Laying Out Stairs: Stringers, Treads, Risers, and Nosing

Why Stair Layout Matters

A stairway that is built even slightly out of proportion is uncomfortable to climb, creates a tripping hazard, and may violate the building code. The key to a safe, code-compliant stair is accurate layout on the stringer before any saw touches the wood. In this lab you will lay out a closed stringer — one that carries both treads and risers and is notched to receive them — using a framing square and stair gauges.

The Vocabulary of a Stair

Before you touch a square, know what you are measuring. Total rise is the vertical distance from finish floor to finish floor. Total run is the horizontal distance the stair travels. Unit rise is the height of one riser; unit run is the horizontal depth of one tread, not counting the nosing. The nosing is the rounded or beveled overhang of the tread that extends past the riser face, typically about 1-1/4 inches for residential work — it increases tread depth for comfort without increasing total run. A stringer (also called a stair horse or carriage) is the diagonal structural member that supports the treads and risers. A closed stringer is notched so treads and risers sit in the notches; the outside face of the stringer still shows the full board profile.

Calculating Unit Rise and Unit Run

Start with the total rise in inches. Divide by the number of risers to find unit rise. A common target is 7 inches; the general residential guideline is a maximum of about 7-3/4 inches per riser and a minimum of about 4 inches, though you must always verify the applicable code for the project.

A useful rule of thumb: unit rise + unit run should equal roughly 17 to 18 inches. Once you choose your unit rise, subtract it from 17 or 18 to get a starting unit run, then verify the total run fits your floor plan. Remember: the number of treads is always one less than the number of risers because the upper floor acts as the top landing.

Example outline (not the worked example below): if total rise = 105 inches and you try 15 risers, unit rise = 7 inches. Using the 17-inch rule, unit run = 10 inches. Total run = 14 treads × 10 inches = 140 inches. Always round unit rise to the nearest 1/16 inch if the division does not come out even, then check that all risers still add up exactly to the total rise.

Setting Up the Framing Square

A stair framing square is an L-shaped steel tool with a body (the long leg, 24 inches) and a tongue (the short leg, 16 inches). Stair gauges are small brass clamp-on buttons that lock onto the square's edges at the exact unit-rise and unit-run dimensions so you can repeat the same position at every step without re-measuring.

1. Set one gauge on the tongue at the unit rise measurement.

2. Set the other gauge on the body at the unit run measurement.

3. Lay the 2×12 stringer stock flat on sawhorses with the crown (slight bow) facing up.

4. Mark the plumb cut at the top first: hold the square so both gauges touch the top edge of the board; draw along the tongue — this is your top plumb cut.

5. Step the square: slide it along the board so the unit-run gauge aligns with the mark you just made. Draw the next unit-rise line and unit-run line. This tread-riser intersection is your first step notch.

6. Continue stepping until you have marked every tread and riser. The last mark at the bottom gives you the bottom plumb cut, which defines how the stringer rests on the floor.

The Tread-Thickness Reduction (Bottom Riser)

Because treads add thickness at every step, the bottom riser is shorter than all others by the thickness of one tread board. If tread stock is 1-1/2 inches thick, cut 1-1/2 inches off the bottom of the stringer at the floor cut. This keeps every visible riser height equal once treads are installed.

Nosing

The nosing is added to the tread after the stringer is cut. It does not change the unit-run layout on the stringer; it simply extends the tread surface. Mark the nosing dimension on your calculation form for reference during tread installation.

Dimensional Tolerance

Your marks must be within 1/16 inch of calculated positions. Use a sharp pencil (not a crayon or marker) for accuracy. Verify each interval with a tape and combination square before moving on.

Safety During This Lab

• Eye protection must be worn whenever lumber is being moved or marked on a workbench shared with others.
• Keep the stringer stock fully supported on sawhorses — a tipping 2×12 can cause a pinch injury.
• Framing squares and stair gauges have sharp corners; handle them with care and store them flat so they do not become trip hazards.
• No cutting occurs in this lab. All layout is pencil-only. If an instructor demonstration involves a saw, stand clear and follow the lockout/tagout and blade-guard rules reviewed in your safety module.
• Call out "board moving" before shifting stringer stock so partners can step back.

Checking Your Work

When layout is complete, your partner measures every tread interval (horizontal) and every riser interval (vertical) with a tape and records the values on the check sheet. Compare each to your calculated unit rise and unit run. Any interval outside 1/16 inch must be corrected by re-marking — not by erasing and guessing. Recalculate and re-step that section of the square. Submit the stringer and the completed check sheet together.