Hotel Just Over the 13R Ceiling: 62 ft 4 in to the Roof Deck

NFPA 13R is a limited-application standard: it applies only to residential occupancies up to and including four stories and not exceeding 60 ft in height above grade plane. Here the story count is satisfied at four, but the height is not. Building height for this purpose is measured from the grade plane to the average height of the highest roof surface, so the 62 ft 4 in roof deck controls and the 4 ft parapet is not a factor either way. Because the building is roughly 2 ft 4 in over the 13R ceiling, the project must be designed to NFPA 13. Two practical checks before you commit: confirm with the architect how the average grade plane was computed at the walk-out side, since a sloped site can move the measurement by a foot or more; and confirm the building official's construction classification, because it drives both the attic question and the concealed-space allowances. If the design can be pulled to 59 ft 6 in by lowering floor-to-floor or removing a story, 13R becomes viable again, and that conversation is worth having before a permit is filed, because it changes the hydraulic demand dramatically.

The two-sprinkler calculation is a 13R method only. Under NFPA 13 the system must be hydraulically calculated for the most demanding remote area at the density and area for the hazard: for this light hazard hotel that is 0.10 gpm/sq ft over 1,500 sq ft, plus a 100 gpm hose stream allowance and a 30-minute duration. The flowing demand at the base of the riser therefore jumps from roughly two sprinklers (on the order of 50 to 80 gpm) to about 150 gpm of sprinkler flow in the remote area plus 100 gpm of hose, and friction losses rise with the square of the flow. Use the flow test to judge the supply: with 71 psi static and 52 psi residual at 900 gpm, the drop is 19 psi at 900 gpm, so pressure available at any other flow follows the 1.85-power relationship, P = 71 - 19 x (Q/900)^1.85. At 250 gpm that is still about 69 psi, but the real question is the pressure at the calculated demand at the riser, including backflow preventer, meter, and private main losses. If the calculated demand lands near or above the residual point of the test, the 6 in main will not carry it without a fire pump or main upgrade. Do not assume the two-sprinkler number; run the full calculation.

The 13R omission list is generous and does not survive the switch. Under NFPA 13, examine at minimum: the wood-truss attic, because 13R permits omission in many combustible concealed spaces and residential attics that NFPA 13 does not treat so broadly, and the omission allowances NFPA 13 does contain for concealed spaces depend on noncombustible construction, limited access, and no fuel-fired equipment or storage; closets and bathrooms, because 13R permits omitting sprinklers in small closets and in bathrooms below a size threshold, while NFPA 13 protects them; elevator machine rooms and electrical/mechanical rooms, which 13R may exclude under conditions; and combustible exterior projections such as wood balconies and soffits, which 13R often does not protect. The attic is the expensive one: adding a wet or dry system at 62 ft with unheated attic space may force a dry-pipe or preaction arrangement, an air supply, and additional hydraulic demand. Document each determination on the plan, because the framing sub starting in nine weeks will close those spaces and the AHJ will not accept a later 'we omitted it like the 13R job' explanation.