Stair Stringer Throat Depth: The Hidden Structural Rule

When you look at a finished staircase, you see treads, risers, and railings. What you do not see is the thin strip of solid wood that carries the entire load: the throat of the cut stringer. On every sawtooth stringer, the tread and riser cuts remove most of the lumber, leaving a narrow bridge of wood at the inside corner of each notch. That bridge is called the throat. It is the smallest cross-section in the stair, and it decides how much weight the stair can safely carry. This guide explains what throat depth is, the code rules that govern it, how to calculate it before you cut, and how to fix a stringer that falls short.

What Exactly Is Throat Depth on a Cut Stringer?

A cut stringer (also called a sawtooth or open stringer) is a board, usually a 2x10 or 2x12, with tread cuts and riser cuts notched into its top edge. Throat depth is the shortest distance from the inside corner of one of those notches down to the bottom edge of the board, measured perpendicular to the bottom edge, not straight down the plumb line. The perpendicular measurement is the smaller, truer number.

On a 2x12 laid out for a comfortable stair with 7-inch risers and 11-inch treads, only about 5.3 inches of solid wood remain at the narrowest point of each notch. Every pound on the stair funnels through that narrow bridge on its way to the supports, which is why experienced builders check the throat before they cut, not after.

Why the Throat Controls the Whole Stair's Strength

Wood is strong along its grain, but a notch interrupts those fibers and creates a sharp inside corner where cracking forces concentrate. Each tread acts like a small lever, and the bending load piles up at that corner. If the throat is generous, the load spreads through enough solid wood. If it is thin, the corner becomes the starting point for a split along the grain.

Residential stairs are designed for a 40-pound-per-square-foot live load under IRC Table R301.5, plus the dead weight of the stair itself. The throat is the one place with the least material resisting that load, and a thin throat flexes more, which is why a bouncy stair is often a thin-throated stair: the fix is usually stiffening the stringers, not the treads.

Code Rules: The 3.5-Inch Minimum and IRC R502.6

The number every stair builder should memorize is 3.5 inches: a cut stringer must retain at least 3.5 inches of solid wood at the throat. Some jurisdictions accept 3 inches, but 3.5 is the safer target. Confirm the exact requirement with your local building official before cutting.

The code logic comes from the notching rules for structural lumber. IRC Section R502.6 limits notches in joists, rafters, and beams to no more than one-sixth of the member's depth, and many inspectors apply the same thinking to stringers. A 2x12 is 11.25 inches deep, so even an aggressive layout leaves a 5-inch-plus throat, while a 2x10 cut for a steep stair can land near the limit. A steeper stair drives the notch corners deeper and shrinks the throat, so always run the math for your actual rise and run. Our guide to laying out and cutting a stair stringer with a framing square covers the marking process step by step.

How to Calculate Throat Depth Before You Cut

With the stock width, the unit rise, and the unit run, the answer falls out of simple trigonometry. The stair angle is the arctangent of rise divided by run:

Throat = stock width minus (unit run times sin of the stair angle), which equals stock width minus (unit rise times cos of the stair angle).

Both forms give the same number. Here are three worked examples using actual lumber dimensions (a 2x10 is 9.25 inches deep, a 2x12 is 11.25 inches deep).

Example 1, a comfortable stair: 2x12 stock, 7-inch rise, 11-inch run (stair angle about 32.5 degrees). Throat = 11.25 - (11 x sin 32.5°) = 5.34 inches. Comfortably above the minimum.

Example 2, a steep basement stair: 2x10 stock, 7.75-inch rise, 10-inch run (angle about 37.8 degrees). Throat = 9.25 - (10 x sin 37.8°) = 3.12 inches, which fails the 3.5-inch standard. Recut the same layout from a 2x12 and the throat becomes 5.12 inches. The steeper the stair, the wider the stock needs to be.

Example 3, a steep 8/9 layout: 2x12 stock, 8-inch rise, 9-inch run (angle about 41.6 degrees). Throat = 11.25 - (9 x sin 41.6°) = 5.27 inches. It passes, but a steep pitch leaves far less margin than Example 1.

Run this check for the steepest flight on the job, because that is where the throat is thinnest. To test different rise and run combinations, use the stair stringer calculator.

Mistakes That Shrink Throat Depth

Cutting a steep stair from 2x10 stock. A steep pitch eats into the throat fast. When the rise approaches the code maximum, step up to a 2x12 as a habit.

Overcutting notch corners with a circular saw. The blade naturally cuts past the corner where the tread and riser lines meet. Cut to the line, then finish each corner with a handsaw or jigsaw.

Measuring down the plumb line instead of perpendicular. The plumb measurement reads longer than the true throat and can make a failing stringer look like it passes. Measure the shortest distance, square to the bottom edge.

Ignoring knots at the notch corners. A knot at the inside corner of a notch is a built-in weak point in the worst place. Keep the notch zone clear of large knots.

Ripping the stringer narrower to gain headroom. Trimming the bottom edge removes throat depth inch for inch. Reframe the opening instead.

Spacing stringers too far apart on a marginal throat. Wide spacing is fine with a generous throat, but a thin throat under a heavily loaded span invites a crack. Our guide to stair stringer spacing covers safe spacing for different loads.

What to Do If Your Throat Is Too Small

If the stringer is already cut and the throat measures under 3.5 inches, do not install it and hope. The cheapest fix is to recut the layout on a wider board. Otherwise, sister a second full-length 2x board alongside the weak stringer, glued and screwed or bolted: it effectively doubles the wood at the throat. Adding intermediate support, such as a knee wall or post under midspan, shortens the span and cuts bending stress.

Cut Stringers vs Solid Stringers: Where the Rule Applies

The 3.5-inch throat rule applies specifically to cut (sawtooth) stringers, where material is removed from the board. A solid stringer, sometimes called a housed or closed stringer, is routed with grooves that accept the treads and risers, so the board keeps its full structural depth and there is no throat to check. Solid stringers are the stronger choice for wide, heavily loaded, or long-span stairs, at the cost of more labor and fussier joinery.

Frequently Asked Questions

What is the minimum throat depth for a stair stringer?

At least 3.5 inches of solid wood, measured perpendicular from the inside corner of the notch to the bottom edge of the stringer. Some jurisdictions accept 3 inches, so confirm with your local building official.

How do you measure throat depth?

Find the inside corner where a tread cut meets a riser cut, then measure the shortest distance from that corner to the bottom edge of the stringer, keeping the tape square to the bottom edge.

Does throat depth matter for solid (housed) stringers?

No. Solid stringers are grooved rather than notched, so the board keeps its full depth and there is no throat. The rule exists only for cut, sawtooth-style stringers.

Can you fix a stringer with too little throat without replacing it?

Often, yes. Sistering a full-length board alongside the weak stringer, adding a midspan support such as a knee wall, or both, are standard repairs. If the installed stringer is already cracking, replacement is safer.

What lumber size should I use for a cut stringer?

Use 2x12 lumber for most stairs, especially steep ones, because the extra depth protects the throat. A 2x10 can work for gentle pitches, but run the throat calculation for your actual rise and run before cutting.

Does the building code mention throat depth directly?

The IRC does not use the word "throat," but inspectors apply the notching limits of R502.6 and the stair geometry rules of R311.7, and the 3.5-inch throat minimum is the standard that satisfies both.

Throat depth is the kind of detail that separates a stair that lasts a century from one that starts talking within a decade. It takes five minutes with a calculator to check and costs nothing to get right on the first cut. Run the math against the 3.5-inch minimum, pick stock wide enough to clear it with margin, and cut your corners clean.