If you’ve ever walked up a staircase, you’ve trusted your weight to a stringer — whether you knew it or not. Stringers are the structural backbone of every set of stairs in the world, and they’re also the single biggest reason custom stairs cost what they do, look how they look, and (critically) don’t move under you.
Here’s what a stringer actually is, what the codes demand of it, and why the choice between a notched 2×12 and a steel mono-stringer can swing a project’s price by an order of magnitude.
The 30-second definition
A stair stringer is the inclined structural member that runs from the bottom of a staircase to the top, carrying the treads (the parts you step on) and the risers (the vertical faces between treads). Most residential stairs use two or three stringers; wider stairs use more.
Think of stringers as the joists of a staircase: invisible to most people, but doing all the work.
The three main types
Cut (sawtooth) stringers are the most common type for deck stairs and many residential interior stairs — typically a 2×12 with triangular notches sawn out so the treads sit on the horizontal cuts and the risers nail to the vertical cuts. After notching, only about 5 inches of the original 11¼-inch board is left as the structural “throat,” and that throat is what’s actually carrying the load.
Closed (housed) stringers are solid boards with routed grooves (dadoes) that accept the treads and risers. Stronger than cut stringers, since the structural depth isn’t reduced, and common on finished interior stairs.
Mono- and double-stringer steel systems are the architectural showpieces — a single central beam (mono) or two parallel beams supporting floating treads. They’re the structural reason a “floating staircase” can exist at all.
What the codes actually require
Stair stringers operate under tighter loading and dimensional rules than almost any other framing element in a house — but it’s worth noting up front that the model codes are unusually thin on prescriptive structural sizing. They specify the loads stringers must carry, but they don’t generally tell you what size lumber to use for an interior stair. That’s left to engineering judgment.
Loading. Per the 2018 IBC Table 1607.1, stairs and exit-ways must resist a minimum live load of either 100 psf distributed or a 300-pound concentrated load, with the controlling case used for design. Residential (one- and two-family) stairs in the IRC drop to 40 psf live load with the same 300 lb concentrated load (IRC Table R301.5). ASCE 7 mirrors these numbers. Deflection under the 2018 IBC is typically capped at L/360 for live load and L/240 for total load.
Stringer dimensions for deck stairs. The American Wood Council’s DCA 6 prescriptive guide — the document the IRC references for residential decks (Section R507) — sets the most concrete numbers. For deck stairs, it allows two prescriptive paths:
- Cut (notched) stringers must be 2x12s spaced no more than 18 inches on center, with a minimum 5-inch throat below the notches and a maximum horizontal span of 6 feet.
- Solid (uncut) stringers carrying butt-attached treads can span up to 13 feet 3 inches for most species, or 16 feet 6 inches for Southern Pine, again at 18-inch maximum tributary width.
For interior stairs, neither the IRC nor the IBC prescriptively requires any of these specific stringer dimensions — they only specify the loads (above) and tread/riser geometry (below). The 2×12-at-18-inches-on-center convention for interior stairs is industry practice, not a code mandate. Stringer sizing on an interior stair is a structural-engineering call made against the 40 psf / 300 lb load and the L/360 deflection limit.
Vertical flight limits. The IBC (Section 1011.8) caps a single flight at 12 feet (144 inches) of vertical rise before a landing is required. The IRC (Section R311.7.3) is slightly more permissive at 12 feet 7 inches (151 inches). These limits matter because they cap how long a stringer can run before it needs an intermediate landing, regardless of whether the lumber could span further.
Treads and risers. The dimensions the stringer has to carry are themselves tightly constrained:
- IRC residential (R311.7.5): max riser 7¾ inches, min tread depth 10 inches with nosing or 11 inches without
- IBC commercial (1011.5.2): max riser 7 inches, min riser 4 inches, min tread depth 11 inches
- Variation in any flight: 3/8 inch maximum between the tallest and shortest riser, or deepest and shallowest tread
- Nosing projection: ¾ inch to 1¼ inches when the tread is under 11 inches deep
- Headroom: 6 feet 8 inches at every point along the stair (both codes)
Width. The IRC requires residential stairs to be at least 36 inches in clear width above handrail height. Below handrail height, a one-handrail stair can narrow to 31½ inches and a two-handrail stair to 27 inches, with handrails projecting no more than 4½ inches per side (R311.7.1, R311.7.8.2). The IBC requires 44 inches minimum on commercial stairs, dropping to 36 inches only for stairs serving an occupant load under 50 (Section 1011.2).
Why this matters: the human factor
Stairs are the most code-intensive element in residential construction, and the data on what happens when codes are missed is sobering.
A 23-year analysis of National Electronic Injury Surveillance System data published in the American Journal of Emergency Medicine (Blazewick et al., 2018) estimated 24,760,843 stair-related injuries treated in U.S. emergency departments from 1990 to 2012 — averaging 1,076,558 per year, or about 2,949 per day. The annual rate worked out to 37.8 per 10,000 U.S. residents. Stair injury rates fell 12.6% from 1990 to 1996 and then rose 24.0% from 1996 to 2012. About 94% of patients were treated and released; 6% were admitted, mostly for fractures or head injuries. Women accounted for 62.4% of injuries, and 67.2% of visits were by people aged 11 to 60.
A separate ergonomic study of 80 stairway fall cases (Cohen, LaRue & Cohen, 2009) found that 80% of the falls occurred during descent, and on 60% of the stairways investigated, riser-height or tread-depth variation exceeded the 3/8-inch (9.5 mm) tolerance allowed by the IBC and IRC. The researchers identified excessive dimensional variation within stairways as a more pervasive factor in falls than individual user variables.
Among adults 65 and older, falls are the leading cause of injury death. CDC WONDER data shows the age-adjusted fall death rate climbed about 21% between 2018 and 2024, from 64.7 to 78.4 per 100,000 older adults. About 1 in 4 older adults report falling each year. Health-care spending for non-fatal falls among older adults reached an estimated $80 billion in 2020 (Haddad et al., Injury Prevention, 2024).
The geometry that the stringer holds in place — riser height, tread depth, uniformity to the 3/8 inch — is the front line of fall prevention.
The takeaway
A stringer isn’t glamorous. Nobody walks into a foyer and admires the throat depth of a 2×12. But every aesthetic choice on a staircase — open risers, floating treads, a sweeping curve, a long unsupported span — comes back to whether the stringer can hold the geometry the designer drew.
For a sculptural feature stair, the stringer is the engineering problem, the visual problem, and a substantial share of the budget — and it’s worth talking through the structural design with someone who’s done it before signing off on a drawing.
Get the stringer right, and the rest of the staircase mostly takes care of itself.
Note: Code references in this post are based on the 2018 and 2021 IBC and IRC, plus AWC DCA 6 (2015). Local jurisdictions adopt and amend codes on their own schedules; always check with your local building official before construction. Cost ranges reflect U.S. national averages from 2024–2026 published guides and will vary materially by region.
