Jon Nisja, Data and Fire Protection Specialist

Ceilings play a significant role in effective fire sprinkler operation. Previous blogs have explored heat transfer and its impact with sprinklers, ceiling configurations (flat, sloped, horizontal, etc.), and ceiling material combustibility (combustible vs. noncombustible). This blog will look at the role that the ceiling (or roof) materials can play when they obstruct sprinkler operation and require different sprinkler placement.

The construction materials that support the ceiling or roof can be fairly large and may serve as barriers to effective sprinkler operation. This blog will look at those features and how NFPA 13, Standard for the Installation of Sprinkler Systems, addresses them.

How Do Structural Ceiling Materials Impact Fire Sprinklers?

There are two primary considerations when dealing with the materials that support ceilings or roofs:

• Do these materials or members prevent heat from getting to the sprinkler and, therefore, delay sprinkler activation?
• Do these members interfere with or obstruct effective sprinkler spray once the sprinkler is activated?

This blog will provide examples of these concerns and describe how NFPA 13 addresses them.

NFPA 13 defines ceiling construction as unobstructed and obstructed.

Unobstructed Construction

Unobstructed ceiling construction is favorable to effective fire sprinkler operation as it does not disrupt heat flow, nor does it tend to interfere with sprinkler spray once a fire sprinkler has activated. One of the primary distinguishing differences between unobstructed and obstructed construction is the distance between structural members. If the distance is more than 7 ½ feet between structural members (with one exception mentioned later), it is considered unobstructed. This concept has been in NFPA 13 for several decades and the terminology for unobstructed and obstructed was added in the 1991 edition.

The annex in NFPA 13 provides examples of unobstructed construction:

• Smooth ceilings

Figure 1 – Example of a smooth ceiling with acoustic ceiling tile

• Bar joist construction without spray-applied fire-resistant material (SFRM)

Figure 2 – Steel bar joists (unobstructed construction)

• Open grid ceilings
• Heavy timber construction –

Figure 3 – Heavy timber construction (members are typically 6 inches or more in size)

• Truss construction of either wood or steel –

Figure 4 – Open wood truss floor/ceiling assembly

In unobstructed construction, the horizontal structural members are mostly open, spaced 7 ½ feet or more apart, and not solid. NFPA 13 defines mostly open as:

• 70 percent or more of the cross-section area is open, and
• The member depth is less than the smaller dimension of the opening

The second bullet point can be a little confusing and difficult to apply. Take, for instance, a wood truss configuration with 2 x 4-inch boards. The openings have to be larger than 2-inches (smaller dimension of the member depth). That can be problematic when applying this definition to triangular-shaped web members since the dimension will be less near the intersection with the top and bottom chords and any gusset plate connectors. This definition is intended to ensure that the spaces are more open than obstructed, and the 70 percent provision is the more critical measurement.

Obstructed Construction

Obstructed construction occurs when the structural members impede heat flow to the sprinkler or obstruct sprinkler spray. When structural members form pockets or bays, these spaces restrict heat from traveling horizontally and activating sprinklers. The heat fills up these spaces and then spills out and fills up adjacent spaces. This delays sprinkler activation. As such, NFPA 13 typically requires less coverage per sprinkler and often closer spacing.

Here are some examples of obstructed construction:

• Concrete T (or Double T) construction –

Figure 5 – Concrete T ceiling (noncombustible obstructed construction)

• Beam and girder construction
• Composite wood joist / “I-joist” construction –

Figure 6 – Composite wood joist (also known as “I-Joist) construction

• Concrete waffle pan –

Figure 7 – Concrete “waffle pan” construction

• Semi-Mill construction
• Bar joist construction with spray-applied fire-resistant material (SFRM) when over 30 percent obstructed –

Figure 8 – Steel bar joists with SFRM (noncombustible obstructed)

• Steel purlins
• Wood truss construction that is obstructed more than 30 percent

NFPA 13 uses the term “panel construction”. This term is not commonly used in general building construction lingo but is germane for fire sprinkler protection. It refers to construction methods that form distinct bays or channels. These can be from trusses, joists, beams, and girders that trap heat. This changes the sprinkler layout rules.

Spacing Based on Construction Type

NFPA 13 prescribes the maximum protection area along with the maximum sprinkler spacing based on the type of construction and structural member spacing (see Table 1).

Ceiling to Deflector Distance Based on Unobstructed vs. Obstructed Ceilings

The distance from the ceiling to the sprinkler deflector also varies based on whether the ceiling construction is unobstructed vs. obstructed. The general rules for distance between ceilings and sprinkler deflectors in unobstructed construction are:

• 1-12 inches below the ceiling for standard spray pendent and upright sprinklers
• 4-6 inches for sidewall sprinklers
• Note: there are certain sprinklers that are listed for different distances

The general rules for distance between ceilings and sprinkler deflectors in obstructed construction are:

• 1-6 inches below the structural members with a maximum distance from the ceiling of 22 inches
• At or above the bottom of the structural members to a maximum of 22 inches from the ceiling
• 1-12 inches from the ceiling when installed in each bay of obstructed construction
• 1-6 inches below composite wood joists (I-joists) with a maximum distance of 22 inches from the ceiling when the joist channels are 300 sq. ft. or less
• 1 inch maximum below the stems of closely-spaced concrete T construction (less than 7 ½-feet apart) with a maximum depth of 30 inches (new in the 2025 edition)
• Sidewall sprinklers are discouraged in most types of obstructed construction

Summary

Ceiling configurations can impact how heat travels to activate sprinklers and also how sprinkler spray, once activated, can be obstructed by the ceiling structural members. Unobstructed construction with smooth ceilings provides few obstructions to heat flow and tend not to obstruct sprinkler spray patterns. NFPA 13 allows the maximum coverage areas and sprinkler spacing for unobstructed construction.

In obstructed construction, ceiling members can impede heat flow to the sprinkler and alter spray patterns. NFPA 13 tends to reduce the maximum coverage area for obstructed construction, especially when the structural members are spaced closer together. The distance between the ceiling and the sprinkler deflector also changes based on the ceiling configuration.

Jon Nisja has been a Data and Fire Protection Specialist with the National Fire Sprinkler Association (NFSA). He began his fire service career as a firefighter in November of 1978. Jon served as a fire marshal in two communities since 1985 and as a state fire safety supervisor for the Minnesota State Fire Marshal Division. From 2005-2022, he supervised the State Fire Marshal’s fire protection, training, and data teams. He is the past president of both the Fire Marshals Association of Minnesota and the International Fire Marshals Association. He retired at the end of a 43-plus year fire service career in 2022 and was hired by NFSA. Jon can be contacted at nisja@nfsa.org.