How Many Stair Stringers Do I Need? (2026 Guide)

The two rules behind every count

A stringer is the notched board running along a staircase's slope that carries the treads and risers; each one supports only the band of tread directly over it. Two rules set the count:

• Both outer edges always get a stringer — the side stringers carry the ends of every tread; skip one and the edge has no support.
• Add middle stringers so no gap runs past 16–18 in — when the span between neighboring stringers reaches about 16–18 in, add a center stringer; add another for every ~16 in more.

That makes two edge stringers alone fit stairs up to roughly 24 in, with 30–36 in stairs taking 3 total — and size, spacing and materials remain structural calls to confirm with your local building code.

Stringers for common stair widths

Builders often phrase the 16-in spacing backwards as "one stringer per 16-in segment of width" — a floor, not a precise formula: 36 ÷ 16 ≈ 2.25 alone wouldn't say 3, because the two edge stringers are mandatory first. Count it as "two edges + one more middle stringer per ~16 in beyond about 24 in," tighten spacing toward 16–18 in for open-riser, composite or heavily used stairs, and follow any maximum your local building code sets.

Worked example: a 36-inch deck stair

1. Measure the clear width

Walking width between the two outer support points: 36 in.

2. Put a stringer on each outer edge

Both edges need a stringer to carry the tread ends: 2 so far.

3. Check the middle span and add 1–2 center stringers

The band between the edges runs nearly the full 36 in — past the 16–18 in limit — so add a center stringer (some framers add two): 2 outer + 1–2 middle = 3–4 stringers. In practice 3, two sides plus one center, is the standard answer; pick 4 for open risers, composite treads or constant traffic. The stair stringer calculator handles the layout.

What each stringer is cut from

Each stringer is one board — almost always a 2×12 (or the engineered equivalent your plan specifies), since the notches need the full board depth. Order one per stringer, plus a spare.

Rough length check when buying: horizontal run ≈ steps × tread run — 13 steps at 11 in per tread projects ≈ 143 in ≈ 12 ft. The stringer runs longer (it climbs the slope), and notches depend on your exact rise and run — treat this as a shopping guide, not a precise cut list.

Stringer tips that keep stairs solid

• Use weather-resistant stock outside — exterior stringers see rain and ground splash: choose pressure-treated or naturally rot-resistant lumber such as cedar or redwood.
• Stagger any splices — if one board won't span the run, splice so joints never line up across the stair.
• Keep tightening past 36 in — every extra ~16 in of width takes another center stringer: 42–48 in stairs run 4–5 total.

Building the whole wood structure? Count how many deck boards you need for the landing and floor, and plan the roof with how many rafters you need.

Calculate your exact stringer layout →