Building stairs is one of the most math-intensive tasks in residential construction, and the tolerances are tighter than most people expect. The IRC (International Residential Code) limits the maximum riser height to 7-3/4 inches, requires a minimum 10-inch tread depth, and allows no more than 3/8-inch variation between the tallest and shortest riser in a flight. Miss any of these and the stairs fail inspection.
This guide walks through the complete process: measuring total rise, calculating the optimal riser and tread combination, laying out and cutting stringers, adjusting for tread thickness, and meeting the code requirements for headroom, handrails, guards, and landing dimensions that most builders know exist but cannot always cite chapter and verse.
Measuring Total Rise Accurately
Total rise is the vertical distance from the top of the finished floor at the bottom to the top of the finished floor at the top. This measurement must be exact because every fraction of an inch of error is multiplied across all the risers. If your total rise is off by 1/2 inch and you have 14 risers, each riser is off by 1/28 inch, which is within tolerance. But if you have a 1-inch error, each riser is off by 1/14 inch, and the accumulated effect is noticeable.
Measure at the location where the stairs will be built, not across the room. Floors sag, slope, and are not always level. Use a level and a long straight edge to verify that both the upper and lower floor surfaces are level at the stair location. If the upper floor slopes 1/4 inch across the stair opening width, account for this in your total rise measurement.
For new construction, measure after the subfloor is installed but before the finished flooring. Add the finished floor thickness to both levels (if different materials, add the correct thickness for each). If the bottom is tile on concrete (0 added) and the top is 3/4-inch hardwood, add 3/4 inch to the total rise. Getting this wrong means the first or last riser will be off after the finished floor goes in.
Measure three times, in three locations across the stair width. Use the longest measurement as your total rise. This gives the most conservative riser height, which is safer than a too-tall top or bottom riser caused by using an average measurement.
Calculating Riser Height and Tread Depth
Divide total rise by your target riser height. If total rise is 108 inches and target riser is 7.5 inches: 108 / 7.5 = 14.4 risers. You cannot have a fractional riser, so round to 14 or 15. At 14 risers: 108 / 14 = 7.714 inches per riser. At 15 risers: 108 / 15 = 7.2 inches per riser. Both are within the IRC maximum of 7.75 inches.
Choose the option that gives the most comfortable step. The classic comfort formula is: 2R + T = 24 to 25, where R is riser height in inches and T is tread depth. At 7.714 inches riser: 2(7.714) + T = 24.5, so T = 9.07 inches. That is below the IRC minimum of 10 inches, so you must use 10 inches minimum. At 7.2 inches riser: 2(7.2) + T = 24.5, so T = 10.1 inches. This satisfies the code minimum and feels comfortable.
The number of treads is always one less than the number of risers (because the top floor surface serves as the final tread). So 15 risers = 14 treads. Total run = 14 x 10.1 = 141.4 inches (11 feet 9.4 inches). Verify this fits in the available horizontal space. If it does not, you need a landing with a turn, a steeper stair (larger riser, smaller tread), or a different stair location.
Laying Out and Cutting Stringers
Stringers are typically cut from 2x12 dimensional lumber (11.25 inches actual width). The stair angle and the riser/tread combination determine how much wood remains at the thinnest point of the stringer after the notches are cut. This remaining wood (the throat) must be at least 3.5 inches for structural adequacy.
Use a framing square with stair gauges clamped at the riser height on one arm and the tread depth on the other. Align the gauges with the edge of the 2x12 and mark the first notch. Slide the square to the next position and repeat for each riser/tread combination. Mark the plumb cut at the top (where the stringer meets the upper floor framing) and the level cut at the bottom (where it sits on the lower floor).
Critical adjustment: subtract the tread thickness from the bottom of the stringer. If your risers are 7.2 inches and your treads are 1 inch thick (3/4 inch actual for dimensional lumber), cut 3/4 inch off the bottom of the stringer. After treads are installed, the first riser from the floor to the top of the first tread will be 7.2 inches, matching every other riser. Without this adjustment, the first step is 7.2 + 0.75 = 7.95 inches, which exceeds the 7.75-inch IRC maximum and fails inspection.
Cut the notches with a circular saw, stopping at the corner of each notch. Finish the cuts with a jigsaw or handsaw. Do not overcut with the circular saw because overcutting weakens the stringer beyond the notch line. After cutting the first stringer, use it as a template for the remaining stringers to ensure all are identical.
IRC Code Requirements Summary
Riser height: Maximum 7-3/4 inches, minimum 4 inches (IRC R311.7.5.1). The greatest riser height within any flight shall not exceed the smallest by more than 3/8 inch.
Tread depth: Minimum 10 inches measured from nosing to nosing (IRC R311.7.5.2). If treads have a projecting nosing, the nosing must project between 3/4 inch and 1-1/4 inches and be uniform within the flight.
Width: Minimum 36 inches clear above the handrail height (IRC R311.7.1). Handrails can project a maximum of 4.5 inches from each side, so the framed opening should be at least 36 inches plus handrail projections.
Headroom: Minimum 6 feet 8 inches measured vertically from the nosing line to the ceiling or header above (IRC R311.7.2). This is often the constraint that forces the stair opening to be longer than the minimum.
Handrails: Required on at least one side of stairs with four or more risers (IRC R311.7.8). Handrail height must be 34 to 38 inches measured from the nosing line. Graspable handrails must have a cross-section of 1-1/4 to 2 inches diameter (circular) or a perimeter of 4 to 6-1/4 inches (non-circular) with a maximum cross-section of 2-1/4 inches.
Guards: Required where the open side of the stair is more than 30 inches above grade. Guard height must be at least 36 inches measured from the nosing line. Balusters must be spaced so a 4-inch sphere cannot pass through (IRC R312.1).
Landings, Turns, and Winders
A landing is required at the top and bottom of every stairway (IRC R311.7.6). The landing depth must be at least as wide as the stair width (minimum 36 inches). The landing provides a safe transition area for entering and exiting the stairway and provides a platform for doors that open over the stairway.
For stairs that turn, the landing at the turn must be at least as deep as the stair width. A 36-inch-wide stair needs a 36x36-inch landing at the turn. This can be a significant space requirement. L-shaped stairs with a single 90-degree turn need one landing. U-shaped stairs with a 180-degree turn need two landings or one large landing equal to the stair width x twice the stair width.
Winder treads (pie-shaped treads at a turn) are allowed by the IRC if they meet specific requirements: the tread depth at the narrow end must be at least 6 inches, and the tread depth at a point 12 inches from the narrow end must be at least 10 inches (IRC R311.7.5.2.1). Winders save space but are less safe than flat landings, especially for elderly occupants and in emergencies.
Spiral stairs are permitted by the IRC for individual dwelling units with specific constraints: minimum 26-inch clear width, minimum 6.5-inch tread depth at 12 inches from the narrow edge, maximum 9.5-inch riser height, and minimum 6-foot-8-inch headroom. Spiral stairs are not permitted as the primary means of egress in most jurisdictions.
Exterior and Deck Stairs
Deck stairs follow the same IRC rise and run requirements as interior stairs. The total rise is from the deck surface to the grade (ground level) at the bottom of the stairs. Because grade is rarely perfectly level, measure the total rise at the exact landing point and verify the grade is stable.
For deck stairs landing on soil, pour a concrete landing pad at the bottom. The pad provides a stable, level surface for the stringer base and prevents the stringers from sinking into soft ground or heaving with frost. A minimum 24x36-inch pad, 4 inches thick on a 4-inch gravel base, is typical. The pad should extend at least 36 inches from the bottom riser (the code-required landing depth).
Exterior stringers must be pressure-treated lumber rated for ground contact if the bottom is in contact with concrete or soil. Galvanized or stainless steel stringer brackets and fasteners are required to prevent corrosion. Regular untreated lumber will rot within 2-5 years at the ground contact point.
Open risers (no vertical board between treads) are allowed by the IRC if the opening between treads does not allow a 4-inch sphere to pass through (IRC R311.7.5.3). With standard tread spacing, this limits open risers to stairs with a riser height under about 7 inches or treads with a nosing overhang that closes the gap. Many deck builders use closed risers or add a horizontal bar between treads to meet the 4-inch rule.