What is the "small room" rule?

In conducting plan reviews or fire sprinkler field inspections you may have heard, or seen, the invocation of the “small room" rule. Though not specifically stated in NFPA 13 as the “small room" rule, it is a combination of several code sections within the standard that can be utilized to provide advantages in hydraulic calculations and flexibility in sprinkler spacing.

To apply the small room rule, the space must meet all the requirements of a small room as defined in NFPA 13. A “small room” is a space that is classified as light hazard, has unobstructed ceiling construction, and is no greater than 800 sq.ft.  Application of the small room rule will affect two critical system design elements, maximum area of protection and sprinkler spacing.

The normal method for determining the area of protection per sprinkler head is to use the formula:

As = S x L

In this case the distance between sprinklers along a branch line (S) and the distance between branch lines (L) is determined and then multiplied to establish the “maximum allowable protection area of coverage for a sprinkler” (As).  The small room allowances of Section permits the sprinkler designer to ignore the rules and requirements of the S x L formula. Instead, an alternate method is allowed. If the space meets the requirements for a small room, then maximum coverage area per sprinkler can be determined by simply dividing the area of the room by the total number of sprinklers in the room.

Sprinkler spacing and distance between individual sprinklers is calculated using the tables and guidance in Section 8.6.3. In these tables, the maximum spacing distance for sprinklers is 15 feet between sprinklers and 7.5 feet from a wall. Utilizing the small room rule requirements, allows sprinklers to be spaced up 9 feet from any single wall.

The small room rule can be stated as:

In a room of 800 sq.ft. or less, having unobstructed ceiling construction, and light hazard contents, sprinklers may be located up to 9’ from any single wall with the maximum area of coverage being determined by dividing the size of the room by the number of sprinklers.

Code summary for small room rule:
NFPA 13:3.3.22 - small room defined
NFPA 13: - max. area of coverage
NFPA 13:8.6.3..4 - sprinkler spacing from a wall