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Closing the Gap: Design Guidance for Smoke Detector Spacing on High Ceilings
By: Jacqueline R. Wilmot, PE & Victoria Hutchison, The Fire Protection Research Foundation, USA
What is a “high” ceiling? That depends not only on whom you ask but also the location of the high ceiling in question. Sprinkler system designers might say 15.2m (50ft), while smoke management designers may say this depends on the ratio of the limiting height in relation to the ceiling height in the mass entrainment equations to maintain tenable conditions. A fire alarm system designer might declare 9.1m (30ft) for heat detectors and 3m (10ft) for smoke detectors in the US or 12.2m (40ft) in Europe and Australia. The fire protection industry has struggled to determine the appropriate spacing requirements when installing smoke detectors above 3m (10ft) since their invention more than a hundred years ago. this, however, is beginning to change.
The Fire Protection Research Foundation (FPRF), the independent research affiliate of the National Fire Protection Association® (NFPA®), has completed a significant amount of work over the years to thread the needle on this issue and provide answers to long-standing questions.
In 2017, FPRF published a Phase I research study1 that reviewed the impact of ceiling height and detector spacing on smoke detection performance. The outcomes of this study indicated that there was limited context to substantiate the varying global requirements and significant knowledge gaps existed that precluded the formulation of scientifically justified prescriptive requirements regarding smoke detector spacing relative to ceiling height, specifically for NFPA 72®, National Fire Alarm & Signaling Code®. To ensure that there was appropriate scientific and engineering basis for future code requirements, additional research was initiated and published by the Foundation in 2023, providing long desired guidance on this issue and identification of the limitations of prescriptive approaches3.
Based on the findings of Phase I and Phase II of the Smoke Detector Spacing for High Ceiling Spaces research program, here are the key takeaways you should know:
Requirements and Substantiations: They’re All over the Map
Imagine you are hired as the fire protection engineer for a new global hotel and conference center chain that will be constructing the same exact building in several different countries. In reviewing the plans, you realize the conference center and the atrium in the hotel lobby call out high ceilings. As the engineer of record, you determine the applicable codes and standards that apply and are enforced in each jurisdiction applicable to the respective new builds. A summary of your analysis is provided in Table 1 (below). In this evaluation, you notice variance in thresholds for different types of detectors, ceiling height limitations and the associated spacing required between the detectors at the various ceiling heights. Why is this and what does this mean? Let’s delve into the variations in global requirements and their rationale to find out.
Table 1 Summary of Ceiling Height and Spacing Limitations
Note: From: Datta, M. 2023. Smoke Detector Spacing for High Ceiling Spaces: Phase II (Report No. FPRF-2023-08). The Fire Protection Research Foundation.
UK: The applicable fire alarm codes used in the UK provides maximum ceiling heights for heat, smoke, and combustion gas detectors to be mounted on. Additionally, there are ceiling height limits for scenarios in which sections of the ceiling exceed prescribed limits, given that the higher parts of the ceiling are not more than 10 percent of the total ceiling area.
Europe & Australia: It appears that Europe and Australia have the same ceiling height limitations of 12 m (39.4 ft) with few differences in spacing requirements for smoke detectors. However, other countries and regions, including Canada, the United States and the Middle East, reference codes and standards that have lower ceiling height thresholds for prescriptive installation requirements for smoke detectors or don’t address higher ceilings at all. By investigating this further to gain better insight into the justification of these values and identify European codes and standards, which reference EN 54-7, Fire Detection and Fire Alarm Systems for technical justification of the smoke detector spacing requirements, it’s clear that this standard reference limited test data in the context of ceiling height.
The limited test data referenced is the 1997 study conducted by NIST titled “Analysis of High Bay Hangar Facilities for Fire Detector Sensitivity and Placement”2. In this experiment, 33 full-scale tests were conducted in two hangars with 15m (49ft) and 22m (72 ft) ceiling heights, using 100kW to 33MW fires using jet propellant fuels (JP)-5 and JP-8 to analyze the behavior of heat and smoke and response times of detection and sprinkler suppression systems. The conclusions made from this study suggested spot-type detector spacing to be increased to 12.2m (40ft) without significant variation in response time and in future research to compare these results to other high ceilings. The study is helpful, but the JP-5 and JP-8 fuels produced sootier smoke than what would typically be projected in a fire scenario for a commercial building.
USA & The Middle East: NFPA 72 does not address spacing considerations for smoke detection on ceilings higher than 3m (10ft). More specifically, NFPA 72 does not provide a prescriptive ceiling height limitation but requires stratification - the phenomenon where the upward movement of smoke and gases ceases due to the loss of buoyancy - to be accounted for.
The current prescriptive installation requirements state that for smooth, flat ceilings, the maximum distance between smoke detectors is a nominal 9.1m (30ft) up to 3m (10ft), and the distance between detectors must not exceed half of the nominal spacing measured from the wall or any partitions extending upward to within the top 15 percent of the ceiling height. Additionally, all points on the ceiling are required to have a detector within a distance equal to or less than 0.7 times the nominal 9.1m (30 ft) spacing.
Canada: In analyzing the applicable fire alarm installation codes in Canada, for ceilings exceeding 3.6m (11.8ft), you must use your engineering judgement in a performance-based design approach to account for the expected fire type, growth rate and the smoke detector manufacturer’s recommendations.
There is a table in NFPA 72 that is specific to heat detectors, but is often misapplied to smoke detectors, which leads to confusion and inconsistencies in design and code enforcement. Table 17.6.3.5.1 requires reductions in heat detector spacing with increasing ceiling heights, but this is specific to heat detectors, not smoke detectors. This table essentially says the higher the ceiling height, the closer the spacing needs to be between heat detectors. This is because as the ceiling height increases, the plume rises, mixing with additional cooler air, affecting its buoyancy and temperature and therefore the time when the heat detector would activate. To account for this, the multiplier in the table is prescribed to reduce the listed spacing of a given heat detector. The basis for the multipliers in Table 2 provided below was determined through experiments under the National Bureau of Standards, now known as National Institute of Standards and Technology (NIST).
Table 2 : 17.6.3.5.1 Heat Detector Spacing reduction Based on Ceiling Height, NFPA 72®, 2022 ed.
Note: Reproduced with permission of NFPA from NFPA 72®, National Fire Alarm and Signaling Code®, 2022 edition. Copyright© 2021, National Fire Protection Association. For a full copy of NFPA 72®, please go to www.nfpa.org. NFPA 72® and National Fire Alarm and Signaling Code® are registered trademarks of the National Fire Protection Association, Quincy, MA. All rights reserved.
Challenges Applying Existing Guidance
Clearly, the specifications for smoke detector spacing, ceiling height limitations and the respective justifications vary globally. As the system designer for international properties, there is a lack of clarity and consistency around appropriate spacing for standard smoke detectors on high ceilings. This is particularly true for the United States and Middle East regions, where guidance has traditionally been limited and the most restrictive for placement of traditional smoke detectors on ceilings greater than 10 ft (3 m) in height. Due to the aforementioned limitations, the use of beam detectors is often considered as an alternative approach. While this approach works for many applications, it can be a fleeting thought as it can be complicated by the presence of structural elements such as trees in the atrium or the moveable partitions specified in the plans of the conference center, which may result in beam detection no longer be an optimal solution. At this point, a designer may opt to use a performance-based design approach and engineering judgement, recognizing that in a high ceiling space, the smoke may stratify and not reach the detectors on a high ceiling. From there, the specific situation - including smoke flows, ceiling jets, ambient airflows, and the heat release rate of a potential fire must be considered to determine the optimal smoke detector system design and placement given the ceiling height and detector performance in the space.
The bottom line is that the design of smoke detectors in ceilings above 3m (10ft) has not necessarily been easy or straightforward.
New Guidance
The impact of ceiling height and detector spacing on smoke detection performance has challenged the fire protection industry for years, and additional information was needed. In 2023, the Fire Protection Research Foundation published the Phase II Smoke Detector Spacing for High Ceiling Spaces study2 which recommended providing prescriptive guidance for the installation of smoke detectors up to 12.2m (40 ft) ceiling heights and to encourage performance-based design for the installation of smoke detectors where ceiling heights exceeding 12.2m (40ft). For ceilings between 3m (10 ft) and 12.2m (40 ft), this report established equations that can be used to determine appropriate spacing based on the ceiling height for spot type and beam type detectors for smooth, flat ceilings.
This developed guidance has been used by the Technical Committee responsible for the development of NFPA 72 in the most recent committee meeting and serves as the technical basis for consideration for changes to the 2025 edition of the code.
For more information, please download the full report or scan the QR code. 
References:
1. Accosta, Robert, Martin, Drew. 2017. Smoke Detector Spacing for High Ceiling Spaces. (Report No. FPRF-2017-20). The Fire Protection Research Foundation https://www.nfpa.org/en/Education-and-Research/Research/Fire-Protection-Research-Foundation/Projects-and-Reports/Smoke-Detector-Spacing-for-High-Ceiling-Spaces
2. Gott, Joseph E, Darren L Lowe, Kathy A Notarianni, and William Davis. 1997. Analysis of High Bay Hangar Facilities for Fire Detector Sensitivity and Placement. National Institute of Standards and Technology.
3. Datta, M. 2023. Smoke Detector Spacing for High Ceiling Spaces: Phase II (Report No. FPRF-2023-08). The Fire Protection Research Foundation. https://www.nfpa.org/education-and-research/research/fire-protection-research-foundation/projects-and-reports/smoke-detector-spacing-for-high-ceiling-spaces-phase-ii