The fire sprinkler installation standard’s latest version offers new protection options and rules for sloped and high ceilings
The 2025 edition of NFPA 13: Standard for the Installation of Sprinkler Systems has numerous revisions and additions that enhance fire protection by adapting to new technology and research. In part one of our series on NFPA 13 2025 updates, we reviewed some of these modifications, including:
- Changes to the rules for dry and preaction systems, including new technologies
- The introduction of “supplemental sprinklers” and new design criteria for their use
In this article, we examine a few additional revisions, along with explanations of key concepts:
- New protection options for sloped ceilings in storage and other settings
- Updated criteria for high ceilings in non-storage areas
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New protection options for sloped ceilings in storage applications and other settings
The fire protection issues presented by sloped ceilings
NFPA and various fire protection organizations continually run experiments to develop evidence-based standards, especially for challenging fire environments like storage facilities. The NFPA 13 2025 changes surrounding sloped ceilings are the product of this evolution; data from the Fire Protection Research Foundation (FPRF) showed it’s possible to protect areas, including storage occupancies, that have more significantly sloped ceilings under the right circumstances. This new data builds on older information and changes how sprinklers can be installed and how designers should complete hydraulic calculations.
The main issue with steeper inclines is heat collection; hot air from a fire rises and collects in the peaks of sloped ceilings, meaning that the sprinklers that activate aren’t necessarily the ones right above a fire. This high and dense collection of heat in the roof’s peak (or within improper channels) can also lead to unreasonable delays in sprinkler activation. So, previous editions of NFPA 13 limited options for protecting storage under sloped ceilings exceeding a 2 in 12 pitch—where the height rises two inches for every foot of horizontal distance—and ceiling-only sprinkler designs were not allowed.
New experiments were needed because the original tests that determined the criteria for storage protection were done using flat roofs. However, later testing showed that if a sloped ceiling did not exceed a 2 in 12 rise-over-run slope (a 16.7% rise), there would be no problems (20.9.1 Enhanced Content).
New research from FPRF has clarified what works in sloped situations, and the NFPA 13 2025 edition includes several options and scenarios.
A review of obstructed vs. unobstructed construction
The key issues the rules in NFPA 13’s new sections on sloped ceilings deal with are how a sloped ceiling may impact heat collection and ensuring enough sprinklers go off when they should in order to handle a fire. Crucially, NFPA 13 provides new protection options in a setting where a ceiling’s slope is beyond 2 in 12 but not more than 4 in 12. A key differentiator in these protection options is whether a ceiling is classified as obstructed or unobstructed construction.
Here’s a basic review of what those terms mean and their impact (some emphasis added):
From the 2025 edition of NFPA 13
3.3.46.1 * Obstructed Construction.
Panel construction and other construction where beams, trusses, or other members impede heat flow or water distribution in a manner that materially affects the ability of sprinklers to control or suppress a fire. (AUT-SSI)
3.3.46.2* Unobstructed Construction.
Construction where beams, trusses, or other members do not impede heat flow or water distribution in a manner that materially affects the ability of sprinklers to control or suppress a fire. Unobstructed construction has horizontal structural members that are not solid, where the openings are at least 70 percent of the cross-section area and the depth of the member does not exceed the least dimension of the openings, or all construction types, with the exception of panel construction, where the spacing of structural members exceeds 71⁄2 ft (2.3 m) on center. (AUT-SSI)
The type of roof is a crucial distinction. A sloped ceiling with unobstructed construction generally has a smoother surface where more heat will naturally travel to the peak and delay heat collection around the sprinklers. In contrast, obstructed construction offers potential avenues to catch heat in channels between purlins, beams, etc.; only if the sprinklers are placed correctly based on obstructed rules will the heat from a fire gather around them and set them off properly.
NFPA 13’s new rules for sloped ceilings in storage applications
Here are the options NFPA 2025’s new section 20.9.1.1 provides for protecting sloped ceilings in storage, interspersed with our explanations:
20.9.1.1* Protection Options.
Where the ceiling slope is greater than 2 in 12 (16.7 percent), storage shall be permitted to be protected by any one of the following methods:
(1) Protect storage with in-rack sprinklers in accordance with one of the options in Section 25.7 provided that no storage is placed above the highest level of in-rack sprinklers.
(2) Install a horizontal false ceiling capable of withstanding an uplift force of 3 lb/ft2 (15 kg/m2) below the sloped ceiling supplemented with ceiling sprinklers below.
Item (1), using in-rack sprinklers in obstructed or unobstructed construction, is not a new solution. In-rack sprinkler designs can avoid the problem of uneven heat collection in a sloped ceiling altogether since the sprinklers are within storage racks.
Item (2) also avoids heat collection problems with sloped ceilings in obstructed or unobstructed construction by installing a false horizontal ceiling that evenly distributes hot gases.
(3) Install sprinklers per guidance in Chapters 20 through 26, which permits a ceiling that exceeds 2 in 12.
Item (3) gives the impression it is directing readers to specific guidance on slopes in all these chapters. It certainly does that in the case of chapter 20, which is the chapter in which this section appears. However, chapters 21–26 have no main sections explicitly mentioning sloped ceilings.
So, this item means designers should follow the other applicable rules for a specific design and sprinklers, including protection for commodity types, storage heights, scenarios for using in-rack sprinklers, sprinkler placement, clearance requirements, and other design considerations. This reference to all these chapters may also serve as a placeholder for future slope-specific rules in different scenarios.
The next option:
(4) For obstructed construction where the ceiling slope does not exceed 4 in 12 and the storage is protected with CMDA sprinkler approach, install sprinklers in every channel.
Item (4), which involves using CMDA (control-mode special application) sprinklers and their accompanying design approach, specifies that the sprinklers must be in every channel within obstructed construction. This placement ensures heat will collect around the sprinklers and set them off when they are needed.
The next option:
(5) For unobstructed construction where the ceiling slope does not exceed 4 in 12, increase the design area of the ceiling sprinklers by 50 percent from the criteria in Chapters 20 through 26 unless [those] criteria [are] specifically for sloped ceilings.
Item (5) uses a design approach that the standard employs elsewhere: increasing the square footage of the design area, which is the most hydraulically challenging place to control a fire in the structure.
Designers calculate how many sprinklers are needed to achieve a specific design density—leading to the water flow and coverage in the space—within the design area based on the occupancy, hazard level, and square footage. Here is the basic formula, solving for flow:
Flow (gpm) = density (gpm/sq. ft.) x design area (sq. ft.)
So, if the design area increases by 50%—say from 2,500 to 3,750 square feet—but the required density of 0.30 for a specific hazard does not change, then the flow will increase from 750 GPM to 1,125 GPM. Thus, more sprinklers that can catch heat and put out more water are in the larger design area, providing a buffer that ensures adequate fire protection in that sloped-roof scenario.
Finally, here’s the last protection option and scenario in section 20.9.1.1:
(6) Use the sprinkler system criteria specified in Chapters 20 through 26 for obstructed construction where the ceiling slope does not exceed 4 in 12 and all of the following conditions are met:
(a) Purlins or beams supporting the roof deck run across the roof slope.
(b) Purlins or beams do exceed 18 in. (450 mm) deep.
(c) Purlins do not exceed 5 ft (1.5 m) on center.
(d) Bays created by the solid structural members are not more than 40 ft (12 m) on center.
(e) Purlin or beam channels are provided with blocking above each solid structural member.
Following conditions (a) through (e) in item (6) ensures that hot gases are caught within appropriate channels in obstructed construction, leading to proper heat collection, timely sprinkler activation, and adequate fire protection.
Along with these criteria for the sprinklers and ceiling layouts, FPRF findings led to some revisions for hydraulic calculations. Since the big focus of the revision was sloped ceilings over storage, one change touches on how design areas are calculated for early-suppression, fast response (ESFR) sprinklers commonly used in these environments:
28.2.4.4.1 Where the design area was increased to 18 sprinklers due to the slope of the ceiling or roof, the design area shall be five sprinklers on the most demanding three branch lines and the three most demanding sprinklers on the next most demanding branch line.
Essentially, this rule ensures that the sprinklers are distributed for a worst-case scenario of how a fire travels—enough sprinklers will be able to cover the most challenging portions of the design area.
Design approaches for sloped ceilings in non-storage applications (spray sprinklers)
In Chapter 19, NFPA 13 2025 also has some changes for sloped ceiling options involving sprinklers in non-storage applications. In the past, section 19.2.3.2.4 simply said that sloped ceilings exceeding 2 in 12 meant the design area for density/area applications should be increased by 30% when using spray, quick response, and CMSA sprinklers.
In the new edition, designers have more options, including installing a false ceiling, meeting specific requirements for obstructed construction similar to the possibilities offered for slopes and storage applications in 20.9.1.1, or (as in the previous edition) increasing the design area by 30% in obstructed or unobstructed construction.
High ceilings in non-storage occupancies
Despite the challenges of delivering water to fires from significant heights, previous editions of NFPA 13 didn’t have any special rules for non-storage sprinklers on very tall ceilings. To address this issue, new criteria have been added for ordinary-hazard and extra-hazard occupancies with ceilings 30 feet and higher.
These rules, found in Chapter 19, restrict the types of sprinklers you can use based on hazard level and ceiling height. Briefly, the rules for ceilings greater than 30 feet are (19.2.3.2.5):
- For Ordinary Hazard (OH) Group 1 occupancies (and higher), no sidewall sprinklers are allowed.
- For OH Group 2 occupancies (and higher), the minimum sprinkler K-factor is 11.2.
- For OH Group 2 occupancies and higher, extended coverage sprinklers must have a K-factor greater than 22.4.
- For OH Group 2 occupancies with ceilings above 40 feet, standard response standard coverage sprinklers aren’t allowed.
Also introduced in the 2025 edition are tweaks to the design area used for hydraulic calculations involving high ceilings (19.2.3.2.5.2). The section references density/area criteria in Table 19.2.3.1.1 while also detailing specific design area increases and minimum densities based on occupancy class and ceiling height.
Fire sprinkler systems and standards evolve to improve fire protection
NFPA 13’s latest edition—and the next ones issued about every three years—clarifies rules, improves them, and adds new approaches to account for experience- and research-based knowledge gains in fire protection.
Specifying which sprinklers work in high-ceiling environments enhances fire protection. Providing more options for protecting sloped roofs accomplishes the same goal while also giving designers and building owners more flexibility. For example, building owners who construct or purchase storage facilities with sloped roofs—but haven’t proactively considered fire sprinkler system design—will reap very practical benefits.
Be sure to check out part one of this series, where we reviewed:
- Changes to the rules for dry and preaction systems, including new technologies
- The introduction of “supplemental sprinklers” and new design criteria for their use
And if you need equipment for your fire sprinkler system, check out QRFS’s selection of commercial and residential fire sprinklers, escutcheons, cover plates, and wrenches.
This blog was originally posted at blog.qrfs.com. If this article helped you, check us out at Facebook.com/QuickResponseFireSupply or on Twitter @QuickResponseFS.