Stairs require exact geometric precision. Learn the IRC 7-11 rule and riser-to-tread math to construct safe, code-compliant residential stair stringers.
Key IRC Building Code Requirements for Stairs
Under the International Residential Code (IRC R311.7):
• Maximum Riser Height: 7-3/4 inches (197 mm).
• Minimum Tread Depth: 10 inches (254 mm) with nosing, or 11 inches without nosing.
• Maximum Riser Variance: The difference between the tallest and shortest riser in a single flight must not exceed 3/8 inch (9.5 mm).
• Minimum Headroom: 6 feet 8 inches (2,032 mm) vertical clearance measured from the sloped plane of stair treads.
The Famous "Rule of 17 to 18" for Comfortable Steps
Generations of master carpenters use the proportional stride formula to design ergonomic, comfortable stairs:
Proportional Rule: Riser Height + Tread Depth = 17 to 18 Inches.
For example: A 7.5-inch riser paired with a 10.5-inch tread equals 18 inches, providing natural, effortless cadence without feeling too steep or shallow.
| Total Rise (Inches) | Risers Count | Exact Riser Height | Treads Count (Risers - 1) | Total Stringer Run (@ 10.5" Tread) |
|---|---|---|---|---|
| 30 in (Deck) | 4 risers | 7.50 in | 3 treads | 31.5 in (2.63 ft) |
| 48 in (Porch) | 6 risers | 8.00 in (Too high! Use 7) | 6 treads | 63.0 in (5.25 ft) |
| 96 in (8ft Story) | 13 risers | 7.38 in | 12 treads | 126.0 in (10.50 ft) |
| 108 in (9ft Story) | 14 risers | 7.71 in | 13 treads | 136.5 in (11.38 ft) |
Stringer Bottom and Top Cut Thickness Adjustment
The most common rookie mistake when cutting stair stringers is forgetting tread thickness:
Because the top of the bottom tread sits on the cut stringer, you must cut off the thickness of one tread (e.g. 1.0 inch for 5/4 deck boards) from the bottom foot of the stringer. If you omit this cut, the bottom step will be 1 inch too tall, and the top step will be 1 inch too short, failing inspection.
Frequently Asked Questions & Answers
Click any question below to expand quick answers, formulas, and expert tips.
Why are there always one fewer treads than risers?
Because the top step lands flush on the upper floor deck or porch framing, which serves as the final landing.
How many stringers do I need for a 36-inch wide deck staircase?
For standard composite or 5/4 wood decking, stringers should be spaced no more than 12 to 16 inches on center. A 36-inch wide stair requires 3 to 4 stringers.
