by Shawn Lee
Next in our heat detector layout series is beam construction. For this post, we will assume that our beamed ceiling is level and the ceiling is 10 feet or less from the finished floor. There are some similarities between heat detector layout on joist ceilings and beamed ceilings with regards to reduced spacing at a right angle to the beams. However, the reduction is a little less. Therefore, we must reduce heat detector spacing by 50% at right angles to the joists — on a beam ceiling our heat detector spacing may have to be reduced to not more than around 66% of the listed spacing at right angles to the beams. This may seem a little confusing because that’s not how NFPA 72 phrases it. We will come back to that in just a bit. Here is what NFPA 72 has to say about heat detector location and spacing for beam ceilings:
- 17.6.3.3.1.1 A ceiling shall be treated as a smooth ceiling if the beams project no more than 4 in. (100 mm) below the ceiling.
- 17.6.3.3.1.2 Where the beams project more than 4 in. (100 mm) below the ceiling, the spacing of spot-type heat detectors at right angles to the direction of beam travel shall be not more than two-thirds of the listed spacing.
- 17.6.3.3.1.3 Where the beams project more than 18 in. (460 mm) below the ceiling and are more than 8 ft (2.4 m) on center, each bay formed by the beams shall be treated as a separate area.
- 17.6.3.3.2 Location. Where beams are less than 12 in. (300 mm) in depth and less than 8 ft (2.4 m) on center, detectors shall be permitted to be installed on the bottom of beams.
Let’s talk about each requirement. The first requirement, 17.6.3.3.1.1, simply tells us that beams that project 4 inches or less from the ceiling are not considered as beams. We treat it no differently than a smooth ceiling. As for where the detectors are mounted, make sure you review 17.6.3.3.2.
The second requirement, 17.6.3.3.1.2, tells us that if our beams do project more than 4 inches from the ceiling, then we must reduce our heat detector spacing at right angles to the beams. That reduced spacing is to be no more than 2/3 or about 66% of the detector’s listed spacing. The reason we say about 66% is because when doing the math, 2/3 of 100% = 66.66666…%. So, depending on whether you choose to round up or round down, you will have a maximum spacing of 66% or 67% of the detector’s listed spacing. That does make a difference in your layout. It is best to be more cautious and use 66%, as using 67% could result in a code violation. Therefore, using 66% or even 66.66666% vs. 67% is recommended. For example, in the case of our heat detector with 30-foot spacing here’s the difference:
- Option #1: S x 0.66
30 x 0.66 = 19.8 feet spacing at right angles to the beam
- Option #2: S x 0.6666666
30 x 0.66 = 19.999998 feet spacing at right angles to the beam
- Option #3: S x 0.67
30 x 0.67 = 20.1 feet spacing at right angles to the beam
30 x 0.66 = 19.8 feet spacing at right angles to the beam
30 x 0.66 = 19.999998 feet spacing at right angles to the beam
30 x 0.67 = 20.1 feet spacing at right angles to the beam