Wood Stair Building Guide: Stringers, Treads, Risers & Code Requirements

Code Requirements

Stairs are heavily regulated because they're a leading cause of household injuries. The IRC (International Residential Code) sets minimum standards that most jurisdictions adopt:

Maximum riser height: 7-3/4 inches (some codes allow 8 inches). This is the vertical distance from the top of one tread to the top of the next.

Minimum tread depth: 10 inches (measured from the front of the nosing to the face of the next riser). Some codes measure from nosing to nosing, requiring 11 inches minimum.

Nosing: If risers are used, each tread must project 3/4" to 1-1/4" beyond the riser below (the nosing overhang). If no risers, no nosing is required.

Minimum width: 36 inches clear between walls or handrails.

Headroom: 6 feet 8 inches minimum clearance measured vertically from the nosing line to any overhead obstruction.

Consistency: The critical rule — all risers must be the same height, and all treads must be the same depth. The maximum variation between any two risers is 3/8 inch. Inconsistent risers are a trip hazard because your body expects each step to be the same.

Handrails: Required on at least one side for stairs with 4 or more risers. Rail height: 34-38 inches measured vertically from the nosing line.

Calculating Rise and Run

Total rise: Measure the total vertical distance from finished floor to finished floor (not subfloor to subfloor — the finish floor thickness matters). This is your total rise.

Number of risers: Divide total rise by your target riser height (typically 7" to 7-3/4"). Round to the nearest whole number. Example: 108" total rise ÷ 7.5" = 14.4, round to 14 risers.

Actual riser height: Divide total rise by number of risers. 108" ÷ 14 = 7.714" (7-11/16"). This is your actual riser height. All risers must be this exact height.

Number of treads: Always one fewer than the number of risers (the top "tread" is the upper floor itself). 14 risers = 13 treads.

Total run: Number of treads × tread depth. 13 treads × 10" = 130" (10 feet 10 inches). This is the horizontal distance the stair occupies.

1. The comfort formula: 2R + T = 24-25 inches (where R = riser height and T = tread depth). A 7.5" riser with a 10" tread: 2(7.5) + 10 =
2. 2
3. This produces a comfortable stair that feels natural to climb.

Stringer Layout

Stringer material: 2x12 lumber (Douglas fir or southern yellow pine for strength). The stringer must be wide enough that at least 3-1/2" of solid wood remains after notching. This is why 2x12 is the minimum — a 2x10 doesn't leave enough material after cutting the step notches.

Layout with a framing square: Clamp stair gauges (small brass clamps) to a framing square at the riser height on one leg and the tread depth on the other. Align the gauges with the top edge of the 2x12 and trace the step pattern. Move the square along the board and repeat for each step.

Adjusting for tread thickness: The bottom of the stringer must be shortened by the thickness of one tread. This is called "dropping the stringer." Without this adjustment, the first riser (from ground to first tread) would be taller than the rest because the tread thickness adds to it. Cut the bottom of the stringer to remove one tread thickness (typically 1" for 5/4 treads or 1-1/2" for 2x treads).

Number of stringers: Minimum two (one on each side). Add a center stringer for stairs wider than 30-36 inches to prevent bounce in the treads.

Assembly

Treads: 5/4x12 (1" thick, 11-1/4" wide) for interior stairs with drywall-enclosed stringers. 2x12 for open stringers and exterior stairs. Hardwood treads (oak is the standard) for finished interior stairs. Bullnose the front edge for a finished look.

Risers: 3/4" plywood or 1x lumber. Paint-grade material is fine since risers are vertical and don't take wear. For open-riser stairs (no risers installed), verify this meets local code — some jurisdictions require closed risers.

Attachment: Glue and screw treads to the stringer notches. Use construction adhesive (PL Premium or similar) and 2-1/2" to 3" screws. The adhesive eliminates squeaks. For finished stairs, fasten from underneath where possible to hide fasteners.

Skirt boards: Trim boards that cover the gap between the stair stringers and the adjacent wall. Typically 1x12 lumber scribed and cut to follow the stair profile.