Running the sprinkler service main piping under a building can be a risky design choice, one that until recently had been further complicated by some confusing points in the relevant standards: NFPA 13, Installation of Sprinkler Systems, and NFPA 24, Installation of Private Fire Service Mains and Their Appurtenances.
The 2013 edition of those standards, however, clarifies the language on this issue. While running piping under buildings remains a debatable practice, the updated standards will help installers and authorities having jurisdiction (AHJs) make informed decisions based on a clear understanding of those standards, rather than on interpretations that may put a system or an entire structure at risk.
Many AHJs, for example, have typically not allowed pipes to be run under buildings for more than 10 feet (3 meters), arguing that a pipe leak or fitting rupture under a building could displace soil and aggregate beneath the structure, rendering it unsafe for occupancy. Even where the water can be shut off quickly and the damage is contained, they argued, accessing the piping and fittings can be a difficult and time-consuming process, forcing the building to essentially be unsprinklered for a long period of time during repairs. Even though having the private service fire main emerge from the ground outside the building was an option, it could be problematic in colder climates where the pipes would be subject to freezing — meaning that routing them under the building was the only way to go.
Chapter 10 of NFPA 13, which is extracted from Chapter 10 of NFPA 24, includes a section on running pipes under buildings, but it has not always been clear on when and how to do it. In the 2010 edition, for example, subsection 10.6.1 stated, “Pipes shall not be run under buildings.” The next section, however, began with the phrase “where pipes must be run under a building,” and continued with design guidance. The standard further stated that running piping under a building is acceptable, provided the underground piping enters the building “adjacent to the foundation,” but the term “adjacent” is not supported by a maximum dimension away from the foundation. This subjective language led to a wide range of interpretations and made it difficult for system installers to bid jobs, since it was difficult for them to know whether or not this approach was acceptable.
In the 2013 editions of NFPA 13 and NFPA 24, the technical committee responsible for this concept revised the section to state that running pipes under buildings can only be done where approved by the AHJ and where special precautions are taken. Those steps include running the underground piping in a covered trench, arching the foundation walls over the underground pipe, and installing isolation valves so that the water can be easily shut off in the event of a break or leak under the building. The section also stated that the trenches and isolation valves are not necessary if the pipe runs under the building for 10 feet or less before turning up through the slab. With the addition of this 10-foot dimension, the AHJ is given the specific criterion necessary to enforce this previously ambiguous concept.
It is important to remember that before proceeding with any design approach where main piping is run under a building, the concept must be reviewed with, and approved by, the AHJ.
Matt Klaus is principal fire protection engineer at NFPA and staff liaison for NFPA 13, 13R, & 13D.