Stair Stringers

A stair stringer is the diagonal structural member that carries the whole staircase. Stringers are the diagonal structural members that support stair treads and risers. Notch them wrong, undersize the lumber, or skip the bottom-drop cut and you’ve got stairs that bounce, fail inspection, or trip somebody on the first step.

Here’s the short version for anyone building or replacing stairs. In residential construction, 2x12 dimensional lumber is the industry standard. While 2x10s are sometimes used, they often leave insufficient wood—known as the throat—after cutting, which can weaken the structure and fail inspection. That throat is the whole ballgame. A minimum throat of 3.5 to 5 inches is typically required for structural integrity.

For the wood itself, match it to the job. Prioritize pressure-treated lumber for outdoor use to resist rot and insects, or kiln-dried Douglas Fir or Southern Yellow Pine for interior applications. That treated requirement matters here. Montana deck stairs sit in snowmelt for weeks at the ground line, and bare untreated lumber won’t last.

2x10 vs. 2x12, and When to Reach for Engineered

The throat decision is why 2x12 wins for most residential stairs. Cut your tread and riser notches into a 2x10 and you may not have enough wood left under the notch to carry the load. The 2x12 gives you the depth to play with. One profile guide lays out both options side by side — a 2x10 measures 1.5” x 9.25” and a 2x12 measures 1.5” x 11.25”. When using the 2x10 layout, that same guide is blunt about it: make sure your throat thickness is a minimum of 5”. The throat thickness of a stair stringer is the minimum distance between the top and bottom edges of the stringer, measured at the narrowest point of the notch. The throat thickness is important for strength and stability. It can be increased by using a smaller riser height or smaller tread width.

Whatever size you cut, don’t get greedy with the saw. Care should be taken not to over-cut the limits of the notch, unnecessarily reducing the effective depth of the member.

Engineered stringers are the other path. While dimension lumber has been and continues to be successfully implemented for stair stringer construction, the structural properties of engineered wood products such as structural composite lumber may be desirable in some situations. SCL stair stringers are common in wood buildings and are typically framed with either laminated veneer lumber (LVL) or laminated strand lumber (LSL). The pitch for LSL and LVL is consistency. These products resist bowing, shrinking, and splitting and stay straight and consistent, with no warp, no wane, and no rot, which means no waste and no need to order extra materials.

The catch with engineered stringers is that minimum throat depth climbs with the product depth. Tolko’s LSL guide spells it out: minimum throat depth runs 3-1/2” for the 1-1/4” x 9-1/2” product, 5-3/4” for the 11-7/8”, and 7-7/8” for the 14”. Deeper stock, more throat required. Plan your layout around that number, not the other way around.

There are pre-cut options too if you’d rather not lay out notches from scratch. Pre-cut stringers are available from various suppliers in both dimension and engineered lumber, and several manufacturers have resources to help specify the correct size — including Versa-Lam LVL guides, LP SolidStart LSL, RigidLam LVL, and the TimberStrand LSL and SturdiStep specifier’s guide.

Stringer Profiles and Sizes

Profile	Actual dimensions	Notes
2x10 sawn	1.5” x 9.25”	Needs min. 5” throat per OC guide; less margin after notching
2x12 sawn	1.5” x 11.25”	Residential standard
TimberStrand LSL 1.3E	widths 1¼”; depths 9½”, 11⅞”, 14”	Engineered, knot-free
TimberStrand LSL 1.5E	width 1½”; depths 9½”, 11⅞”, 14”	Engineered
TimberStrand LSL 1.55E	width 1¾”; depths 9½”, 11⅞”, 14”	Engineered
Tolko T-TEC LSL 1.35E	width 1¼”; depths 9½”, 11⅞”, 14”	Throat min. scales with depth

Western Building Center carries pre-cut treated stringers in 2-step and 3-step configurations, a 4-step treated stringer at 47.71” long by 11¼” wide, and an OSB engineered stringer at 1-1/4” x 11-7/8” x 20’.

Spacing, Span, and Load

Get the number of stringers and the spacing right or the stairs feel springy underfoot. The width threshold is the rule to remember. A third stringer is required when the width of the stairs is more than 36 inches. For wide stairs that means left, right, and center. Use at least three stringers spaced no more than 16 inches apart, and if using flexible tread materials like composite, decrease spacing to 9 to 12 inches.

Loads drive the rest. For residential projects the live load is usually 40 PSF. Pair that with dead load and a deflection target. The larger the L/XXX number, the stiffer the stairs. For reference, wood usually follows an L/360 recommended deflection. The code minimum, per the engineering references, sets deflection typically limited to L/360 for live load and L/240 for total load.

Span gets read off a table, and remember which direction it measures. This span value is the horizontal dimension that the stringer travels, not vertical. As a working anchor from the Owens Corning table at 40 PSF total load: a 2x12-equivalent stringer at 12” spacing allows roughly a 7’-5” span at L/180. If the span is too long, consider adding an intermediate support or landing.

Layout, the Bottom Drop, and Attachment

Start with the full layout before any cutting. That includes total rise, total run, number of steps, tread width, riser height, and stringer spacing — and the layout depends on building code requirements, materials selected, and desired layout. Marking is straightforward gear: once the layout is determined, mark the tread and riser lines using a framing square and stair gauge, or a prefabricated stair stringer template.

Now the step most people botch. The bottom drop. Subtract the thickness of your tread material from the bottom of the stringer to ensure all steps, including the first, are uniform. Failing to do this is a common code violation that creates a tripping hazard. Uniformity isn’t a suggestion — the greatest riser height within any flight of stairs shall not exceed the smallest by more than 3/8 inch.

Stay inside the geometry limits while you’re at it. U.S. building codes establish that the riser height shall not be more than 7-3/4” and the run shall be a minimum 10”. Always confirm against your local jurisdiction, since riser heights exceeding 7-3/4 inches or tread depths under 10 inches are typically prohibited.

Attachment is safety-critical. Connection of the top of stair stringers to landings is often accomplished using manufactured steel hangers, and several companies make adjustable slope hangers for both dimension and engineered wood products. The Tolko detail names the hardware directly — use a Simpson Strong-Tie LSC adjustable stringer connector, connect with seventeen 0.148” x 1-1/2” nails, and follow Simpson installation requirements. For outdoor deck stairs, the connectors have to be rated for exterior exposure.

A couple of Montana-specific notes worth burning into memory. Where a stringer meets concrete — common with basement and slab landings around here — the stair stringer shall be protected by a vapor barrier if it is in contact with concrete. That barrier stops moisture wicking up into the wood. And understand the limits of any published span table for exterior stairs: those calculations are run for indoor conditions and don’t factor in freeze-thaw cycling, UV, or temperature swings — exactly the punishment a Montana deck stair takes every winter.

Frequently Asked Questions

Why is 2x12 the standard instead of 2x10? Once you cut the tread and riser notches, what’s left under the notch — the throat — carries the load. A 2x12 (1.5” x 11.25”) leaves more wood after notching than a 2x10 (1.5” x 9.25”). A minimum throat of 3.5 to 5 inches is typically required, and the 2x10 often comes up short, which weakens the stair and can fail inspection.

What is the bottom drop and why does it matter? The bottom drop means subtracting the thickness of your tread material from the bottom of the stringer. Skip it and your first step ends up taller than the rest. Since the greatest riser height in a flight can’t exceed the smallest by more than 3/8 inch, an uneven first step is both a code violation and a tripping hazard.

How many stringers do I need and how far apart? A third stringer is required once stair width exceeds 36 inches. General guidance is at least three stringers spaced no more than 16 inches apart. With flexible tread materials like composite, tighten that spacing to 9 to 12 inches so the treads don’t flex.

What lumber should I use for outdoor deck stairs versus interior stairs? Use pressure-treated lumber outdoors to resist rot and insects. For interior stairs, kiln-dried Douglas Fir or Southern Yellow Pine works. Wherever a stringer contacts concrete, separate it with a vapor barrier to stop moisture from wicking into the wood.

Are engineered stringers worth it over sawn lumber? LSL and LVL resist bowing, shrinking, and splitting, run straight and knot-free, and cut down on waste. The trade-off is that minimum throat depth increases with product depth — for example, a 1-1/4” LSL needs a 3-1/2” throat at 9-1/2” depth but a 7-7/8” throat at 14”. Lay out to those numbers from the start.

What live load do residential stairs need to handle? Residential stair design generally uses a 40 PSF live load, paired with dead load and a deflection target. Wood typically follows an L/360 deflection recommendation for a stiffer feel. Read your required span off a load table — and remember span is the horizontal travel of the stringer, not the vertical rise.