Four-Story Apartment Building: Is NFPA 13R Permitted, and Which Omissions Are Acceptable?
A developer proposes a 4-story, 48 ft high wood-frame apartment building with 60 dwelling units in Maryland. The building will be protected by an NFPA 13R system supplied by a 4 in combined riser. The water supply has 60 psi static and 55 psi residual at 1,000 gpm. The architect wants to omit sprinklers in the attic, in 8 ft² closets, and on open balconies. The local fire official asks whether 13R is permitted, whether those omissions are allowed, and whether the 4 in riser and water supply are adequate. The building has no separate standpipe, and fire department access is at the front only.
1. Is NFPA 13R permitted for this building? What conditions must be confirmed?
Read the worked answer
NFPA 13R is generally permitted for residential occupancies up to and including four stories above grade plane, subject to the building code edition and local amendments adopted in Maryland. This building is four stories, so it is at the upper limit and must be checked carefully.
The designer must confirm the height and story limit in the adopted code, the occupancy classification, and any Maryland or county amendments. The presence of a 4 in combined riser and no separate standpipe also must be evaluated against the building code and fire department requirements. A four-story building is not automatically disqualified, but the approval must be documented rather than assumed.
2. How should the proposed omissions in the attic, the 8 ft² closets, and the open balconies be evaluated?
Read the worked answer
NFPA 13R permits limited omissions, but each omission has conditions. The 8 ft² closets may qualify only if they meet the exact size, construction, and combustibility limits in the adopted edition; an 8 ft² closet is not automatically exempt. The attic may be omitted only where the code permits it and the attic has no occupancy or hazardous storage; combustible construction and storage can change that answer.
Open balconies are sometimes permitted to be omitted, but the designer must show that the balcony is open, noncombustible, and does not create a concealed combustible space. The fire official should require a code-by-code citation for each omission, not a blanket statement that 13R allows it.
3. Is the 4 in combined riser and the water supply adequate? What must the developer submit?
Read the worked answer
The 4 in riser and water supply cannot be approved from the static and residual pressures alone. The developer must submit a hydraulic calculation for the 13R system using the actual pipe sizes, elevations, sprinkler demand, and water supply curve. A residual pressure of 55 psi at 1,000 gpm is a useful indicator, but the demand point must be compared with the supply at the required pressure.
The calculation also must account for the combined riser demand and any fire department connection or standpipe requirements. If the building code requires a standpipe or a larger combined riser, the 4 in pipe may be inadequate even if the sprinkler calculation alone works. The reviewer should require the flow test, calculation, and code analysis before accepting the design.