2x10 vs 2x12 Stair Stringers: Which Lumber Size Do You Actually Need?

The stringer carries every step, and once it is cut, the wood left below each notch is all that holds the stair together. That is why many builders default to 2x12 lumber. The right choice depends on riser height, span, tread material, and how the stringer is cut.

A 2x10 is not 10 inches deep, and a 2x12 is not 12 inches deep. Those are nominal sizes, the names lumber is sold under. The actual dressed size of a 2x10 is 1.5 inches thick by 9.25 inches deep, and a 2x12 is 1.5 inches thick by 11.25 inches deep. The difference is exactly 2 inches of depth, and on a cut stringer those 2 inches are the margin between plenty of solid wood below the notches and a thin strip.

What a Stringer Actually Carries

A cut stringer, sometimes called a sawtooth stringer, is a board with right-angle notches cut into its top edge. Each notch forms one riser and one tread. Load travels through the tread into the stringer, then down to its supports. The weakest point is usually the inside corner of a notch, where the remaining wood, called the throat, is thinnest. If the throat is too thin, the stringer can split along the grain, especially at the middle steps.

A closed stringer is different: treads sit in housings routed into the board, or on cleats fixed to it, so it is not notched through and keeps more of its depth. Before you size anything, fix your rise and run. Under IRC R311.7.5, the maximum riser is 7.75 inches, the minimum tread depth is 10 inches, and variation across a flight must not exceed 3/8 inch. A taller riser cuts deeper and leaves less throat, so a stair at the code maximum is exactly where a 2x10 is most likely to run short.

The Throat Depth Rule, in Plain Terms

The throat is the solid wood left below the deepest notch, measured to the bottom edge of the stringer. Carpenters commonly aim for at least 3.5 inches of throat on a residential cut stringer. That is a framing rule of thumb, not a single line in the IRC, so local code has the final word, but it explains the whole debate.

Take a normal stair with a 7.5 inch riser. Cut that notch into a 2x10, which is 9.25 inches deep, and very little solid wood remains. Cut the same notch into a 2x12, which is 11.25 inches deep, and you gain a full 2 inches, keeping the throat comfortable and leaving room for a small cutting error. For a low stair the logic reverses: with a 6 inch riser, a 2x10 leaves considerably more throat and may be fine. The question is never just board size. It is board size minus cut depth, set by your riser.

A Worked Example: An 8-Foot Total Rise

Suppose total rise is 96 inches, from one finished floor to the next. Divide by a target riser near 7.5 inches and you get 12.8 risers, so round up to 13, giving an actual riser of about 7.38 inches. With a 10.5 inch tread, the angle is roughly 35 degrees, and total run is 12 treads times 10.5 inches, or 126 inches.

Now test the boards. A 2x10 gives 9.25 inches of depth to absorb a 7.38 inch riser cut plus the angled bottom edge, leaving a thin throat well below the 3.5 inch target. A 2x12 gives 11.25 inches for the same cut, so the throat stays sound, with margin to spare. On this ordinary 8-foot stair, the 2x12 is the right call. With a 36 inch total rise and 6 risers of 6 inches each, the same 2x10 leaves a much healthier throat. Run your own numbers first with a stair calculator, then test the board against that riser, not a guess.

Span, Spacing, and Bounce

Depth is not only about the throat. A deeper board is stiffer, so a stair on 2x12 stringers feels more solid and is less likely to bounce or squeak as wood dries. That matters most on long, unsupported flights of 12 feet or more. On a wide stair, add a center stringer rather than asking two outside stringers to carry a long tread span alone. Keep spacing at 36 inches or less on center for typical residential treads, and closer for thin treads, stone treads, or heavy traffic. A stringer must also bear fully on a solid header, landing, or footing at both ends.

When a 2x10 Is Enough, and When to Use a 2x12

A 2x10 is legitimate in a few clear cases. Short flights with low risers are the first: deck steps with three or four steps, a porch step, or a garden stair with a riser of 6 inches or less and a short span will usually be fine, provided throat and supports are sound. Closed stringers are the second case, since the board is not notched through. The trap is using a 2x10 because it is cheaper or already in the garage, then cutting a full-height stair from it. The saving is often only a few dollars per board, while replacing a cracked stringer after the stair is finished is expensive.

Use a 2x12 for a full flight between floors, for risers near the 7.75 inch code maximum, for long spans without intermediate support, for wide stairs, and for heavy treads such as stone, thick hardwood, or tile over a backer. Use it too for heavy traffic, weaker species or lower grades, and outdoors, where moisture can weaken a thin throat. If neither board works, change the structure: a very long flight may need engineered lumber, a steel stringer, or a landing that breaks the span.

Lumber Choice, Grade, and Cutting Tips

Species and grade matter as much as size. A straight, dense board with no large knots at a notch will outperform a wider board with a knot at the weakest point. Reject any board with a large knot, split, or heavy twist where a notch will fall. Outdoors, use lumber rated for ground contact where it touches soil or concrete, and keep the stringer bottom off bare soil with a proper footing or post base.

Cut carefully: a 2x12 only helps if the throat you cut matches the throat you planned. Lay out with a framing square and stair gauge, mark every riser and tread before cutting, and do not overcut the inside corner. Finish each notch with a handsaw or jigsaw, test-fit the first stringer before cutting the rest, and check riser uniformity. For the full layout method, see our guide on how to calculate stair stringer length.

Frequently Asked Questions

Can I use a 2x10 for a standard indoor staircase?

Usually not if risers are near 7.5 inches. A standard indoor flight leaves too little throat on a 2x10, so a 2x12 is safer. A 2x10 can work on a short, low-rise stair, but check your riser height first.

What is the minimum throat depth for a cut stringer?

Many carpenters aim for at least 3.5 inches of solid wood below the deepest notch on a residential stair. Check local code, since the IRC sets riser, tread, and uniformity limits rather than a single throat figure.

Does a 2x12 cost much more than a 2x10?

The gap is usually modest, a few dollars per board depending on species, length, and region. Against the cost of replacing a failed stringer, the upgrade is one of the cheapest strength improvements in the project.

How far apart can stair stringers be spaced?

Keep spacing at 36 inches or less on center for typical residential stairs, and add a center stringer on wide stairs or where treads are thin or heavy.

Is a solid stringer stronger than a cut stringer?

A closed, solid stringer keeps more of its board depth because it is not notched through, so it is generally stronger at the tread connection. A cut stringer is easier to build and common for decks, but it depends on the throat.

Should I use engineered lumber for a long staircase?

Consider it when a 2x12 still leaves a thin throat or the span is long and unsupported. Engineered and steel stringers span farther and stay straighter, but need proper connections and sometimes an engineer's review.

The Bottom Line

For a normal flight between floors, choose the 2x12. It costs little extra, keeps a sound throat under a standard riser, and spans farther without bounce. Save the 2x10 for short, low-rise steps and closed stringers where the throat math clearly works. Size the board from your actual riser height, check throat and span, and support both ends properly.