Stair Stringer Spacing: How Far Apart Stringers Should Be

A stair tread that sags, bounces or cracks underfoot is almost always a spacing problem, not a lumber problem. Stair stringer spacing depends mostly on what the treads are made of. Typical examples: around 12 inches or less for composite and PVC treads, about 16 to 18 inches for 5/4 wood deck boards, and up to roughly 24 to 36 inches for full 2x dimensional lumber treads, depending on species and local guidance. Stair width then tells you how many stringers it takes to stay under that maximum.

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A standard 36-inch-wide deck stair with 2×6 wood treads usually needs three stringers: one at each side and one in the middle. Switch to composite treads and the same stair often needs four. Wider stairs scale up from there. Tread material, stair width and stringer style all feed into the final count, and small framing details decide whether treads stay solid for years.

Stair Stringer Spacing by Tread Material

Treads behave like tiny joists spanning between stringers. Thin, flexible materials need supports closer together, while thick lumber can span farther. The ranges here are common examples. Composite and PVC makers publish their own stair spacing requirements, which are often tighter than for the same board used as flat decking, and those requirements govern.

Why do composite treads need such tight spacing? Composite boards are heavier and less stiff than wood, and stair treads see concentrated point loads from a single foot landing near the nosing. Many composite makers require 12-inch spacing on stairs even when 16 inches is allowed on the deck surface.

How Many Stringers Does Your Stair Need?

Divide the stair width by the maximum spacing for your treads, round up, and add one. That gives you the number of stringers. Measure spacing center to center.

• 36-inch stair, 5/4 wood treads (18-inch max): 36 divided by 18 equals 2, plus 1 equals 3 stringers.
• 36-inch stair, composite treads (12-inch max): 36 divided by 12 equals 3, plus 1 equals 4 stringers.
• 48-inch stair, 2×6 wood treads (24-inch max): 48 divided by 24 equals 2, plus 1 equals 3 stringers.
• 60-inch stair, composite treads: 60 divided by 12 equals 5, plus 1 equals 6 stringers.

Space the stringers evenly across the width. Outer stringers often sit inside a pair of side skirt boards or are trimmed with fascia.

When a middle stringer is needed

Any time the gap between the two outer stringers exceeds the allowable span for the treads, a middle stringer is needed. In practice, nearly every exterior stair 36 inches or wider uses at least one center stringer. Interior stairs also commonly use three stringers for 36-inch widths, especially when treads are 1-inch stock rather than full 2x lumber. Even when two stringers might technically carry the load, a third stiffens the stair and reduces squeaks.

Cut Stringers vs Solid Stringers

Cut (notched) stringers have the rise and run sawn out of a 2×12, leaving a sawtooth profile. After cutting, the remaining material at the throat (the narrowest point) is what carries the load, so many guides call for at least about 5 inches of solid wood at the throat. Cut stringers are also more prone to splitting and rot at the notches if water sits there.

Solid stringers stay uncut and carry treads on cleats or metal tread brackets fastened to their inside faces. They are stronger for a given board size and can sometimes allow longer spans between supports along the stair length. Many builders use solid stringers on the outside for a clean look and cut stringers in the middle.

Long stair runs also need support along their length. A common limit for cut stringers is roughly 6 to 7 feet of horizontal span before a post or intermediate support is required, while solid 2×12 stringers may run a bit farther. Check local guidance for your exact situation.

Tools and Materials

• Pressure-treated 2×12 lumber for stringers, ground-contact rated near grade
• Tread material (wood, composite or PVC) and riser boards if used
• Circular saw for cutting stringers and treads, plus a handsaw or jigsaw to finish notches
• Framing square with stair gauges
• Structural screws, stringer hangers or brackets rated for treated lumber
• Concrete pad, pavers or footings for the stair landing
• Level, tape measure and clamps

A circular saw handles most of the cutting. Stop short of the inside corners of each notch so you do not overcut the throat, and finish the cut with a handsaw. Overcuts weaken the stringer exactly where loads concentrate.

Step by Step: Spacing and Installing Stringers

1. Confirm tread spacing requirements. Check the maximum stair spacing for your tread material and choose the stringer count.
2. Cut the first stringer. Lay out rise and run, cut it, and test fit it against the deck and landing. Use it as a template for the rest.
3. Mark positions on the rim. Mark each stringer center on the deck rim or header so spacing is even and parallel.
4. Attach at the top. Fasten stringers to the rim with rated stringer hangers or brackets, or to a support header, following the hardware maker’s fastener schedule.
5. Support the bottom. Set stringers on a concrete pad or footing, not bare soil. Anchor them so they cannot kick out.
6. Check level and alignment. Each tread seat across all stringers should be level and in the same plane. Shim or plane any stringer that sits high.
7. Add blocking if needed. Blocking between stringers near mid-length keeps tall stringers from twisting.
8. Install treads. Fasten treads to every stringer, keeping consistent overhang (often about 1 inch of nosing) and gaps for drainage.

Trip Hazard and Stair Safety Details

Stairs are where small inconsistencies cause falls. Keep risers uniform. Most codes limit variation between the tallest and shortest riser in a flight to about 3/8 inch, so cut and check carefully. Treads should be level side to side and slightly pitched for drainage only if allowed.

• Use slip-resistant treads or add non-slip strips on exterior stairs, especially those that see rain, ice or algae.
• Install graspable handrails where required, typically on stairs with four or more risers.
• Light the stairs so each nosing is visible at night.
• Keep landing pads flat and sized for the stair width, with no lips or loose edges at the bottom step.
• Secure any outdoor mats so their edges lie flat.

Structural Safety

Stairs carry concentrated loads, and stringer connections are a common failure point. Fasten stringers into solid framing with rated hardware, support the bottom on concrete or proper footings, and add posts for long runs. Call 811 before digging footings for a landing. Stairs attached to elevated decks usually fall under the deck permit, and inspectors check riser height, tread depth, handrails and guards closely. Design for snow loads where they apply.

Troubleshooting

Treads bounce in the middle

Stringers are too far apart for the tread material. Add a stringer between existing ones, or switch to thicker treads.

One tread rocks

One stringer is slightly high or low at that step. Plane or shim the tread seat so all stringers support the tread evenly.

Stringers split at the notches

Overcut notches or water trapped at the cuts are common causes. Seal cut ends, use joist tape on tread seats, and replace split stringers.

Costs and When to Call a Pro

Treated 2×12 stringer stock is moderately priced, and adding one more stringer usually costs far less than replacing sagging treads later. Precut stringers are available in common sizes, but custom-cut stringers fit better. Professional deck stair builds commonly range from a few hundred dollars for short runs to well over a thousand for wide or tall stairs with landings and railings.

Bring in a deck builder or carpenter for stairs taller than about 8 feet, stairs needing an intermediate landing, or any stair where riser heights are hard to keep consistent. They will check stair stringer spacing, throat depth and connections together, which is what keeps a stair safe over time.