Stairs look simple until you start building them, and then the math and code requirements hit all at once. Getting the rise and run right is not just about passing inspection. Consistent tread height is what keeps people from tripping. This guide covers the key numbers, the code requirements most homeowners don't know about, and how to lay out a stringer before you make your first cut.
Every staircase is defined by two dimensions: the rise (the vertical height of each step) and the run (the horizontal depth of each tread). The International Residential Code (IRC), which most U.S. jurisdictions adopt, sets these limits: maximum riser height of 7-3/4 inches, minimum tread depth of 10 inches. The most comfortable stairs fall around a 7-inch rise and 11-inch run. This is sometimes called the '7-11 rule' among builders. The total floor-to-floor height divided by your riser height gives you the number of risers. The number of treads is always one less than the number of risers, because the top landing counts as the final tread.
The IRC also requires that no riser in a flight vary by more than 3/8 inch from any other riser. This rule exists because people calibrate their stride to the first few steps they take. If the bottom step is 6.5 inches and all the others are 7.5 inches, someone will eventually miss that step. When you're calculating, divide the total rise as evenly as possible across your risers. Round to the nearest 1/16 inch and check that no two risers differ by more than 3/8 inch. This often means making a small adjustment to the top or bottom riser.
The IRC requires a minimum of 6 feet 8 inches of headroom measured vertically from the nosing of any tread to the structure above. This is the dimension that catches a lot of basement stair projects. The ductwork or the floor joist above can eat into headroom quickly. Measure from the nosing of the bottom tread, draw an imaginary vertical line upward, and make sure nothing (including finished ceiling material) falls within 80 inches of that line along the entire stair run. If you're tight, you may need to lower the starting point of the stair, modify framing above, or relocate ductwork.
Handrails are required on any stair with four or more risers. The graspable portion of the handrail must be between 34 and 38 inches above the stair nosing, measured vertically. Guardrails (the full-height barriers at open sides of landings and elevated decks) are required when the drop exceeds 30 inches and must be at least 36 inches high, measured from the walking surface. Guardrail balusters must be spaced so a 4-inch sphere cannot pass through. This is the child safety requirement. Many homeowners confuse handrails and guardrails; you often need both on an open staircase.
Not just any piece of lumber qualifies as a graspable handrail. The IRC requires a profile with a perimeter between 4 and 6.25 inches, or a circular cross-section with a diameter between 1.25 and 2 inches. A standard 2x4 laid flat does not meet this requirement. Use a purpose-made handrail profile from the molding aisle, or verify your lumber profile against the code dimensions.
A stringer is the diagonal board that supports the treads and risers on each side of the stair. Most residential stairs use three stringers: two outer and one center. Use a framing square with stair gauges (small clamps that attach to the square's legs at your rise and run dimensions) to step off the cuts along the length of the board. Mark each step carefully, then cut with a circular saw and finish the corners with a handsaw. Never overcut with the circular saw, as this weakens the stringer at its thinnest point. After layout, check the remaining depth at the narrowest part of the stringer (the notched section); code requires at least 3.5 inches of solid wood remaining.
The most frequent stair violations inspectors flag: inconsistent riser heights (usually from a poorly measured total rise), a missing or non-graspable handrail, handrail height outside the 34-38 inch range, inadequate headroom, and treads with a nosing that protrudes more than 1.25 inches (nosing projection must be between 3/4 and 1.25 inches under IRC). If your project requires a permit, expect the inspector to measure riser consistency with a tape measure on every step. Do the math before you build, not after.
