How to Build Wood Stairs: Stringer Layout, Rise & Run Calculations, Code Requirements

Understanding Stair Anatomy

Every staircase consists of the same basic components: stringers (the diagonal structural members that support the steps), treads (the horizontal boards you step on), risers (the vertical boards between treads), and a handrail assembly. Understanding these components and how they work together is essential before cutting any lumber. The stringer is the most critical element — it must be precisely laid out and cut to produce uniform steps that meet building code requirements.

IRC Code Requirements

The International Residential Code (IRC) sets strict dimensions for stairs. Maximum riser height: 7-3/4 inches (most builders target 7 to 7-1/2 inches for comfort). Minimum tread depth: 10 inches (measured from the nose of one tread to the nose of the next). Minimum stair width: 36 inches clear. Maximum variation between any two risers or treads: 3/8 inch (this is critical — uneven steps are a tripping hazard and will fail inspection). Minimum headroom: 6 feet 8 inches measured vertically from the stair nosing to any overhead obstruction. Handrails are required on at least one side for stairs with 4 or more risers, at 34 to 38 inches above the stair nosing.

Rise and Run Calculations

Measure the total rise (the vertical distance from the finished floor at the bottom to the finished floor at the top). Divide by your target riser height (7.5 inches is ideal for comfort) to get the number of steps. Round to the nearest whole number. Divide the total rise by the number of steps to get the exact riser height. Example: total rise of 108 inches divided by 7.5 equals 14.4, round to 14 steps, so 108 divided by 14 equals 7.714 inches per riser. Each tread will be a standard 10-1/4 to 11-1/4 inches deep (a standard 2x12 works perfectly for treads, providing 11-1/4 inches of depth). The total run (horizontal distance) equals the number of treads times the tread depth. With 14 risers you have 13 treads (the top floor acts as the final tread), so 13 times 10.25 inches equals 133.25 inches or about 11 feet 1 inch of horizontal run.

Stringer Layout

Use a framing square with stair gauges (small brass clamps that attach to the square). Set one gauge at your riser height on the tongue (short side) and one at your tread depth on the blade (long side). Starting at one end of a 2x12, align the gauges with the top edge of the board and trace the first step. Slide the square along and trace each subsequent step, making sure the gauges stay tight against the board edge. You'll trace the number of risers you calculated. Cut the stringer with a circular saw, stopping just short of the inside corners, then finish with a jigsaw or handsaw to prevent overcutting (overcutting weakens the stringer). The minimum remaining throat depth (the narrowest point of the stringer after cutting) must be at least 3-1/2 inches per code.

For exterior deck stairs, use pressure-treated 2x12s for stringers. For interior stairs, Douglas fir or SPF 2x12s are standard. Maximum stringer spacing is 16 inches on center for 2x treads, which means a 36-inch-wide staircase needs 3 stringers (one on each side plus one in the center). Wider stairs (48 inches) need 4 stringers.

Tread and Riser Materials

Exterior deck stairs: 5/4x6 composite or pressure-treated decking (two boards per tread, or use purpose-made stair treads). Risers are often left open on deck stairs. Interior stairs: 1-inch-thick hardwood treads (oak is the standard — strong, durable, beautiful grain) in 11-1/4-inch depth, available as pre-made bullnose stair treads from lumber yards. Risers are typically 3/4-inch plywood, MDF, or solid wood, painted white. For a more finished look, use return treads (the tread wraps around the open side of the stringer) available pre-made from stair parts suppliers.

Handrail Assembly

A complete handrail system includes the rail itself (graspable profile, 1-1/4 to 2 inches in cross-section diameter), balusters (vertical spindles, spaced so a 4-inch sphere cannot pass through — typically 4 to 5 inches on center), newel posts (the structural posts at the top and bottom of the rail), and fittings (goosenecks, volutes, turnouts for transitions). For deck stairs, use 2x4 or 2x6 rails with 2x2 balusters. For interior stairs, pre-made rail profiles and turned balusters are available from stair parts distributors in oak, maple, poplar, and other species.