How to Design a Staircase: IRC Code Requirements and Layout

Staircase diagram showing total rise, total run, riser height, tread depth, nosing, and headroom with IRC code requirements

Start With the Total Rise

Every staircase begins with one measurement: the total rise from finished floor to finished floor. Not subfloor to subfloor — finished floor to finished floor. This distinction matters because floor finish thickness (hardwood, tile, carpet) affects the first and last riser heights. If you measure to subfloor and the finish flooring hasn’t been installed yet, add its thickness at both levels.

Measure total rise in at least three locations (left, center, right of the stair opening) because floors are never perfectly level. Use the largest measurement to ensure no riser exceeds code.

IRC Dimensional Requirements

The International Residential Code (IRC) sets these limits for residential stairs:

Maximum riser height: 7-3/4 inches (7.75”). No riser may exceed this. Minimum tread depth: 10 inches, measured horizontally from nosing to nosing. Maximum riser variation: 3/8 inch between the tallest and shortest riser in the staircase. Minimum headroom: 6 feet 8 inches measured vertically from the nosing line. Minimum width: 36 inches clear above the handrail.

The 3/8-inch variation limit is the one that catches carpenters. If your total rise doesn’t divide evenly, every riser must still be within 3/8 inch of every other riser. Sloppy division means a failed inspection.

The Calculation

Step 1: Divide total rise by your target riser height (typically 7 inches for comfort). 108 inches ÷ 7 = 15.43

Step 2: Round to the nearest whole number. This is your riser count. Round(15.43) = 15 risers

Step 3: Calculate actual riser height. 108 ÷ 15 = 7.2 inches per riser ✓ (under 7.75” maximum)

Step 4: Calculate total run. Treads = Risers - 1 = 14 treads Total run = 14 × 10 inches = 140 inches (11 feet 8 inches)

Step 5: Verify you have enough horizontal space for the total run plus a landing at top and bottom.

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Stringer Layout

The stringer is the structural member that carries the treads and risers. For a standard residential staircase, 2×12 lumber is typical, with the throat (remaining wood after cutting the tread/riser notches) providing structural depth.

Mark the stringer using a framing square with stair gauges clamped at the riser height and tread depth. Walk the square along the stringer, marking each step. Cut the top and bottom to account for the tread thickness — the top loses one tread thickness where it meets the upper floor, and the bottom loses one tread thickness where it sits on the lower floor. This is the most common layout error: forgetting to adjust the first and last risers for tread thickness.

Minimum Three Stringers

For stair widths of 36 inches, two stringers (left and right) are technically adequate for light loads. In practice, a center stringer eliminates bounce and flex under foot traffic. Most builders use three stringers as standard practice for widths up to 36 inches, and add a fourth for wider stairs.

Common Mistakes

Measuring to subfloor instead of finished floor. If the finish floor isn’t installed, add its thickness. A 3/4-inch hardwood floor at the top level changes a 108-inch total rise to 108.75 inches — enough to change the riser count or push an already-tight riser over the 7.75-inch limit.

Not checking headroom. A staircase that meets all riser and tread requirements can still fail inspection if the headroom clearance is under 6’8” where the upper floor framing crosses above the stair.

Inconsistent risers at top and bottom. The first and last risers must include the tread thickness adjustment. Without it, the bottom riser will be one tread thickness too tall and the top riser one tread thickness too short.

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