Clarifying the “small room rule” of NFPA 13
NFPA Journal®, July/August 2011
While the “small room rule” has been around for 40 years, it remains a source of confusion in the application of NFPA 13, Installation of Sprinkler Systems. The rule is never clearly stated but exists as a combination of special allowances made for rooms no larger than 800 square feet (74.3 square meters) in size.
One area of confusion is between the small room rule and the “room design method.” The room design method is an alternative to the use of a density/area method when deciding which sprinklers constitute the “design area,” the reasonably worst-case arrangement of sprinklers expected to operate simultaneously during a fire. The size of the design area determines the minimum water supply for the area.
In its allowance for a room design method, NFPA 13 recognizes the benefit of enclosures and fire resistive construction in limiting the number of sprinklers that will open, leading to more economical sprinkler system installation. Unlike the small room rule, however, the room design method has no fixed maximum size for a room.
The special allowance for “small rooms” first appeared in the 1971 edition of the standard and applied to “small rooms such as rest rooms, toilets, closets, and offices with smooth ceilings,” limited to 800 square feet (74.3 square meters) in area for light hazard or 520 square feet (48.3 square meters) for ordinary hazard. The original allowance was for sprinklers to be located a maximum distance of 7 feet, 6 inches (2.29 meters) away from any two walls in the room, provided that the total area of the room divided by the number of sprinklers did not exceed the regular maximum protection area per sprinkler.
In the 1972 edition of NFPA 13, the area limit per sprinkler for hydraulically calculated systems was increased from 200 square feet (19.6 square meters) to 225 square feet (20.9 square meters) for light hazard, negating the benefit of the special allowance. In the 1976 edition, the allowance was limited to light hazard only and allowed sprinklers to be spaced a maximum of 9 feet (2.7 meters) from a single wall.
In the intervening 35 years, the definition of “small room” retained the light hazard restriction and the 800 square feet (74.3 square meters) maximum area bounded by walls and a ceiling. However, the definition now also allows openings to adjacent spaces if they have a minimum lintel depth of 8 inches (200 millimeters) from the ceiling and the total opening width does not exceed 8 feet (2.4 meters). A single opening up to 36 inches (900 millimeters) wide is permitted without a lintel.
That definition of “small room” is found in Section 3.3.17 of the 2010 edition of the standard. Section 188.8.131.52.4 contains the allowance to locate sprinklers up to 9 feet (2.7 meters) from a single wall. Section 184.108.40.206.2 allows the protection area of coverage for each sprinkler in a small room to be determined by dividing the room area by the number of sprinklers in the room. Together, those three sections constitute the small room rule.
Like the room design method, the small room rule takes advantage of the compartmentalization of the enclosure. However, its advantage is not a reduction in water supply but flexibility in spacing. Building features such as lighting fixtures or ceiling fans can force sprinklers out of their idealized locations in a room ceiling. By stretching the maximum distance from a single wall, the small room rule can provide acceptable sprinkler positioning without the need for additional sprinklers in the room.
Russell P. Fleming, P.E., is the executive vice-president of the National Fire Sprinkler Association.