Shower Compartment Size (IPC 417.4)
Two compliance paths, each with TWO conditions, and area alone satisfies neither. IPC 417.4 wants at least 900 sq in of finished interior area AND at least 30 in in least dimension; the exception allows a least dimension down to 25 in only if the area reaches 1,300. So a 28 x 36 stall has 1,008 sq in -- past the 900 -- and FAILS both paths, because 28 is under 30 and 1,008 is under 1,300. Adding length does nothing for the short side, and for a rectangle the 30 in disc test IS the least-dimension test. A 30 x 30 passes exactly; a 25 x 52 passes exactly by the exception. Measured from the FINISHED interior at the threshold, exclusive of valves, showerheads, soap dishes, and grab bars, and it must hold to 70 in above the drain -- so tile and a bench come out of the number and a low soffit can fail a stall that measures fine at the curb.
Formula and source
area = width x depth; least dimension = min(width, depth); base path passes when area >= 900 sq in AND least dimension >= 30 in; the exception path passes when area >= 1,300 sq in AND least dimension >= 25 in; the compartment complies if either path passes. The other side needed at a given least dimension = max(minimum dimension, minimum area / least dimension) on each path.
IPC 417.4 - shower compartments not less than 900 sq in in interior cross-sectional area and not less than 30 in in least dimension, measured from the finished interior dimension at a height equal to the top of the threshold and at a point tangent to its centerline, exclusive of fixture valves, showerheads, soap dishes, and safety grab bars or rails, continued to a height not less than 70 in above the shower drain outlet; with the exception permitting a least dimension of not less than 25 in where the compartment has not less than 1,300 sq in of cross-sectional area.
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