NFPA 13: Sprinkler installations and obstructions

In every fire sprinkler systems installations, regarding sprinkler obstructions, usually every project site brings new challenges during sprinkler installation. Typically, one of the main areas of concern are the large mechanical rooms, such rooms are found with HVAC ducts greater than 4 ft wide. More often, sprinkler coverage is found only above such large ducts, with no sprinkler coverage below the ducts. 

There are two main criteria for the location of sprinkler heads. First, sprinklers must be located so they are within the hot-gas layer that develops near the ceiling during a fire because activation occurs when the sprinkler head reaches a certain temperature. Second, they must be located so that the water that is discharged from the sprinkler is not disrupted or affected by construction elements such as beams, trusses, or soffits.

NFPA 13 has a detailed set of criteria as to what constitutes an “obstruction” and when the obstruction is deemed too disruptive to the spray pattern of the sprinkler. These rules vary based on whether the obstruction is non-continuous (the three and four-times rules) or continuous (the beam rule) as well as the size of the obstruction and distance from the deflector. However, the configurations in which the spray is excessively disrupted can vary greatly based on the specific sprinkler, K-factor, and operating pressure , as well as the characteristics of the obstruction. It is generally understood that all of these variables may not be sufficiently addressed with the present rules as currently outlined in NFPA 13.

The Beam Rule

The “beam rule” states that there must be at least 1 foot (0.3048 m)of separation between the sprinkler and the obstruction if the deflector is any distance above the bottom of the obstruction. As the distance  increases from the sprinkler to the obstruction the greater the allowable distance from the deflector to the bottom of the obstruction. It is worth noting that the criteria for CMSA/ESFR and EC/Residential sprinklers, while similar in concept do have different spacing rules as would be expected as the spray patterns are different for each sprinkler type.

The Three Times Rule.

NFPA 13 (2019) §10.2.7.2.1.3 lays out the “Three Times Rule” which states that unless specific requirements are met that “sprinklers shall be positioned away from obstructions a minimum distance of three times the maximum dimensions of the obstruction”, up to a maximum of 24 inches (0.6096 m) though the “maximum clear distance does not apply to obstructions in the vertical orientation (e.g., columns)” . The intent is to provide guidance for situations where the expectation is that the spray can reach both sides (through) the obstacle without a significant impact from the obstruction. The “Four Times Rule” is really just an extension of the “three times rule” however it covers extended coverage sprinklers, and the maximum clearance is 36 inches (0.9144 m).

The Four-Foot and Wide Obstruction Rule

For obstructions wider than 4 feet (1.2192 m), sprinkler protection is required below the obstruction with deflectors to be located less than 12 inches (0.3048 m) from the bottom of the obstruction. For obstructions less than 4 feet (1.2192 m) in width sprinklers may not be required underneath the obstruction. Several exceptions to the rule also exist when the obstruction is less than 24 inches (0.6096 m) from the floor, not fixed in place (ie tables), or if the room is smaller than 400 ft3. It should be noted that ducts less than 4 feet (1.2192 m), though continuous, do not require sprinkler protection underneath them if the deflector is greater than 18 inches (0.4572 m) from the obstruction.