Stringer Spacing: How Many Stringers Does Your Stair Need?

A stair 36 in / 914 mm wide — the IRC code minimum — needs a minimum of 3 stringers when framed with cut 2x12s. That number comes directly from the American Wood Council’s DCA6 prescriptive limit: cut stringers can’t be spaced more than 18 in / 457 mm on center, and 36 in of width divided by 18 in leaves exactly two spaces, which always resolves to three stringers. Wider stairs, thinner tread material, or composite decking all push that number up.

The Stringer Spacing Formula

The math is simple once you know the governing limit. Take the stair’s total width, divide by the maximum on-center spacing for your stringer type and tread material, round the result up to the next whole number of spaces, then add one to convert spaces into a stringer count.

For standard cut 2×12 stringers under solid wood treads, that maximum spacing is 18 in / 457 mm. For thinner composite or PVC decking, manufacturers frequently tighten that to 12–16 in / 305–406 mm on center because the thinner board flexes more between supports — always check the deck board manufacturer’s span rating rather than assuming lumber spacing applies.

Worked example: a 42 in / 1067 mm wide interior stair using standard 2×12 cut stringers under 1x hardwood treads. 42 ÷ 18 = 2.33, rounded up to 3 spaces, plus one gives 4 stringers. Run your own width and tread material through the Stringer Spacing Calculator to get an exact count and center-to-center layout before you cut lumber.

Stringer Counts by Common Stair Widths

The table below covers the widths that come up most often in residential and light-commercial work, using the 18 in maximum spacing for standard cut lumber stringers under solid wood treads.

Stair Width	Spaces (÷18 in, rounded up)	Stringers Required	Notes
36 in / 914 mm	2	3	IRC code-minimum width
42 in / 1067 mm	3	4	Common interior main stair
48 in / 1219 mm	3	4	Wide interior or entry stair
60 in / 1524 mm	4	5	Grand or dual-handrail stair
36 in, solid stringers, ≤13 ft span	1	2	Solid uncut stringers only — DCA6 exception

That last row is the exception, not the rule: the two-stringer allowance only applies to solid, uncut stringers on a 36 in wide stair spanning within DCA6’s solid-stringer limit — 16 ft 6 in for southern pine, 13 ft 3 in for most other species. Any notched, cut-profile stringer at any width still follows the 18 in spacing rule, and any width above 36 in needs a third stringer regardless of stringer type.

Common Mistakes

Assuming two stringers is always enough. The two-stringer detail gets copied from one project to the next without checking whether the new stair is still 36 in wide and still using solid, uncut stringers. Widen the stair to 42 in or notch the stringers for treads, and two is no longer code-compliant. The fix is to recheck the width and stringer type every time, not to reuse a prior layout.

Ignoring tread material stiffness. Lumber spacing rules get applied to composite or PVC tread stock without checking the manufacturer’s own span rating, which is frequently tighter than 18 in. The result is a tread that deflects visibly underfoot even though the framing technically meets the lumber-based spacing rule. Always cross-check the decking manufacturer’s span table against the framing spacing, and use whichever is more restrictive.

Spacing stringers unevenly. Stringers get placed to match convenient joist locations on the structure below rather than evenly across the stair width, leaving one bay wider than the calculated maximum. An 18 in average spacing with one 24 in gap does not satisfy an 18 in maximum — the largest single gap is what code and the tread material actually see. Lay out stringers to an even spacing first, then adjust the supporting structure to match.

Landing intermediate stringers on unsupported blocking. An added center or intermediate stringer needs to bear on something continuous — a beam, ledger, or footing — not on a piece of blocking nailed between joists with no load path to the ground. A stringer that’s correctly spaced but poorly supported at either end still fails the same way an absent stringer would.

Related Calculators You Might Need

Once the stringer count is set, the next question is usually what those stringers land on. The Stair Beam Span Calculator sizes the beam or ledger carrying the full stringer load, and the Stair Footing Size Calculator sizes what that beam sits on at grade. If you’re estimating lumber for the stringers themselves, the 2×12 Stringer Board Calculator converts your stringer count and rise into a board count and cut list. For anything beyond standard residential foot traffic, confirm the finished frame against the Stair Load/Weight Capacity Calculator, and run the final layout through the IRC Stair Code Checker before you finalize the cut list.

Frequently Asked Questions

How many stringers do I need for a 4 foot wide stair? A 48 in / 1219 mm wide stair using standard cut 2×12 stringers at the 18 in maximum spacing needs 4 stringers: 48 ÷ 18 = 2.67, which rounds up to 3 spaces, plus one. If the stair uses composite or PVC treads, check the decking manufacturer’s span rating first — it may require an additional stringer even at this width.

What is the maximum spacing between stair stringers? For cut, notched 2×12 stringers under standard wood treads, the DCA6 prescriptive limit is 18 in / 457 mm on center, adopted by most jurisdictions since the IRC itself doesn’t publish a separate stringer span table. Thinner or composite tread material often requires tighter spacing — check the tread manufacturer’s rating against this maximum and use whichever number is smaller.

Can you build a wide staircase with only two stringers? Only if the stair is 36 in / 914 mm wide or narrower and the stringers are solid and uncut, spanning within the DCA6 limit of 16 ft 6 in for southern pine or 13 ft 3 in for most other species. Any stair wider than 36 in, or using notched cut stringers at any width, needs a minimum of three stringers regardless of span length.

Do composite deck stairs need more stringers than wood stairs? Often yes. Composite and PVC decking typically deflects more than solid lumber at the same span, so manufacturers frequently rate their boards for 12–16 in on-center support rather than the 18 in lumber maximum. Check the specific product’s span table — using lumber spacing rules with composite treads is one of the most common stair-building errors on deck stairs. Verify with the Stringer Spacing Calculator using your actual tread material.

Do stringers need to be evenly spaced? Yes — the maximum on-center spacing rule applies to the largest single gap between any two adjacent stringers, not an average across the stair width. Spacing stringers unevenly to match existing framing below can leave one bay exceeding the maximum even if the overall count looks correct on paper.