Wooden exterior stairs with cut stringers and open treads beside a deck

Reference

How far apart should deck stair stringers be?

The tread decides the spacing. A 2x or 5/4 wood tread can bridge more than a composite board, so wood stairs get away with fewer stringers. The figures below are from the American Wood Council deck guide (DCA 6), which many US building departments use for decks, and from Trex's installation guide.

Stringer spacing: up to 18 in on center for 2x or 5/4 wood treads on cut 2x12 stringers. Trex and most other composite 1x boards need closer stringers — 12 in for most Trex 1 × 6 boards, 9 in for Enhance. A 36 in wide stair needs at least 3 stringers in wood and 4 in Trex.

Deck stair stringer spacing chart: wood and composite

Tread	Maximum stringer spacing	Source
2x or 5/4 wood on cut stringers	18 in · 46 cm	AWC DCA 6, Figure 29
Wood on solid stringers with 2x4 ledgers (tread size per table below)	36 in · 91 cm	AWC DCA 6, Figure 29
Trex Signature, Transcend and Select 1 × 6; Transcend 1 × 4	12 in · 30 cm	Trex 2026 installation guide
Trex Enhance 1 × 6	9 in · 23 cm	Trex 2026 installation guide
Trex Transcend and Select 2 × 6	16 in · 41 cm	Trex 2026 installation guide

Other composite and PVC brands publish their own stair spans, and they can be tighter than for the same board on a deck: Trex, for example, allows 16 in joists but only 12 in stringers for the same 1 × 6. Use the figure printed in your board's installation guide; it overrides the wood numbers above. Trex also asks for treads at least 11 in deep, an overhang (nosing) of no more than 3/4 in, and a 1/4–3/8 in gap between tread boards.

How many stair stringers do I need?

One stringer at each side and one every spacing between them: stair width ÷ spacing, rounded up, plus one. Stairs must be at least 36 in wide, and the AWC guide never allows fewer than 3 cut stringers.

Stair width	18 in o.c.	16 in o.c.	12 in o.c.	9 in o.c.
36 in · 3 ft	3	4	4	5
42 in · 3 ft 6 in	4	4	5	6
48 in · 4 ft	4	4	5	7
60 in · 5 ft	5	5	6	8

Trex states its own minimum, which overrides the 16 in column for Trex 2 × 6 treads: 4 stringers for a 36 in stair and 5 for anything wider, with every tread fastened across at least four of them. If a railing mounts on top of the treads, the stair may need to be wider than 36 in to keep the clear width.

Stair stringer size and span

Rule	AWC DCA 6
Stringer lumber	2x12 minimum
Cut stringer, max span (horizontal)	6'-0"
Wood left below the notches (throat)	5 in minimum
Solid stringer, max span (horizontal)	13'-3"
Intermediate landing required	total rise over 12 ft (IRC R311.7.3 allows 151 in per flight)

A cut stringer longer than 6'-0" in plan needs a 4x4 post on a footing partway down, or a landing, to shorten the span. A 2x12 is the smallest board that keeps 5 in of wood below a 7½ × 10½ in notch; a 2x10 leaves about 3 in. The deck stair calculator works out the number of steps, riser height and stringer length for your rise, and counts the stringers for your width.

Lay out your stairs →

Minimum tread size for wood stairs

Species	On cut stringers (18 in max)	On solid stringers (36 in max)
Southern pine	2x4 or 5/4	2x8
Douglas fir-larch, hem-fir, SPF	2x4 or 5/4	2x8 or 3x4
Redwood, western cedars, ponderosa pine, red pine	2x4 or 5/4	2x10 or 3x4

DCA 6 Table 6, for No. 2 lumber in wet service. Fasten each tread at every stringer with (2) 8d threaded nails or (2) #8 screws at least 2-1/2 in long at each stringer for 2x or 5/4 treads, or (2) 16d threaded nails or (2) #8 screws at least 3-1/2 in long at each stringer for 3x treads. Board sizes and lengths are in the deck board guide.

Figures: American Wood Council, Prescriptive Residential Wood Deck Construction Guide (DCA 6-15, 2018 printing), STAIR REQUIREMENTS, Figures 28 and 29 and Table 6; 2026 Trex Decking Installation Guide, p. 36 "Spanning Requirements for Trex Stairs". IRC 2021 R311.7 for stair width and R311.7.3 for the 151 in maximum rise of a flight. Local codes vary; your building department has the final word.

Good to know

Stair stringer questions

Yes for wood: 2x or 5/4 treads on cut stringers can go up to 18 in apart. For composite, only boards rated for it — Trex Transcend and Select 2 × 6 allow 16 in; their 1 × 6 boards need 12 in (9 in for Enhance).

4 at 16 in on center for wood, 5 at 12 in for composite (Trex asks for at least 5 on any stair over 36 in). A 36 in stair needs 3 in wood.

2x12, with at least 5 in of wood left below the notches. A cut stringer may span 6'-0" horizontally before it needs a post or a landing.

12 in for Signature, Transcend and Select 1 × 6; Transcend 1 × 4; 9 in for Enhance 1 × 6; 16 in for Transcend and Select 2 × 6.

6'-0" for a cut stringer and 13'-3" for a solid stringer, measured horizontally.

The rest of the deck

These tables set how far the frame can reach. Every deck tool on the site works inside them.

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