Thursday, May 2, 2019

Smoke Detector Spacing with Beams

Smoke Detector Spacing for Smooth Ceilings


Let's start with the basics of smoke detector spacing.  Based on NFPA 72, there is not a listed spacing and you are instructed to consult with the smoke detector's published documentation.  However, NFPA 72 2016 edition section 17.7.3.2.3.1 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 NFPA 72, we have spaced each detector at 15 feet from walls and 30 feet apart.    

smoke detector spacing for smooth ceilings
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 2016 edition section 17.7.3.2.3.1 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 spacing template for 30 feet
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.

NFPA 72 Smoke Detector Spacing Irregular Areas
NFPA 72 Smoke Detector Spacing Diagram



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 2016 edition section 17.7.3.2.4.2 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 


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 cal 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 2016 section 17.7.3.2.4.2 (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 2016 section 17.7.3.2.4.2 (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. 

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