Masonry Fireplace Flue Area (IRC R1003.15.1)
Whether the flue is big enough for the opening it serves, by the code's Option 1 ratio test. Shape decides the fraction: a ROUND flue needs 1/12 of the opening area, square or rectangular under 2:1 needs 1/10, and a rectangle 2:1 or worse needs 1/8, because a narrow flue wastes cross section in its corners and boundary layer. A 36 x 29 in opening is 1,044 sq in, so a square liner must net 104.4 sq in and a 12 in nominal clay liner (11.5 x 11.5 actual = 132 sq in) clears it, while a 10 in round pipe at 78.5 sq in does NOT. Two traps: a clay liner's NOMINAL size is its outside size, so sizing off it overstates the flue, and all three ratios only apply to a chimney at least 15 ft tall measured from the firebox FLOOR. Also reports the smallest compliant round and square flue for the opening. Area only, not a draft calculation.
Formula and source
opening area = width x height; required net flue area = opening / 12 (round), / 10 (square or rectangular under 2:1), / 8 (rectangular 2:1 or greater); net area = pi d^2 / 4 round, a x b rectangular; smallest compliant round diameter = sqrt(4 (opening/12) / pi), smallest compliant square side = sqrt(opening/10).
IRC R1003.15.1 (Option 1), Flue area (masonry fireplace), with the identical text at IBC 2113.16.1. The ratios are conditioned on a chimney at least 15 ft high measured from the firebox floor to the top of the chimney flue.
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