System Layout and Design
Spacing and coverage are two separate tests
NFPA 13 limits both the maximum protection area per sprinkler and the maximum spacing between sprinklers, plus distance to walls and ceilings. Familiar values: light hazard 225 ft² and 15 ft; ordinary hazard 130 ft² and 15 ft; extra hazard 100 ft² and 12 ft. Both limits must be satisfied at once - a 15 ft grid in ordinary hazard satisfies spacing but exceeds 130 ft² per head. On sloped ceilings, spacing is measured along the slope, and the last sprinkler must be within half the allowable spacing of a wall, generally no closer than 4 in.
Placement details
Deflectors of standard spray pendent and upright sprinklers sit 1 in. to 12 in. below the ceiling. Obstruction rules - the three-times rule and the 4-ft-wide limitation - determine whether a head can throw past a beam, duct, or soffit, or whether a baffle or additional head is needed. Pipe sizing follows the hydraulic calculation, but hanger spacing is prescriptive: 12 ft for 1 in. and smaller pipe, 15 ft for 1¼ in. and larger.
Traps: designing to area alone; ignoring obstructions and sloped-ceiling geometry; forgetting that extended-coverage sprinklers change both spacing and required density; omitting heads in small rooms or closets where the standard requires them; mixing light-hazard spacing assumptions into an ordinary-hazard area.
Rule of thumb: a 30 ft × 30 ft light-hazard room is 900 ft², so 900 ÷ 225 = 4 sprinklers; a 15 ft × 15 ft grid leaves each head 7.5 ft from the wall, exactly half the allowable spacing.