The term “shadow area” isn’t actually defined in the body of NFPA 13, but it’s clear that most users have an understanding of what it means. Simply put, a shadow area is the theoretically dry space beyond an obstruction, such as a structural column or soffit, that will not be covered by water discharged from a sprinkler.
It is important to understand that the idea of shadow areas on paper is different than what can happen during a discharge event. Sprinkler discharge is a complex phenomenon that becomes even more complicated during a fire event due to airflow and pressure dynamics. Due to these factors, areas that would remain dry in theory can in fact see some water. It may not be enough water to suppress a fire that originates in that area, but it would allow for some pre-wetting of these areas if they were not initially involved.
While NFPA 13 does not define “shadow area,” this doesn’t mean that water must reach every square foot of the defined coverage area for a sprinkler. The obstruction rules in NFPA 13, like the so-called “three and four times rules” (written to address shadows created by small, noncontinuous obstructions), and the “beam rule” (addressing shadows created by solid, continuous obstructions) will allow for some inherent shadow areas on the floor below. These areas will vary depending on both the vertical and horizontal distance between the obstruction and the sprinkler.
During the development of the 2013 edition of NFPA 13, the Technical Committee on Sprinkler System Installation Criteria considered the concept of adding maximum shadow area values to the standard, including 15 square feet (using the three times rule) for standard spray sprinklers and 21 square feet (using the four times rule) for extended coverage sprinklers. The committee was not in favor of this approach out of concern that it could invite users of the standard to ignore the long-standing, tried-and-true obstructions rules that have served the industry for many years. The committee revisited the issue in its discussions over the 2016 edition and arrived at the same conclusion. While these maximum shadow areas have not been included in the standard as prescriptive options, they may be presented to the authority having jurisdiction as an equivalency through Chapter 1 of NFPA 13.
This was not the same path taken by the Technical Committee on Residential Sprinkler Systems. The two NFPA residential sprinkler standards, NFPA 13D, Installation of Sprinkler Systems in One- and Two-Family Dwellings and Manufactured Homes, and NFPA 13R, Installation of Sprinkler Systems in Low-Rise Residential Occupancies, do not contain the same comprehensive obstruction rule package found in NFPA 13. As such, the concept of simply providing a maximum theoretical shadow area for each sprinkler was appealing. For these reasons, NFPA 13D and NFPA 13R both state that, for each sprinkler, the allowable shadow area must not exceed 15 square feet.
It is important to note that this area is cumulative, so if there are multiple obstructions to the same sprinkler, the shadows created by each obstruction must be added together. Furthermore, the shadow area must be within the coverage area of the sprinkler. This rule cannot be used to omit coverage for a portion of the floor area being protected.