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Using Smoke and Heat Detectors to Protect Difficult Areas

A ceiling fire alarm in commercial space.
Published On
Sep 11, 2020

I often get questions regarding detector placement in difficult areas, and most of the time the answer lies in the Annex A to NFPA 72 of the National Fire Alarm and Signaling Code. I will cover the most common questions here.

The first question is about smoke protection in an elevator machine room.

Most contractors know the general purpose of the elevator machine room smoke detector when actuated is to provide a flashing fire hat and send the elevator cars to the appropriate floor. But is the purpose of the smoke detection to protect the equipment or to protect the room? If the room is approximately 20-feet-by-20-feet with multiple ceiling pockets deeper than 12 feet, is there a requirement to put a smoke detector in each of these pockets? Or would a smoke detector in the middle of the room be enough?

The goal of the smoke detection in the room is to protect the room. The description of the room did not include the height, but the ceiling appears to resemble a “waffle-type ceiling” so with that information, take a look at Chapter 17 of NFPA 72-2019.

Section states in part:

“For level ceilings, the following shall apply:

  1. For ceilings with beam depths of less than 10% of the ceiling height (0.1 H), smooth ceiling spacing shall be permitted. Spot-type smoke detectors shall be permitted to be located on ceilings or on the bottom of beams.
  2. For ceilings with beam depths equal to or greater than 10% of the ceiling height (0.1 H), the following shall apply:
    1. Where beam spacing is equal to or greater than 40% of the ceiling height (0.4 H), spot-type detectors shall be located on the ceiling in each beam pocket.
    2. Where beam spacing is less than 40% of the ceiling height (0.4 H), the following shall be permitted for spot detectors:
      1. Smooth ceiling spacing in the direction parallel to the beams
        and at one-half smooth ceiling spacing in the direction perpendicular to the beams
      2. Location of detectors either on the ceiling or on the bottom of the beams
  3. *For beam pockets formed by intersecting beams, including waffle or pan-type ceilings, the following shall apply:
    1. For beam depths less than 10% of the ceiling height (0.1 H), spacing shall be in accordance with
    2. For beam depths greater than or equal to 10% of the ceiling height (0.1 H), spacing shall be in accordance with”

There is an asterisk next to number 3 that informs the reader to refer to Annex A for additional information that generally will explain why the requirement is allowed.

Another common question I receive is what spacing should be used when installing spot-type smoke detectors on ceilings that are twenty feet or higher?

Always think a question through. For example, why are smoke detectors installed in buildings? Usually for early warning of small fires. As ceiling heights increase above the normal of 10–12 feet, what size fire will be detectable by the same smoke detectors? It stands to reason the fire size must be larger. Therefore, those spot-type smoke detectors will not meet our fire protection goal.

Additionally, if we need staging to install spot-type smoke detectors on a ceiling, when do you think they will be serviced or tested after the staging is removed? (Answer: when they continually false alarm!) Others will tell me the code has a table that reduces the spacing of smoke detectors as ceiling height increases so they follow that. There is no such table. They are referring to the table for heat detector spacing reduction as ceiling height increases, which is applicable only for flaming fires.

The bottom line is never install spot-type smoke detectors on ceilings 20 feet or higher under any circumstances. In high ceiling environments, first determine what fire protection goal you are trying to meet. Then choose the appropriate smoke detection. The options available today include linear beam smoke detection and aspirating smoke detection.

I mentioned the table for heat detector spacing reduction above. This table was developed based on research for flaming fire detection only. The requirement in the code requires that on ceilings 10-30 ft high, heat detector spacing must be reduced in accordance with referenced table prior to any additional reductions for beams, joists or slope, where applicable.

That last phrase is important. For example, if I am going to use a listed 50-foot heat detector and I plan to install it on a 20-foot ceiling, I refer to Table, Heat Detector Spacing Reduction Based on Ceiling Height, and find that I must multiply my listed spacing by 0.64. My new spacing is now 32 feet on center. If there are beams, joists or a sloped ceiling, I now use the number 32 in all those spacing reductions. The table does not apply to line type or pneumatic rate-of-rise tubing heat detectors.

Bottom line is the code provides the guidance for almost all the protection situations you may encounter. The key is to know that often the help in understanding the requirements often appear in the Annex A of the code. If you are serious about being in this profession, I strongly recommend you purchase a personal copy of the NFPA 72.

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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