Stair rise and run: IRC baseline vs the amended states
The same floor-to-floor height produces a different stair depending on which riser and tread pair your state adopted. Riser is measured between the leading edges of adjacent treads; neither riser nor tread may vary more than 3/8″ within a flight.
What this diagram shows
Two side-by-side stair cross-sections drawn to the same scale from the same 112-inch floor-to-floor height. Left, the IRC baseline with a 7 3/4 inch maximum riser and 10 inch minimum tread: 15 risers at 7.47 inches, consuming 140 inches of floor. Right, the amendment used in New York, New Jersey, Pennsylvania, Massachusetts, Connecticut and Rhode Island with an 8 1/4 inch riser and 9 inch tread: 14 risers at 8.00 inches, consuming 117 inches of floor. One riser fewer and 23 inches less run for the same height.
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