NFPA 13 – 2025 updates for supplemental sprinklers under obstructions

The 2025 edition of NFPA 13 introduces significant updates regarding supplemental sprinklers under obstructions, particularly clarifying their definition, requirements, and hydraulic calculations. Key changes include defining "supplemental sprinklers," specifying when they are needed, and providing guidelines for their placement, spacing, and characteristics.

Definition and Scope:

• The 2025 edition introduces the term "supplemental
  sprinklers" to refer to sprinklers installed specifically below obstructions.

• These sprinklers are often required under obstructions like ducts, beams, or catwalks that can impede the spray pattern of ceiling-mounted sprinklers.

• The definition clarifies that supplemental sprinklers are intended to address coverage gaps created by obstructions.

Yes, obstructions in buildings can significantly hinder the effectiveness of fire sprinkler systems. These obstructions, whether they are structural elements, stored items, or even decorations, can prevent water from reaching the fire, delay sprinkler activation, or disrupt the intended spray pattern, potentially leading to increased fire damage and hazards.

Here's why obstructions are problematic:

• Delayed Activation: Obstructions, especially those close to the sprinkler head (within 18 inches), can block the heat from reaching the sprinkler's heat-sensitive element, delaying its activation.

• Disrupted Spray Pattern: Obstructions can interfere with the sprinkler's ability to distribute water effectively, potentially creating "shadows" where the water doesn't reach, leaving areas vulnerable to fire spread.

• Reduced Water Coverage: When obstructions block the sprinkler's spray pattern, the area covered by the water is reduced, limiting the sprinkler's ability to suppress or extinguish the fire.

• Structural Damage: In some cases, obstructions, like heavy items hung from sprinkler pipes, can damage the system itself, leading to leaks or even system failure

Key Changes and Clarifications:
Non-Flat and Non-Solid Obstructions:
The standard clarifies when supplemental sprinklers are needed for non-flat and non-solid obstructions, such as cable trays with openings.

Shielding: Supplemental sprinklers under non-flat, non-solid, or open grating obstructions require a shield or water shield to protect them from the heat of the fire.

Sprinkler Characteristics: The 2025 edition allows supplemental sprinklers to differ from ceiling sprinklers in terms of K-factor, orientation, and coverage area under certain conditions.

Hydraulic Calculations: NFPA 13-2025 clarifies when and how supplemental sprinklers need to be hydraulically calculated, especially when they are not supplied by the same piping as the ceiling sprinklers.

Spacing: New spacing guidelines permit supplemental sprinklers to apply unobstructed building spacing, rather than requiring them to be spaced as if they were ceiling sprinklers.

ESFR and CMSA Sprinklers: Specific guidelines are provided for ESFR and CMSA sprinklers, which are particularly sensitive to obstructions, with closer spacing requirements for non-flat and solid obstructions.

Concrete Tee Obstructions: The 2025 edition limits the depth of concrete tees to 30 inches and requires the deflector to be at least 1 inch below the bottom plane.

To mitigate these risks, it's crucial to:
Follow NFPA 13 Guidelines: Adhere to the National Fire Protection Association's (NFPA) guidelines regarding sprinkler placement and spacing relative to obstructions.

Maintain Clear Space: Ensure a clear space of at least 18 inches below the sprinkler head and maintain adequate spacing from any obstructions according to the "three times rule" (the distance from an obstruction should be at least three times the obstruction's maximum dimension, up to a maximum of 24 inches).

Communicate with Designers: Involve fire protection system designers early in the design and construction process to optimize sprinkler coverage and account for potential obstructions.

Proper Storage Practices: Avoid storing items too close to sprinkler heads, especially in areas like walk-in refrigerators or freezers.

Regular Inspections: Conduct regular inspections of sprinkler systems to identify and address any potential obstructions or damage.

How sprinklers work and fire sprinkler obstruction types?
To understand obstructions, we must first review how sprinklers work. Their job is to wet surfaces (especially walls and floors) in their design (coverage) area, and most do it by spraying water in a roughly umbrella-shaped pattern.

An obstruction is anything that blocks the spray from a sprinkler disrupting the spray pattern from developing, preventing water from reaching the hazard, or both.

Types of Obstructions
Obstructed construction (3.3.139) refers to exposed structural members (beams, trusses, rafters, etc.) that block water spray or heat distribution. NFPA 13 dedicates special sections (10.2.6.1.2) to the arrangement of sprinkler heads in obstructed construction.

A noncontinuous obstruction (3.3.140) affects water spray from one sprinkler only. This could be a small object like a light fixture or smoke detector. The above item contrasts with continuous obstructions (3.3.140.1), such as soffits, beams, or ducts, which block water spray from two or more adjacent sprinklers.

Storage is an obstruction category covered more in-depth by NFPA 25: Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems. The basic concept involves keeping boxes, pallets, or other stacked storage sufficiently far from sprinkler heads. The standard minimum clearance required is 18 inches (NFPA 25, 2023: 5.2.1.2.1), though different situations call for more room.

From the 2022 edition of NFPA 13
9.1.1* The requirements for spacing, location, and position of sprinklers shall be based on the following principles: […]

(3)* Sprinklers shall be positioned and located so as to provide satisfactory performance with respect to activation time and distribution.

9.5.5.1* Performance Objective.
Sprinklers shall be located so as to minimize obstructions to discharge as defined in 9.5.5.2 and 9.5.5.3, or additional sprinklers shall be provided to ensure adequate coverage of the hazard.
(See Figure A. 9.5.5.1.)

So, it might be helpful for building owners and any design professionals in charge to prioritize solutions in terms of positioning obstructions instead of sprinklers. The reasoning: if an inspector regards anything as a violation, it’s easier to move something like a smoke detector (even if it’s hard-wired) than a sprinkler connected to fixed supply pipes.

A good place to start: “the three times rule”

Many fire protection pros know the “three-times rule” articulated in section 10.2.7.3.1.3 of NFPA 13 (2022 edition). The gist is that sprinklers should be spaced away from an obstruction at a distance at least three times the object’s maximum dimension (its height or width, whichever is greater) up to 24 inches of separation, maximum. (As long as something is further than 24 inches away, the dimensions of that object don’t matter unless it’s a big vertical column).

The three-times rule is an important measurement to consider, and here’s a diagram illustrating the concept:

However, it isn’t always necessary to use the three-times rule. NFPA 13 intends the section to handle obstructions where water is expected to be able to throw on oth sides or around an object, whether that’s a column, light fixture, or smoke detector. The rule is generally focused on ensuring the sprinkler’s spray pattern development in those upper 18 inches.

In addition, there are exceptions to having to use the three-times rule: NFPA 13 states that in light and ordinary hazard occupancies, “structural members only shall be considered” when using it (10.2.7.3.1.4) meaning a column qualifies and not a smoke detector, for example. The standard points readers dealing with light or ordinary hazard occupancies to other rules in sections 10.2.7.2 and 10.2.7.4.2. (We’ll cover both of these scenarios below: “the beam rule” and “handling bigger obstructions,” respectively)

NFPA 13 provides various scenarios for using this rule in Table 10.2.7.2(a) and Figure 10.2.7.2(a). We’ve reproduced both for you below. In short, the closer your sprinkler is to the obstruction’s side, the more level the sprinkler must be with the bottom of the object:

Figure 10.2.7.2(a) illustrates how to use the beam rule and Table 10.2.7.2(a). The smaller the horizontal distance between the sprinkler and the obstruction (A), the smaller the vertical distance between the sprinkler deflector and the obstruction’s bottom (B) must be. For example, referencing the above table reveals that if a sprinkler is less than one foot away from the side of the obstruction, its deflector must be completely level with the bottom of the obstacle. This allows the sprinkler to spray under it without impacting the pattern. Image source: NFPA 13

Figure/Table 10.2.7.2(a) above applies to obstructions away from walls. And there are similar but unique rules (with a more complex formula) for obstructions against walls. They are:

• If the obstruction is less than 24 inches wide, there is no limit to how close a pendent or upright sprinkler can be to it, provided other rules about sprinkler spacing from walls are followed.

• If the obstruction is less than or equal to 30 inches wide, you can use a formula to determine how far away the sprinkler should be. This calculation is based on the size of the potential obstruction and the vertical distance between the item and the sprinkler.

The obstructions against walls directives are illustrated in Figures 10.2.7.2b-c:

Figure 10.2.7.2(c) shows that there is no limit to how close a pendent or upright sprinkler can be to the bottom of an obstruction that is against a wall if the object is 24 inches wide or less, provided other rules about sprinkler spacing from walls are followed.

Figure 10.2.7.2(b) shows the formula for calculating the sprinkler distance from a wall obstruction as long as it is not over 30 inches (750 mm) in width. The calculation is based on the width of the object and the comparative height of the sprinkler’s deflector.

Hallways can be tricky environments, with little room to arrange sprinklers and plenty of small things like light fixtures and smoke detectors that get in the way. Per NFPA 13’s Figure 10.2.7.2(c), if:

• The hallway is no wider than 6 feet, and

• An obstruction is no bigger than 12 inches across

… a sprinkler can be placed on either side of the object, at least 12 inches away. This image helps explain it:

Things can get tight in hallways. However, they also limit the space sprinklers need to protect. So, NFPA 13 has lighter rules for small obstructions (1 foot or less across) in small hallways (6 feet or less wide). Image source: NFPA 13.