A common question we get about NFPA 13, Installation of Sprinkler Systems, and where sprinklers should be located has to do with non-combustible concealed spaces. When you walk down the hallway in any office building, hospital, or other commercial property with a suspended ceiling, you will likely see sprinklers protecting the occupied space below. But are they needed for the space above? Chapter 8 provides a comprehensive list of concealed spaces above these drop ceilings that do not require sprinklers. The most common arrangement on this list is also the one that raises the most questions.

NFPA 13 permits sprinklers to be omitted from concealed spaces that are formed by noncombustible and limited combustible materials provided that they have “minimal combustible loading.” In other words, in a concealed space without sprinklers, the floor, ceiling, walls, and structural elements of the space must be constructed with noncombustible or limited combustible materials. This does not prohibit other combustibles from being in the space as long as they do not make up the floor, ceiling, walls, or structural elements, and are not of such a quantity that they can no longer be considered “minimal” to the authority having jurisdiction (AHJ).

The concern is that combustibles will propagate a fire through that space and allow the fire to spread through the building and jump from room to room or from a corridor into a room. Items like wire ties or end caps on cable trays in an otherwise noncombustible space are not sufficient to propagate fire through that space. Similarly, a few computer cables spread around an otherwise limited-combustible space will not propagate a fire through that space.

At some point, though, a few cables becomes dozens, and dozens can become hundreds, in which case we may have a situation where the loading can no longer be considered minimal. In some buildings and structures these cables can weigh hundreds of pounds and have the ability to spread a fire through a concealed space. In these instances, the concealed space would require sprinkler protection, according to NFPA 13. The standard allows sprinkler protection in a concealed space to be limited to the area above the combustibles if they are confined to a certain area within the space.

Clear guidance on when sprinkler protection is required in concealed spaces has been elusive. The NFPA 13 Technical Committee has been unable to come to a consensus on exactly where to draw the line between those spaces that have sufficient measurable quantities of combustibles to require sprinklers and those that do not. Therefore, every situation must be evaluated on a case-by-case basis. The language in 8.15.1.2.1 is important because it informs the user that a small amount of combustibles is permitted in an unsprinklered concealed space. Without the language in NFPA 13, some AHJs would require sprinklers in any space that contains any amount, however small, of combustibles.

The committee responsible for this language wanted to prevent that heavy-handed approach. While this rule does require a certain amount of subjectivity, in most cases people can agree on whether or not a proposed amount of combustibles will propagate a fire through a space. The horizontal nature of most concealed spaces makes fire propagation extremely difficult because the hot gasses from the fire do not pass over the combustibles to pyrolize them and prepare them for combustion. When in doubt, a simple test using a mock-up of the concealed space and its materials can be performed to determine if the materials, in the form they will be used in the concealed space, will burn sufficiently to allow the fire to break out of the room of origin.

Keep in mind that the ultimate decision is up to the AHJ. When contractors and engineers aren’t sure whether or not what they are seeing on plans for a concealed space is considered minimal or not, they should contact the AHJ for confirmation. Waiting until the job has been installed and hoping that your interpretation matches that of the plan reviewer and inspector is not always a wise economic decision.

MATT KLAUS is NFPA technical services lead for fire protection engineering.