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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It was a Saturday at closing time on March 25, 1911 in the heart of New York City. Young women (mostly immigrants) and some men were preparing to begin their single day off (a result of recent major labor reform) when a fire broke on the 8th floor of the Asch Building in Manhattan, endangering many and ending as the greatest workplace disaster in the US for the following 90 years.
David Von Drehle's 2003 non-fiction account of the Triangle Waistcoat Factory fire offers a thorough investigation of the social struggle for labor rights and a deep depiction of the era in which the awful event occurred.
The book focuses on the major labor disputes at the time, recognizing early beginnings of "sweat shops" (named for owners who would 'sweat' or cut pay for employees after they complete work and had earlier agreed to higher wages). Large immigrant influxes composed the early manufacturing labor in often cramped, poor conditions with 7-day weeks and long working hours.
The focus then shifts to the fire itself, detailing the development from a likely discarded cigarette to rapid growth from heaps of discarded textiles which ended up taking 146 lives. Locked exits (which were intended to funnel exiting and prevent theft), inadequately planned and installed fire escape, no sprinklers, ladder trucks which couldn't reach the height of the building, and severe lack of drills and warning about the fire all contributed to the disaster.
After the fire the book focuses on the trials of the owners, a Factory Investigation Commission, and the social reform for workplace condition improvement brought about by the labor unions formed in that era. Following the fire and recommendations for the independent commission, New York State legislature passed thirty-eight new laws regulating labor, wages, and safety, including mandates for exit door locking and swing direction, fire escape construction and design, egress access, and installations for alarm and sprinkler systems. Many states followed suit thereafter.
The book is a vivid account of the era, although it spends much more time in social injustice and labor reform than on the fire event and consequences of the fire than a fire protection professional may prefer.
Have you read it?
p.s. If you're interested in reading along with us, our next book is Chicago Death Trap by Nat Brandt, an account on the Iroquois Theater Fire of 1903. We'll review that volume on September 27th.
A 3-in-12 pitch to a ceiling or overhang might not appear that dramatic, but I came across a reminder again this past week as to the importance of paying attention to ceiling and overhang slopes.
We had a project with a corridor that had a high roof where routing pendents would be impractical. The slope of the corridor was 3 in 12 (3 inches vertical for every 12 inches horizontal), and so we evaluated use of sidewall sprinklers to protect the corridor.
Here's where there's three important points to remember came into play that offered a good refresher for us:
Sidewall sprinklers are required to have the deflector aligned parallel to ceiling or roof slopes (NFPA 13 2002-2016 Editions Section 220.127.116.11), and, where the slope exceeds 2 in 12 the sprinkler must be located at the high point of the slope and be positioned downward (NFPA 13 2002-2016 Editions Section 18.104.22.168.2). Additionally, as with all slopes, the sprinkler coverage is measured along-the-slope, not in floor area (NFPA 13 2002-2016 Editions Section 22.214.171.124.2).
Those can be easy-to-miss rules and I probably didn't pick up on them for longer than it should have taken when I first started designing fire sprinkler systems.
Other considerations that often pop up in these scenarios include:
(1) Use of extended coverage sprinklers may have specific limitations on how dramatic of a slope they can handle,
(2) Sidewall sprinklers must be listed for use when they are lower than 6 inches down from the ceiling or roof (NFPA 13 2002-2016 Editions Section 126.96.36.199.1.2). This listing often involves different required pressure and coverage. Reference the product data sheets to be sure installations match their listings.
(3) Sidewall sprinklers can't be located more than 6 inches from the wall on which they are mounted (NFPA 13 2002-2016 Editions Section 188.8.131.52.2.2)
What issues do you look for with sloped ceilings? Post in the comment section below.
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Joseph Meyer, PE, is a Fire Protection Engineer in St. Louis, Missouri. See bio on About page.