Stair Stringer Spacing: How Far Apart Can Stringers Be?

If you have ever climbed outdoor steps and felt the treads flex under your feet, you have felt what wrong stringer spacing does. Stringers are the sloped boards that carry every step, and their spacing decides whether a staircase feels rock solid or bouncy after a single season. Get it right and the stairs stay quiet and level for decades. Get it wrong and you get cracked treads, squeaks, and an inspector asking hard questions.

What Stringer Spacing Actually Controls

A stair tread is a small beam. Every footstep bends it slightly across the gap between two stringers. The wider the gap, the more the tread deflects, and that deflection is the bounce you feel. Repeated flexing loosens fasteners, cracks wood, and opens joints where water gets in.

Spacing also decides how much load each stringer carries. Residential stairs must handle a 40 pound per square foot live load under IRC Table R301.5, and that load travels from the tread into the stringers and down to the supports. Closer spacing gives each stringer a smaller share, which matters most for cut stringers, since the saw cuts remove much of the board's strength.

The Short Answer: Spacing Limits by Tread Material

The code does not hand you one stringer spacing number. The real limit comes from the tread material, because the tread is what spans between the stringers. These are the maximums builders commonly work to:

• 5/4 deck boards as treads: 16 inches on center. Deck boards are thin and will cup and bounce on wider spans.
• 2x6 or 2x8 treads (two boards per step): 24 inches on center. This is the most typical deck stair setup.
• Full 2x10 or 2x12 treads (one board per step): up to 36 inches on center is sometimes used, but 24 inches on center is the safer choice most inspectors prefer.
• Cut (sawn) stringers: keep to 16 inches on center when you can. The saw cuts weaken the board, so closer support compensates.
• Stone, tile, or concrete treads: follow the manufacturer's span tables or an engineer's direction. Brittle materials do not forgive wide spacing.

These are maximums, not targets. Closer is always stiffer. If you are still working out rise and run, run your numbers through a stringer calculator so the layout and the spacing come from the same dimensions.

What the Code Says (and Does Not Say)

Knowing where the code speaks and where it stays silent keeps you out of trouble at inspection.

Geometry is covered by IRC Section R311.7. Stairways must be at least 36 inches wide in clear width (R311.7.1). Risers are limited to 7-3/4 inches maximum and treads must be at least 10 inches deep (R311.7.5.1 and R311.7.5.2). Within any flight, the largest and smallest riser cannot differ by more than 3/8 inch, and the same uniformity rule applies to tread depths. Nosing projection, where required, must fall between 3/4 inch and 1-1/4 inches (R311.7.5.3).

Structural load is covered by IRC Table R301.5, which assigns stairs a 40 psf live load. Your stringer size, spacing, and connections must be adequate for that load, but the code never states the spacing that achieves it. That judgment comes from span tables, manufacturer data, and accepted trade practice.

In practice: the inspector measures risers, treads, and width against R311.7 and judges the framing against the 40 psf requirement. If your spacing looks wide for the tread material, expect questions. When in doubt, add the extra stringer.

Cut Stringers vs. Solid Stringers

The two stringer types behave very differently, and spacing rules treat them accordingly.

Cut (sawn) stringers have the familiar zigzag profile sawn from a 2x10 or 2x12. Each notch removes wood where bending stress is highest, leaving a thin "throat" of solid lumber along the bottom edge. That is why cut stringers want tighter spacing, 16 inches on center is the widely used rule of thumb, and why they should be cut from 2x12 stock whenever possible. For the full marking procedure, see the guide on how to lay out and cut stair stringers with a framing square.

Solid (housed) stringers are full-depth boards with treads housed into routed grooves or carried on cleats, so nothing is sawn away. They are far stronger for their size and can follow the tread material limits above without the extra penalty cut stringers carry. They take more labor, which is why you see them mostly on interior stairs and higher-end work.

How to Lay Out Stringer Spacing, Step by Step

Do this on paper before you cut a single board.

Step 1: Measure the finished stair width. Outside to outside of the treads. A typical deck stair is 48 inches.

Step 2: Pick your maximum spacing from the tread material. With two 2x6 boards per tread, the maximum is 24 inches on center.

Step 3: Divide the width into equal spaces. Divide the width by the maximum spacing and round up to get the number of intervals: 48 / 24 = 2 intervals. Stringers = intervals + 1, so 3 stringers, spaced at 48 / 2 = 24 inches on center.

Step 4: Check the result. The outer stringers sit at or just inside the tread ends. If the division gives a fraction, round the interval count up and recompute even spacing. Example: a 36-inch stair with the same treads gives 36 / 24 = 1.5, round up to 2 intervals, 3 stringers at 18 inches on center. Tighter than the maximum is always fine.

Step 5: Add one more for long or busy stairs. More than about 12 risers, or heavy daily traffic, justifies an intermediate stringer. It is cheap compared to rebuilding bouncy stairs.

Step 6: Fasten every tread to every stringer. Use at least two fasteners per crossing and keep on-center spacing consistent within 1/4 inch. Perfect spacing loses its stiffness if treads are only fastened at the ends.

Common Spacing Mistakes

• Two stringers on a wide stair. Fine for narrow utility steps, but on a 36-inch or wider stair the tread ends go unsupported. Use three or more.
• Wide spacing with 5/4 decking. The most common cause of bouncy deck stairs. With 5/4 treads, 16 inches on center is the limit.
• Over-cutting the throat. Notches sawn too deep leave a thin strip of wood carrying the whole load. Cut from 2x12 stock.
• No mid-span support on long runs. Very long stringers can need a bearing support partway down. Spacing alone does not fix excessive span.
• Uneven spacing. Stringers at 16, 19, and 22 inches on center load treads unevenly. Even spacing is stronger and easier to inspect.

Frequently Asked Questions

How far apart can stair stringers be at most?

As practical maximums: 16 inches on center for 5/4 deck-board treads, 24 inches on center for 2x lumber treads, and up to 36 inches on center only with full-depth 2x10 or 2x12 treads. Keep cut stringers at 16 inches on center. These are trade-practice limits, not a single code number, so confirm with your local building official at the maximums.

How many stringers does a 36-inch-wide stair need?

Three stringers at 18 inches on center is standard and suits any common tread material. With cut stringers or 5/4 decking, consider four at 12 inches on center for extra stiffness.

How many stringers does a 48-inch-wide stair need?

Three at 24 inches on center is typical with 2x treads. With 5/4 decking, use four at 16 inches on center, which is also the premium choice for any heavily used 48-inch stair.

Does the building code specify stringer spacing?

No. The IRC sets stair geometry in Section R311.7 and the 40 psf live load in Table R301.5, but gives no direct spacing dimension. Spacing is governed indirectly: the layout must safely carry the required load with the materials used.

Can thicker treads span wider stringer spacing?

Yes, within reason. Thicker treads span farther, which is why full 2x12 treads can go wider than 5/4 deck boards. But thicker treads do not fix a weak cut stringer, and most inspectors still like to see 24 inches on center or less on residential work.

Should outer stringers sit flush with the tread ends?

On deck stairs with cut stringers, the outer stringers sit flush with or just inside the tread ends. On housed-stringer stairs they are often inset 1 to 2 inches for appearance. Either way, keep the on-center spacing even across the full width.

The Bottom Line

Stringer spacing is a judgment call driven by tread material and stringer type, not a code lookup. Memorize the two numbers that cover most residential work: 16 inches on center for 5/4 decking and cut stringers, 24 inches on center for 2x treads. Lay the positions out evenly, fasten every tread to every stringer, and when a stair feels borderline, add the extra stringer. Lumber is cheap. Rebuilding stairs is not.