Stair Rise, Run & IRC Code Compliance
Rae must build 13 stairs from the basement slab to the kitchen subfloor — a total rise of 104 1/4". IRC code caps each riser at 7 3/4".
- Tape
- Framing square
- Stair gauges
- 2×12 stringer stock
- 1
Measure total rise floor-to-floor
Subfloor → subfloor, not slab → drywall. Mistake here = code violation.
- 2
Divide by max riser
104.25 ÷ 7.75 = 13.45 → round UP to 14 risers (so each riser stays legal).
- 3
Recalculate riser height
104.25 ÷ 14 = 7.45" per riser. All risers must be within 3/8" of each other.
- 4
Pick a run that meets code
Tread run ≥ 10". Use 2×12 (11 1/4" deep) — gives 1" nosing overhang.
Build a stair from a 48" deck to a patio. IRC R311.7.5 caps riser ≤ 7 3/4", requires tread ≥ 10", and limits variation between any two risers to 3/8". The classic 'rule of 25' (2R + T ≈ 25) gives comfort. Compute step count, exact riser, total run, and verify code in one pass.
Tap Show next step to reveal the math one piece at a time.
Stairs from a 48″ deck
Given: Total rise = 48″, target riser ≈ 7″
- 1
Trial step count
48 ÷ 7 = 6.857
Comfort check (17–18″ rule)
Given: Riser 7.5″, tread 10″
- 1
Add riser + tread
7.5 + 10 = 17.5″