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IRC Code Requirements (R403.1, R311.7)

• Max riser height: 7.75 inches (197 mm)
• Min tread depth: 10 inches (254 mm) — nosing adds 0.75" to 1.25"
• Rise + Run formula: (2 × Riser) + Tread = 25" to 27"
• Max variation between risers: 3/8 inch
• Min headroom: 6'8" (80 inches) clear — IRC R311.7.2
• Min stair width: 36 inches (residential), 44 inches (commercial)
• Handrail height: 34"–38" above tread nosing — IRC R311.7.8
• Baluster spacing: Max 4" openings — IRC R312.1
• Landing depth: Min 36" at top and bottom — IRC R311.7.5
• Max step count without intermediate landing: 14 risers

Stair Type Comparison

Formulas

Worked Example — Straight Stair

Inputs: Total Rise = 108 in (9 ft), Tread Depth = 10 in, Stair Width = 36 in, Ceiling = 120 in

• Risers needed: 108 ÷ 7.75 = 13.9 → 14 risers
• Actual riser height: 108 ÷ 14 = 7.71 in (PASS — under 7.75")
• Treads: 14 − 1 = 13 treads
• Check: (2 × 7.71) + 10 = 25.42 in (PASS — 25–27")
• Total run: 13 × 10 = 130 in
• Stringer: √(108² + 130²) = 168.8 in ≈ 14.1 ft
• Angle: arctan(108/130) = 39.8° (slightly steep — consider 9" run)
• Headroom: Using 10" run, 8 treads deep = 80 in vertically → 120 − 80 = 40 in (PASS)
• Handrail: 168.8 in + 12" top + 12" bottom = 192.8 in ≈ 16.1 ft (one side)
• Balusters: (10 × 13) ÷ 4 = 32.5 → 33 balusters

Stringer Count Guide

• 2 stringers: Standard for stairs ≤ 36" wide with solid treads
• 3 stringers: Recommended for stairs 36"–48" wide, or with composite/stone treads
• 4 stringers: Required for stairs > 48" wide, heavy traffic, or concrete treads

Common Mistakes to Avoid

• Measuring only the visible rise — include the top landing thickness
• Using different riser heights on the same flight (code violation — max 3/8" variation)
• Ignoring nosing projection in tread calculations
• Forgetting headroom clearance under intermediate landings
• Placing balusters more than 4" apart (child safety hazard — IRC R312.1)
• Setting handrails below 34" or above 38" — both are code violations
• Skip the landing at floor transitions — 36" minimum required
• Assuming spiral stairs meet residential code — check local amendments

Material Selection Guide

• Hardwood treads: Oak, maple — premium look, $15–30/lf, needs finishing
• Composite treads: Trex, Veranda — low maintenance, $20–40/lf, weather-resistant
• Concrete treads: Indoor/outdoor, $8–15/lf, heavy — needs 4 stringers
• Stone treads: Marble, granite, slate — luxury, $30–80/lf, very heavy
• Open risers: Modern look, improves light flow, must have ≤ 4" gaps
• Closed risers: Traditional, code-compliant everywhere, can be painted/stained

Tips

• Use a string line from finished floor to finished floor for accurate rise measurement
• Pre-cut stringers save time — mark riser and tread locations carefully with a framing square
• Treat stringers with preservative if they contact concrete or are in damp locations
• Always verify local code amendments — some jurisdictions are stricter than IRC baseline
• For winder stairs, the narrowest point of the tread at the walking line must be ≥ 10"
• Spiral stairs: headroom must be measured at the outer edge of the treads
• L-shaped and U-shaped stairs need an intermediate landing — minimum 36" × stair width
• Factor in finishing time: stain, paint, or seal each tread and riser before installation