Smoke Detector Spacing with Beams

Smoke Detector Spacing for Smooth Ceilings

Let's start with the basics of smoke detector spacing.  Based on 2022 NFPA 72, there is not a listed spacing for smoke detectors. Instead, you are instructed to consult with the smoke detector's published documentation.  However, NFPA 72 2022 edition section 17.7.4.2.3.1 states the following, "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 feet 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 foot spacing.

What does this mean?

Number 1 above states you must have a smoke detector within one half the nominal spacing from walls.  One half of 30 feet is 15 feet.  In the image below you will see a total of six yellow circles each with a 30 foot diameter.  These circles represent the area covered by a spot type smoke detector based on a nominal spacing of 30 feet. As required by the 2022 NFPA 72, we have spaced each detector at 15 feet from walls and 30 feet apart.    

Smoke Detector Spacing with a Smooth Ceiling

What about the white areas not covered?

If you notice in the image, there are areas not covered by the yellow circles representing the smoke detector coverage.  This is where NFPA 72 2022 edition section 17.7.4.2.3.1 criteria number two comes into play.  

With a nominal spacing of 30 feet, you must insure that all areas of the ceiling have coverage within 0.7 times this 30 foot amount.  To find this distance, simply multiply 30 feet by 0.7 to get 21 feet.

If you use Pythagorean's Theorem you will come up with a surprising result.  Remember Pythagorean's Theorem is used to find the unknown side of a right triangle and is expressed as A squared + B squared = C squared.  In this case we have a right triangle in each quadrant of the yellow circles.  Each quadrant is 15 feet our and 15 feet up.  We can write this equations as:

15 squared + 15 squared = C squared 

225 + 225 = 450 squared

450 squared = 21.2132 feet

In the image below you will see a cleared depiction of how this all comes together. With this, it is clear that we have met the intent of the standard by mounting our smoke detectors 15 feet from the walls, 30 feet apart and still achieve 0.7 times the nominal spacing (21 feet) coverage at all points of the ceiling.

Smoke Detector Coverage for 30 Feet

NFPA and Smoke Detector Spacing Distances

The Annex of NFPA 72 provides us with a diagram to assist in smoke detector spacing. Note that smoke detectors are not listed for spacing.  Use the smoke detector's published installation documents and the spacing breakdown below to assist in your design.

For areas/corridors 10 feet wide, smoke detectors can be spaced at 41 feet.

For areas/corridors 15 feet wide, smoke detectors can be spaced at 39 feet.

For areas 20 feet wide, smoke detectors can be spaced at 37 feet.

For areas 25 feet wide, smoke detectors can be spaced at 34 feet.

For areas 30 feet wide, smoke detectors can be spaced at 30 feet.

Smoke Detector Spacing with Beam Construction

If your ceiling configuration involves beams, your smoke detector coverage can get a bit more tricky.

NFPA 72 2022 edition section 17.7.4.2.4.2 deals with level ceilings with beams. In a nutshell, this is how it breaks down:

• If the beam depth is LESS than 10% of the overall ceiling height, then smooth ceiling spacing for smoke detection can be applied.  Also note that in this scenario, you can choose to install the smoke detectors on the ceiling or the bottom of the beams. Reference the above text and images for smooth ceiling spacing.
• If the beams are are equal to or greater than 10% of the overall ceiling height, two scenarios are possible:
• If the beam depth is equal to or greater than 10% but less than 40%, use smooth ceiling spacing PARALLEL to the beams and one half spacing PERPENDICULAR to the beams.  With this scenario, the smoke detectors can be mounted on the ceiling or the bottom of the beams.
• If the beam depth is equal to or greater than 40%, a smoke detector shall be placed on the ceiling within each beam pocket.  Keep in mind that more than one smoke detector may be required to cover a given beam pocket.

How to Calculate Smoke Detector Spacing with Beam Construction 

Did you know the 2021 International Fire Code now requires 120 VAC single and multiple station smoke alarms to produce a 520 Hz low frequency audible tone? 

To calculate the beam depth for smoke detector spacing, convert your overall ceiling height into inches.  For example, if your ceiling is 12 feet it would convert to 144 inches.  Take 144 and multiply it by 0.1 to get 10%.  144" x 0.1 = 14.4".  In this case, ant beam depth of 14.4" or more would require an altered smoke detector lay out.  If you want an easier way to work this calculation and remember what spacing requirements go with the different percentages, we have you covered.  Download a FREE copy of our Excel Fire Alarm Calculation Tool and use the "SD BEAMS" tab.  All you need to do is input your ceiling height and beam depth in inches and the calculator will give you a color code for which spacing requirement is required (see image below).  Email us with any questions.   

Smoke Detector Spacing in Corridors with Beams 

What do you do if you have a corridor that is equal to or less than 15 feet in width with beams running perpendicular to the length of the corridor?  Consult NFPA 72 2022 section 17.7.4.2.4.2 (4).  This section of the standard allows you to use smooth ceiling spacing and the smoke detectors can be mounted on the ceiling, bottom of the beams or on the sidewall.

Smoke Detector Spacing in Rooms 900 Square Feet or Less

NFPA 72 2022 section 17.7.4.2.4.2 (5) allows the use of smooth ceiling spacing for smoke detection coverage in rooms that are equal to or less than 900 square feet.  You can also install the smoke detector on the ceiling or bottom of the beam. 

NFPA 72 2007 Mistake with Smoke Detector Spacing

After looking into the National Fire Protection Association NFPA 72 2007 edition, I came across what I consider a mistake.  Now this might be something that was intentional but I will explain my reasoning below.

NFPA 72 2007 section 5.7.3.2.3.5 states that for smooth ceilings, all points of the ceiling shall have a detector within 0.7 times the selected spacing.

Now this is no big surprise.  We know that smoke detector spacing is determined by the manufacturer's recommendations which is typically around 30'.  If we take 30' and multiply it by 0.7 we arrive at a new spacing for smoke detectors of 21'.  This is the reason, NFPA 72 clearly states that for corridors or rooms 10' in width or less, smoke detectors can be located 20.5' feet from the end wall and spaced 41' between detectors.

Knowing this information, "How many smoke detectors would be required to cover a room that measures 25' by 34'?".

To find the answer we need to use the Pythagorean Theorem of A squared + B squared = C squared.  This is the formula used to determine the distance of an unknown side of a triangle   For example or room measures 25' x 34' so we will need to solve for 25 squared + 34 square = C squared.  This equates to 625 + 1156 = The square root of 1781.  The square root of 1781 is 42.2019.  Now for someone who is new to this, it might seem kind of crazy to do all of this work to find out the quantity of smoke detectors required to cover a specific room size.  However, it is quite simple.  All we did by using the Pythagorean Theorem is determine the distance diagonally across the room measuring 25' x 34'.  We now know that the room measures 25' Long x 34' Wide x 42.2019' Across Diagonally.

Now using the 0.7 rule we know that all points in the room shall have a smoke detector within 21'.  Clearly out diagonal distance of 42.2019' split is half is greater than 21'.  With this information, we know that 2 smoke detectors are required to cover this space.

This is where it gets interesting:

NFPA 72 2007 section 5.7.3.2.4.2 deals with level ceilings of solid joist or beam construction.  Note #5 within this code section specifically states that if the room is 900 square feet or less, only one smoke detector is required.

If we take the manufacturer's recommended spacing of 30' for smoke detectors we can cover 900 square feet since 30' x 30' is 900'.  Keep in mind that this is not incorporating the 0.7 rule found in NFPA 72 2007 section 5.7.3.2.3.5.

This is where in my mind, the mistake is located.  According to NFPA 72 2007 edition, 2 smoke detectors are required to cover a room measuring 25' x 34' if the room has smooth ceilings.  However, if the room has solid joist or beam ceilings, you are only required to install one smoke detector.  Keep in mind both rooms are under 900 square feet as we are using the same measurements for both scenarios.  If anything  this rule should be reversed allowing only one smoke detector in rooms with smooth ceilings measuring 900 square feet or less.  Rooms with joists and beams will slow the movement of smoke across the ceiling to a point where additional smoke detectors may be required to achieve the same response time of the same room with smooth ceilings.  

The kicker is that NFPA must have caught this mistake as it has been corrected in the NFPA 72 2010 edition.  Now if you look at section 17.7.3.2.4.2(5) in the 2010 edition, it states that for rooms measuring 900 square feet or less, smooth ceiling spacing shall apply.

This takes us back to the smooth ceiling spacing section which requires a detector within 0.7 times the selected spacing to all points on the ceiling.

To recap, NFPA 72 2007 edition requires 2 smoke detectors for a room with smooth ceilings measuring 25' x 34' yet only requires 1 smoke detector if the room has solid joist or beam construction ceilings.  NFPA 72 2010 edition requires 2 smoke detectors in both scenarios.

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