Stair Code Basics — IRC Riser, Tread & Handrail Rules
Stair inspections fail on a handful of numbers, and every one of them is checkable before the inspector arrives. This guide is the IRC residential list in plain language, with the deck-stair specifics that trip DIY builds.
Quick answer: IRC R311 residential numbers: riser max 7¾ in, tread min 10 in, headroom 6 ft 8 in, handrail 34-38 in above the tread nosings, guards 36 in where the drop exceeds 30 in, and riser heights may vary no more than 3/8 in within a flight.
The riser and tread numbers
- Riser: 4 in minimum, 7¾ in maximum (R311.7.5)
- Tread: 10 in minimum, measured nosing to nosing (R311.7.5.2)
- Nosing: ¾-1¼ in where the tread is under 11 in
- Consistency: the largest riser may exceed the smallest by no more than 3/8 in in one flight — the classic failed-stair rule
Headroom, width, landings
- Headroom: 6 ft 8 in minimum above the tread nosings (R311.7.2)
- Width: 36 in clear above the handrail (R311.7.1)
- Landings: 36 in deep at exterior doors; a stair may not descend more than 7¾ in directly outside a door without a landing
Handrails and guards
- Handrail: 34-38 in vertically above the tread nosing, returned or continuous, grip-able profile (R311.7.8)
- Guard: 36 in minimum height where the walking surface is more than 30 in above the grade below (R312)
- Guard openings: a 4-in sphere must not pass through (this is the baluster-spacing rule)
- Deck stairs over 30 in drop need guards on both sides with the same 4-in sphere rule
The deck-stair layout that passes
Divide total rise by 7¾ and round up for the riser count, then set every riser to rise ÷ count so they're identical within the 3/8 tolerance. Cut stringers from 2×12 stock (the stringer calculator outputs the length), keep the throat of the stringer solid, and verify the first step off the deck matches the rest — deck stairs that start with a 9-inch riser because someone forgot the decking thickness are the most common DIY fail.
Local amendments
IRC is the model code; states and cities amend it. Some jurisdictions keep a 4-in sphere exemption on older deck guards, some tighten tread minimums. The numbers here are the model-code baseline as of September 2026 — your building department's handout is the one that counts.