Jon Nisja, Data and Fire Protection Specialist

In a previous blog, the role of heat transfer (especially convection) and its relation to ceilings and fire sprinklers was discussed. This blog will review ceiling types and their definitions from NFPA 13, Standard for the Installation of Sprinkler Systems.

It was pointed out earlier that fire sprinklers tend to operate best when placed close to a smooth, flat, horizontal ceiling that stretches from wall to wall. Many modern buildings have decorative architectural features or structural support features at ceiling level that can impede heat flow to the fire sprinklers.

NFPA 13 Ceiling Types and Construction Definitions

NFPA 13 has provisions for dealing with variations of ceiling configuration but, unfortunately, it uses different terminology than what is used for describing general construction terms for roof or ceiling types. This is due to NFPA 13 dealing with heat flow to the fire sprinkler and possible obstructions to sprinkler spray caused by the ceiling arrangement rather than how the roof or ceiling is constructed or arranged.

While the architect or building plans may show a steel bar joist roof assembly, the fire sprinkler contractor and authority having jurisdiction (AHJ) will need to determine if it is unobstructed or obstructed construction. Partial ceilings or cloud ceilings are examples of architectural or decorative features that can impact sprinkler installation or performance.

Ceiling Types

NFPA 13 has four definitions for ceiling types. These terms are used throughout the standard and can either allow or restrict the use of certain types of fire sprinklers:

• Flat ceiling,
• Horizontal ceiling,
• Sloped ceiling,
• Smooth ceiling.

According to the definitions in NFPA 13, a flat ceiling is a “continuous ceiling in a single plane.” Language was added in the 2019 NFPA 13 Handbook to clarify that the term “flat ceiling” did not mean that the ceiling was horizontal (parallel to the floor); this was often assumed in previous editions of NFPA 13.

A horizontal ceiling is defined in NFPA 13 as a “ceiling with a slope not exceeding 2 in 12 (16.7 percent).” It is somewhat common to have a ceiling that has some slope to it, especially if it follows a roof line where drainage and runoff is intended. The slope is limited to 2:12 pitch to be considered horizontal as testing and research has shown that a small degree of slope does not negatively impact sprinkler activation and performance.

Sloped ceilings are defined as a “ceiling with a slope exceeding 2 in 12 (16.7 percent).” Steeper ceiling and roof pitches have been shown to negatively impact sprinkler activation and performance. Heat from a fire travels vertically and:

• May not activate lower sprinklers even though they are closer to the fire and
• May activate sprinklers higher up the slope even though they are remote from the fire and may not spray onto the fire

Therefore, NFPA 13 imposes some limitations and requirements when sloped ceilings are present. The following are some examples from NFPA 13 (2025 edition) where sloped ceilings impact sprinkler placement or design:

• Section 9.5.4.2 – Sprinkler deflectors to be parallel to sloped ceilings to prevent the discharge from hitting the ceiling.
• Section 10.2.7.1.4.4 – Higher minimum pressure is required for standard spray sprinklers in steeply pitched slopes of combustible concealed spaces.
• Sections 10.3.5.2.1 and 11.3.5.2.1 – Sidewall sprinklers to be installed at the high point of sloped ceilings.
• Section 13.2.1.2 – CMSA sprinkler installation is limited to ceiling slopes of 4:12 or less.
• Section 13.2.6.1.2 – CMSA sprinklers have specific installation criteria in obstructed construction having sloped ceilings.
• Section 19.2.3.2.4 – Requires a design area increase of 30 percent for sloped ceilings.

The following photos show examples of sloped roofs.

NFPA 13 defines a smooth ceiling as a “continuous ceiling free from irregularities, lumps, or indentations greater than 4 in. (100 mm) in depth.”  This definition was changed in the 2022 edition by adding the 4-inch measurement. Previous editions only stated that ceilings had to be free from “significant irregularities, lumps, or indentations” which allowed for inconsistent application and enforcement.

The following photos show examples of smooth ceilings.

Here are some examples from NFPA 13 where smooth ceilings come into play:

• Section 10.3.2 (1) – allows sidewall sprinklers under smooth, horizontal ceilings in light hazard occupancies.
• Section 10.3.2 (3) – allows sidewall sprinklers under smooth, horizontal ceilings in ordinary hazard occupancies when:
  • Listed for such an application, and,
  • The ceiling height is 30 feet or less.

Future blogs will discuss how NFPA 13 treats obstructed ceiling construction.

Summary

NFPA 13 has definitions for different ceiling types. These definitions play a role in determining when certain fire sprinklers are allowed to be installed or how they are to be positioned. A pitch of 2:12 is used to separate roof types as horizontal (less than or equal to 2:12) or sloped (greater than 2:12).

Fire sprinklers tend to operate quickest under smooth, flat, horizontal ceilings as there are few impediments to heat reaching the sprinkler. NFPA 13 allows small changes of 4-inches or less when defining smooth ceilings.

With sloped ceilings, the heat tends to travel up the slope and may skip fire sprinklers that are located lower on the slope or activate sprinklers further from the fire. To compensate for that, NFPA 13 requires that the design area be larger.

Jon Nisja has been a Data and Fire Protection Specialist with the National Fire Sprinkler Association (NFSA). He began his fire service career as a firefighter in November of 1978. Jon served as a fire marshal in two communities since 1985 and as a state fire safety supervisor for the Minnesota State Fire Marshal Division. From 2005-2022, he supervised the State Fire Marshal’s fire protection, training, and data teams. He is the past president of both the Fire Marshals Association of Minnesota and the International Fire Marshals Association. He retired at the end of a 43-plus year fire service career in 2022 and was hired by NFSA. Jon can be contacted at nisja@nfsa.org.