#127 – Fire Alarm Detector Layout Part 3: Solid Joists

by Shawn Lee

To continue our posts about detector layout, we will review how to configure a fire alarm system layout on ceilings that have joists, beams, other obstructions, or a combination of ceiling surfaces. In this post, we discuss solid joist construction and how that affects your heat detector layout.

First, read NFPA 72’s definition of a solid joist ceiling surface. In Chapter 3, Definitions, we find Solid Joist Construction. It states: “Ceilings that have solid structural or solid nonstructural members projecting down from the ceiling surface for a distance of more than 4 in. (100 mm) and spaced at intervals of 36 in. (910 mm) or less, center to center.”

Now, review what NFPA 72 says about spacing heat detectors in a space with solid joist construction. NFPA 72,, titled Spacing states: “The design spacing of heat detectors, where measured at right angles to the solid joists, shall not exceed 50 percent of the listed spacing.”

Let’s look at this requirement. Since we already know how to perform a heat detector layout on a smooth ceiling and the calculations involved, we only need to do one more calculation on a ceiling with solid joists…reduce the spacing at right angles to the joist by 50 percent (or one-half). Why do we have to do this? It is because of the way heat will travel across the ceiling. The “pockets” formed by the solid joists will impede the rate of speed the heat travels to get to the area of the heat detector. To offset the negative effect joists have on how quickly the heat will travel across the ceiling, we space the heat detectors closer together, so the response time is shortened. This will ensure occupants are notified of a fire as soon as possible.

If we go back to our illustration of a 30-foot by 30-foot room with a smooth ceiling and a heat detector with 30-foot spacing, we get this:

Fire Sprinkler Head used to hang picture

We will only require one heat detector for this area. However, what if the ceiling were not smooth, but had solid joists on it? In the direction of solid joist travel, or parallel to the joists, spacing is the same as we have previously discussed. The spacing will be no more than one-half the heat detector’s listed spacing from the walls, measured at a right angle. However, at a right angle to the solid joist, the heat detector spacing is further reduced by 50 percent of its normal spacing…in this case, instead of being able to space the heat detector at 15 feet from the nearest wall, we may only space it a maximum of 7.5 feet from the wall. Look at the illustration below to get an idea of our room with no detectors placed in it yet.

Fire Sprinkler Head used to hang picture

Because of the requirement to reduce the spacing at right angles to the joist, I always start my heat detector layout at right angles to the joists. This makes it easier to know where my heat detectors will end up. Before placing heat detectors, we will need to know the spacing between the joists. For our example above, assume that the joists are centered at 36 inches (3 feet) between them with 18 inches (1.5 feet) between the walls and the center of the nearest joists.

Placing the first heat detector is illustrated below. Keep in mind that at a right angle to the joist, we must reduce the detector’s spacing by 50 percent. So, since we normally have a maximum of one-half the detector’s spacing from the wall, and we also need to reduce the spacing by 50 percent for the joists, we end up with our spacing divided by 4 for the detector nearest the wall. In this case, for our 30-foot detector, the maximum distance from a side wall at that right angle would be 30 feet ÷ 4 = 7.5 feet. Look at the illustration below for how to lay out the first heat detector.

Fire Sprinkler Head used to hang picture

Now that we have placed our first heat detector, we will place our second detector. And we know we need another detector since the detector is more than 7.5 feet away from the wall at a right angle to our joists. There is still 22.5 feet from the first detector to the furthest (or eastern) wall.

Fire Sprinkler Head used to hang picture

We’ve placed a second heat detector and now we need to figure out if we need any additional detectors. So here goes: each heat detector is spaced no more than 7.5 feet when measured at a right angle to the joists. Check. The detector spacing running parallel with the joists are at normal spacing. The detectors are no more than one-half the listed spacing from the walls…in this case no more than 15 feet. Check. One last thing to look for is the distance between the detectors. Since we are using a 30-foot detector and we are looking at the distance between the detectors at a right angle to the joists, our spacing between the detectors must be at 50 percent of the normal listed spacing per the requirements of That means there shall be no more than 15 feet between detectors. Check.

Now that we’ve laid out the detectors, we need to double check that we meet the location requirement of NFPA 72. The next statement in NFPA 72 is, Location. It states: “Detectors shall be mounted at the bottom of the joists.” In our case it works out since the joists we placed the detectors on are spaced at 3-foot intervals with the first joist being 1.5 feet from the wall. However, there will be times when the joists are not spaced in a way that you will be able to place a detector exactly where you want to. In those cases, you may have to get closer to the sidewalls. This may open up the distance between your detectors, forcing you to place one or more additional detectors for your system. It’s just something to keep in mind.

Placing detectors on a ceiling with joists is not overly complicated if you know the basics of detector layout. We just need to remember that at a right angle to the direction of joist travel, your detector spacing is reduced by 50 percent. In the direction parallel to the joists, the spacing is not reduced.

For our examples, we used 30-foot spacing for our heat detectors. There are other listed spacing distances for detectors as well. You might decide to spec out a detector with a different spacing or you might be limited in what you can spec out due to AHJ or manufacturer limitations. It really doesn’t matter so long as you follow the basic rules of doing detector layout. The number of detectors may be different, but the area will have the correct coverage.

To read the next article in this series, Link to: #130 – Fire Alarm Detector Layout Part 4: Beam Construction

To read the previous article in this series, Link to: #119 – Fire Alarm Detector Layout Part 2: Irregularly Shaped Areas