lines indicating changes are nothing more than fine-tuning, and are neither changes nor additions. For instance, in section 5.7.3.2.3.2 the word “documented” is replaced with “published” and now reads, “In all cases, the manufacturer's published instructions shall be followed.” The change from using “documented” to “published” added about four feet of vertical lines in the margins of the 2007 edition of NFPA 72.

Another section that seems to have been re-worked to incorporate additional requirements brings up the concepts of performance-based and prescriptive-based layouts – neither of which is new. NFPA 72, 2002 at 5.7.3.2.3.1 stated that smoke detectors shall be permitted to be located using a 30-foot spacing as a guide . The 2007 change now has that section beginning with “In the absence of specific performance-based design criteria…” This means that now, when you are required to comply with the 2007 edition, you must either designate your detector placement layout as “performance based” or use the simple, prescriptive, 30-foot spacing. The “as a guide” has been dropped. The very next rule, 5.7.3.2.3.2, still states that “in all cases” the manufacturer's instructions must be followed. I'm guessing that manufacturers will all state it should be < 30 feet since anything more would increase their liability. A larger spacing would also decrease the number of smoke detectors needed for a project, and that would mean a decrease in sales. So my take is: spot-type smoke detectors will be spaced at 30 feet unless the AHJ allows you to do a performance-based design (fat chance), or unless the manufacturer states the spacing must be less than 30 ft. (better chance) because 30 ft. is now a rule, not a guide .

That thinking seams logical; that is until you get to the next rule, which was left unchanged, and turns the 30 foot back into “a guide”: “5.7.3.2.3.3 Other spacing shall be permitted to be used depending on ceiling height, different conditions, or response requirements.” Confused? Expect more vertical lines in future editions when they try to better explain these 2007 changes.

FYI: If you want to submit performance-based information you would need to provide for review and approval “…documentation, in an approved format, of each performance objective and applicable scenario, together with any calculations, modeling, or other technical substantiation used in establishing the proposed design's fire and life safety performance.” The following must be evaluated :

(1) Structural features, size, and shape of the rooms and bays

(2) Occupancy and uses of the area

(3) Ceiling height

(4) Ceiling shape, surface, and obstructions

(5) Ventilation

(6) Ambient environment

(7) Burning characteristics of the combustible materials present

(8) Configuration of the contents in the area to be protected

Real changes were made to section 5.7.3.2.4 regarding spacing to be used in rooms or spaces with solid joist or beam construction. To show the changes, I have inserted the 2002 wording in brackets, next to the underlined 2007 wording.

5.7.3.2.4.2 For level ceilings the following shall apply:

(1) For ceilings with beam depths [ of < 1 foot and ceiling heights of < 12 feet] of less than 10 percent of the ceiling height (0.1 H), smooth ceiling spacing shall be permitted.

(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 < 15 feet wide and has beams or joists running in any direction, you can ignore them. The bounding walls of the hallway force the heat/smoke to flow over the projections with virtually no loss in detection response time.

(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 < 900 square feet, the elevator recall smoke detector is the only required detector.

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.