While smoke detectors often have recommended spacing of 30 feet (identified in manufacturer's product data), spacing 30-feet on center is not the only way to space smoke detectors. NFPA 72 offers two methods for spacing smoke detectors on smooth ceilings:
The first method is simply to provide detectors at their listed spacing (often 30 feet), center-to-center, and within half the distance (which is 15 feet) to walls. [NFPA 72 2002 220.127.116.11.3(A-B), 2007 18.104.22.168.3.1-.2, or 2010-2016 22.214.171.124.1(1)]
The second, often lesser-known method, is to provide smoke detectors such that all points on the ceiling are within a distance of 0.7 times the listed spacing, or less [NFPA 72 2002 Section 126.96.36.199.3(E), 2007 188.8.131.52.3.5, or 2010-2016 184.108.40.206.1(2)].
Applying the Method
In practice, this simply results in drawing a 21 foot circle (0.7 x 30-foot spacing = 21 feet) around each detector and making sure that every point on the ceiling is covered. On site, it would simply result in making sure every spot on the ceiling is within 21 feet of a smoke detector.
This second method becomes important for complex room configurations, long and narrow corridors, or as a way to simply provide smoke detectors at their most efficient coverage.
A corridor which is 100-feet long and 10-feet wide, for instance, would require 4 smoke detectors under their listed spacing (30-feet spacing on center and 15-feet to the corridor ends). Using the second spacing method allowed by NFPA 72, these smoke detectors can be spaced nearly 41 feet center-to-center, requiring only 3 smoke detectors to be used.
Using the Second Method
Fundamentally, the theory is that smoke production will fill a ceiling based on the area of the ceiling. For a long, narrow corridor, smoke will be limited in it's spread in the narrow dimension, forcing travel down the corridor. As a result, smoke detector response time is dependent upon the amount of area the detector covers, not necessarily the spacing between detectors.
Matching smoke detector layouts to the nature of smoke transport and this code allowance could result in a simpler approach and often the need for less smoke detectors overall.
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