When I first started learning about sprinkler design, I was using the then current 1972 edition of National Fire Protection Association (NFPA) 13 . That was the first version of the standard that had density/area curves for light, ordinary and extra hazard occupancies and at the time, the only design option was the Density/Area Method. The Room Design Method was only introduced a couple of years later, in the 1974 edition. Then, a year after that, for the 1975 edition, the provision was added that the room must be enclosed with construction having a fire resistance rating equal to the water supply duration. Additionally, the corresponding rules for the protection of openings (ie. doorways) were first provided in NFPA 13-1975.
The wording has been tweaked numerous times since then, for example, the standard used to refer to the “largest room”, whereas today the verbiage references the “room that creates the greatest demand”. However, the application of the Room Design Method is essentially still the same, today as it was back then.
The concept of the Room Design Method is that all sprinklers within a room will operate, along with some sprinklers in adjacent rooms of light hazard occupancy, if the openings aren’t protected. NFPA 13 doesn’t define a “room” though, and that may be the cause some misunderstandings, such as the one I recently encountered.
I was recently speaking with a colleague, who was reviewing an apartment building, whose sprinkler design was intended to be based on the Room Design Method. The hydraulic calculations incorporated 6 sprinklers for the living room/dining/kitchen area and some additional sprinklers were included for communicating spaces, such as the bedrooms and the closets, which didn’t have protected openings (ie. self-closing doors). For a light hazard occupancy, it sounded good at first, until we realized that while there was adequate separation from the building’s corridors and the adjacent apartment suites/dwelling units, the interior walls within the unit were unrated. With this, we had to conclude that the Room Design Method was being misapplied. The designer based the calculations on the individual (unrated) walls within the apartment suite/dwelling unit, whereas the “room” that should have been considered for the analysis was the entire suite, with all sprinklers that are bounded by the fire rated construction.
Often the Room Design Method can be applied to a single room, such as an ordinary hazard mechanical room in an otherwise light hazard occupancy or perhaps for a classroom in a school. However, even more often, spaces such as hospital rooms and hotel rooms will have sprinklered washrooms and/or closets within. With such situations, because of unrated construction, those spaces cannot be considered as separate or communicating rooms, they must be taken as part of the overall “room”.