The NFPA 13 Small Room Rule: Frequently Asked Questions
Common issues when evaluating and applying the fire sprinkler small room rule
Most sprinkler designers know about and use the small room rule found in NFPA 13: Standard for the Installation of Sprinkler Systems. It simplifies certain area-of-coverage and sprinkler placement requirements for easy-to-protect “small rooms.” But while the rule is pretty straightforward, even a simple section on paper can create confusion in practice.
NFSA’s Expert of the Day service has answered many questions about using this provision, so we’ve put together some FAQs about the small room rule, including:
- When does NFPA 13 allow the small room rule?
- How can the small room rule be applied?
- What sprinklers can be used with the small room rule?
- How does the small room rule affect hydraulic calculations?
Read on for a detailed exploration of these questions! Note: All references from NFPA 13 in this piece are referencing the 2019 edition of the standard.
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Designers can apply the small room rule for any compartment that is:
- 800 ft.2 (74 m2) or less in size
- Classified as light hazard
- Unobstructed construction
NFPA 13 explicitly calls such an area a “small room” (3.3.196; 2019 edition). And the NFPA 13 Handbook explains that “Fires in small rooms of a light hazard occupancy present a lesser challenge to the sprinkler system” because of their small size and low fuel-load. Thus, the standard allows sprinkler designers some leeway in these circumstances.
The exact definition of “light hazard” occasionally causes confusion. NFSA once received a question from a member who was told by their state fire marshal that they could not use the small room rule for nursing home bedrooms because the threat was not “light hazard.” The confusion arose because there are different uses of the term “hazard” in different codes and standards. The fire marshal was probably thinking of the definition in NFPA 101: Life Safety Code, which takes “hazard” to mean “threat to life.” The marshal was incorrect as far as NFPA 13 is concerned—for sprinkler systems, “hazard” means “severity of fuel load.”
As long as a room meets the hazard, construction, and size requirements, the small room rule can be used. But if any of those conditions change, NFPA 25: Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems is unequivocal. The property owner or their representative must evaluate (22.214.171.124) whether the protection is adequate and, if not, correct the deficiency (126.96.36.199).
The rule is used in particular situations, often when other elements of the room make installing sprinklers tricky. For example, many offices have lights installed right in the middle of the ceiling. This placement prevents putting in a single sprinkler there that would comply with standard requirements for a maximum distance of 7.5 ft. from any wall. If the small room rule exception didn’t exist, installers would have to place sprinklers on either side of the light. With the rule, installers can still install one sprinkler.
The small room rule gives designers two main advantages:
- They can place sprinklers up to 9 ft. from a single wall
- They can use area divided by the number of sprinklers as the sprinkler protection area
Placing sprinklers 9 ft. from a single wall
Again, normally, standard spray sprinklers can be placed up to 7.5 ft. (2.3 m) from any wall. This stipulation is found in section 10.2.5.2.1 of NFPA 13, which states that the distance from the sprinkler to the wall can be no more than one-half the allowable distance between sprinklers. The max distance between standard spray sprinklers in light hazard settings is 15 ft. (4.6 m) (with exceptions for light-hazard, combustible concealed spaces).
The small room rule allows sprinklers to be placed up to 9 ft. from one wall—a significant design accommodation:
10.2.5.2.3.1 Sprinklers shall be permitted to be located not more than 9 ft (2.7 m) from any single wall.
Confusion sometimes arises about the meaning of “any single wall.” Some individuals mistakenly think this means “any wall” or “all walls.” But the proper meaning is that designers may select one wall and place sprinklers up to 9 ft. from that wall. From any other wall, the max distance allowed is still 7.5 ft.
Coverage area calculations under the small room rule
For sprinkler system design and hydraulic calculations, area of coverage is an important concept. Depending on hazard and obstruction, of course, sprinklers have a maximum area that they can effectively handle. Under normal circumstances, NFPA 13 says that the area covered by a sprinkler in a room is calculated as follows (188.8.131.52):
As=S x L
As is the area covered by a sprinkler
S is either the longest distance to an adjacent sprinkler or twice the distance to the furthest wall/obstruction (whichever is greater) along sprinkler’s branch line
L is—perpendicular to the sprinkler’s branch line—either the longest distance to the sprinkler on the next branch line or twice the distance to the furthest wall/obstruction (whichever is greater)
When the small room rule is applied, calculations become much more straightforward (10.2.4.1.2.1). Merely divide the area of the room by the number of sprinklers to get the area of coverage for each sprinkler.
Do note that the maximum protection area rules stay the same under the small room rule. In the unobstructed light hazard spaces covered in Table 10.2.4.2.1(a), the maximum area one sprinkler can protect is 225 ft.2 (20 m2):
Sometimes, designers and AHJs wonder precisely what kinds of sprinklers can be used with the small room rule. Are quick-response sprinklers allowed? What about residential? Extended coverage?
Here’s the short answer: the small room rule is found in section 10.2 of NFPA 13, which applies to standard-spray upright and pendent sprinklers in commercial systems only. Sidewall, extended coverage, and residential sprinklers do not apply.
Quick-response sprinklers are required in light-hazard settings and acceptable as long as they are standard-spray; you can use the design-area reduction for QR sprinklers and the small room rule at the same time. The small room rule can also be used in dry-pipe and preaction sprinkler systems, as long as they use the correct kind of sprinkler.
It depends. On the one hand, the small room rule provides a hydraulic advantage by making assessments more practical. But if the most remote sprinkler is in a small room and this advantage is used, designers should watch out for pitfalls.
To illustrate these concepts, we’ll refer to the following example discussed by Kenneth Isman, NFSA’s former Vice President of Engineering:
The small room rule provides an advantage on hydraulic calculations
The small room rule presents an advantage by making area of coverage calculations realistic. Without the rule (instead, using As=S x L), a designer might have to assume sprinklers are protecting more space than is actually in the room. This can inflate the flow and pressure demands of the sprinklers in that area.
Consider Room A in the above example. Under the small room rule, Sprinkler 1 in Room A would cover 122.5 ft.2 (245 ft.2 ÷ 2 sprinklers). Without the small room rule, that number would be 210 ft.2 (S=14 ft., L=15 ft.).
This gives the designer a hydraulic advantage because it makes it easier to achieve the required density. For a light-hazard, 1,500 ft.2 design area, the commonly-chosen design density is 0.1 gpm/ft.2. Compare the flow required from Sprinkler 1 with and without the Small Room Rule:
|Without the small room rule:
Area of coverage= 210 ft.2
Design density= 0.1 gpm/ft.2
Required flow= 210 ft.2 x 0.1 gpm/ft.2 = 21.0 gpm
Required pressure= (21 gpm ÷ 5.6)2 = 14.1 psi
|With the small room rule:
Area of coverage= 122.5 ft.2
Design density= 0.1 gpm/ft.2
Required flow= 122.5 ft.2 x 0.1 gpm/ft.2 = 12.25 gpm
Required pressure= (12.25 gpm ÷ 5.6)2 = 4.8 psi, increased to 7 psi and 14.8 gpm to clear the pip cap/seat
Following the small room rule, the pressure required to achieve design density in this sprinkler’s area of protection is so low that it has to be increased to 7 psi (with flow increased to 14.8 gpm, accordingly) in order to push the pip cap/seat out of the sprinkler. Even with that increase, the required pressure is still half of what would be needed without applying the small room rule.
With the small room rule, be careful of undercalculating the design pressure
The hydraulic advantage from the Small Room Rule can be a double-edged sword. Designers may undercalculate the flow and pressure for the rest of the design area if the most remote sprinkler is in a small room (as is the case in this example). In other words, the most remote sprinkler might not be the most demanding sprinkler if it’s in a small room.
This can happen because other sprinklers, even if they have access to more pressure, may have to protect a larger area.
In this example, Sprinkler 4 is not in a small room and has to cover an area of 225 ft.2 (S= 15 ft., L= 15 ft.). To deliver the design density, Sprinkler 4 must flow at a minimum of 22.5 gpm (0.1 gpm/ft.2 x 225 ft.2).
This requires over 16 psi (compared to Sprinkler 1’s 7 psi). Unless the designer can find an additional 9 psi between Sprinkler 1 and Sprinkler 4 (at a distance of 35 ft., this is unlikely), Sprinkler 4 won’t be able to achieve the required density. The pressure and flow at Sprinkler 1 would need to be adjusted upward to account for this.
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