What makes a stair step legal and comfortable — IRC riser/tread limits and the Blondel rule

Code sets the limits — riser ≤ 7¾″, tread ≥ 10″ (11″ with no nosing), nosing ¾–1¼″ — and Blondel’s rule sets comfort: 2R+T = 24–25″ and R+T = 17–18″. A 7″/11″ step hits both. Risers must also vary by ≤ 3⁄8″.

What this diagram shows

One stair step drawn large in section with the three numbers code constrains: the riser must be no taller than 7¾ inches, the tread must be at least 10 inches deep (11 inches if there is no nosing), and the nosing overhang is ¾ to 1¼ inches. Alongside it, the Blondel comfort rule: two risers plus one tread should equal 24 to 25 inches, and one riser plus one tread should equal 17 to 18 inches. A 7-inch riser with an 11-inch tread checks out at 2×7+11 = 25 and 7+11 = 18. Other limits: headroom at least 6 feet 8 inches (80 inches), stair width at least 36 inches, and risers and treads within a flight may vary by no more than 3⁄8 inch.

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Code-compliant stair geometry, stringers & materials per 2021 IRC R311.7 / 2024 R318.7. Straight, L, U, winder, spiral & curved. AWC DCA 6 stringer spans. Free.

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Read the guide: Stairs Building Guide