What’s the Correct Measurement? Solving a smoke detection spacing dilemma

Published On
Dec 15, 2021

A county fire inspector and a city fire inspector disagreed about the proper spacing and locations of system smoke detectors in a residential building. They questioned whether the spacing should be from center to center or outermost edge of the rough-in boxes.

The inspectors were concerned that the measurements may be off. They have found that, once smoke detectors are installed, the spacing between edges of the detectors does not meet the 30-foot spacing requirements of NFPA 72, National Fire Alarm and Signaling Code. To be clear, they were using a corridor placement scenario.

As I have emphasized before, it is important to read the entire code when determining compliance. For new construction, the code requires that the location and spacing of smoke detectors be based on the anticipated smoke flows due to the plume and ceiling jet produced by the anticipated fire. This means that the design should account for the following factors at a minimum:

  • Ceiling shape and surface
  • Ceiling height
  • Configuration of contents in the protected area
  • Combustion characteristics and probable equivalence ratio of the anticipated fires involving the fuel loads within the protected area
  • Compartment ventilation
  • Ambient temperature, pressure, altitude, humidity and atmosphere

Section 17.7.1.11 of the code requires that “the effect of stratification below the ceiling shall be taken into account.”

Annex A states that “Stratification occurs when air containing smoke particles or gaseous combustion products is heated by smoldering or burning material and, becoming less dense than the surrounding cooler air, rises until it reaches a level at which there is no longer a difference in temperature between it and the surrounding air.”

Air stratification in a space can prevent the air containing smoke particles or gaseous combustion products from reaching ceiling-mounted smoke detectors. It occurs in high-ceiling areas, which means you should never place spot-type smoke detectors on high ceilings. In my opinion, anything over 15 feet qualifies as a high ceiling. The code recommends in Annex A for “high-ceiling areas, such as atriums, where spot-type smoke detectors are not accessible for periodic maintenance and testing, projected beam–type or air sampling–type detectors should be considered where access [to the detectors] can be provided.”

Where the smoke detection system is designed for smoldering or small fires and where the possibility of stratification exists, consider detector location and spacing very carefully. Small or smoldering fires are the most difficult to detect because there is no thermal lift to the smoke and early warning therefore becomes exceedingly difficult. That said, the use of aspirating-type or beam-type smoke detectors could help meet the detection goals for early warning.

To settle the inspector’s dilemma, we look to these NFPA 72 requirements:

“17.7.3.2.3 On smooth ceilings, spacing for spot-type smoke detectors shall be in accordance with 17.7.3.2.3.1 through 17.7.3.2.3.4.

“17.7.3.2.3.1* In the absence of specific performance-based design criteria, one of the following requirements shall apply:

“(1) The distance between smoke detectors shall not exceed a nominal spacing of 30 ft (9.1 m) and there shall be detectors within a distance of one-half the nominal spacing, measured at right angles from all walls or partitions extending upward to within the top 15 percent of the ceiling height.

“(2)* All points on the ceiling shall have a detector within a distance equal to or less than 0.7 times the nominal 30 ft. (9.1 m) spacing (0.7S).

“17.7.3.2.3.3 Other spacing shall be permitted to be used depending on ceiling height, different conditions, or response requirements.”

An asterisk indicates there is additional guidance in Annex A. For Section 17.7.3.2.3.1, it states that “The 30 ft. (9.1 m) spacing is a guide for prescriptive designs. The use of such a spacing is based upon customary practice in the fire alarm community. For the purposes of this section, ‘nominal 30 ft.’ should be determined to be 30 ft. + or –5%.”

For number 2, Annex A provides further guidance for when smoke detectors are used in building corridor applications. Specifically, it states that the 30-foot spacing may be extended up to 41 feet on center, depending on the corridor width.

All the above applies to smooth ceiling spacing. There are additional requirements for nonsmooth ceilings where joists or beams are present.

The inspector’s dilemma is solved by the code: moving a detector a few inches one way or another will not affect detection.

About the Author

Wayne D. Moore

Fire/Life Safety Columnist

Wayne D. Moore, a licensed fire protection engineer, frequent speaker, writer and expert in the life safety field, has been a principal member and chair of NFPA 72, Chapter 24, as well as a former principal member of NFPA 909 and NFPA 914. He is the...

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