What Is a Stringer on Stairs? Structural Types, Layout Math, and Building Codes

Structural Anatomy and Engineering Role of the Stair Stringer Backbone

The engineering integrity of any residential or commercial staircase depends entirely on the accuracy and structural strength of its underlying stringers. A staircase consists of three core components: the horizontal steps that occupants walk upon (treads), the vertical face boards spanning between each step (risers), and the inclined stringers supporting them from beneath. When an individual steps onto a stair tread, that concentrated dynamic load does not rest in midair; it transfers directly into the notched or routed timber shoulders of the stringers, which channel forces downward to bottom thrust plates and upper floor headers.

Stair stringers are classified into distinct architectural typologies based on how the steps interface with the structural lumber. Traditional exterior decks and utilitarian basement stairways predominantly utilize cut open stringers, where triangular notches are sawn into 2x12 boards so treads sit directly atop horizontal cutouts. In contrast, refined interior architectural staircases frequently employ routed closed stringers (or housed stringers), where treads and risers slide into precision-routed mortises hidden behind solid hardwood face stringers. Contemporary minimalist homes often showcase engineered steel mono stringers, where a single central steel spine supports cantilevered hardwood treads.

Compare the structural characteristics, aesthetic profiles, and installation complexities across major stair stringer types.

Stair Stringer Typologies: Cut Open, Routed Closed, and Modern Mono Stringers

Mastering stair stringer layout requires precise geometric calculation to ensure every single step within a flight possesses identical dimensions. The human brain quickly establishes subconscious muscle memory within the first two steps of ascending or descending stairs. If a carpenter cuts stringers with an irregular height variance as minute as 1/4 inch at the top or bottom landing, occupants will instinctively misjudge their footfall, leading to dangerous stumbles and catastrophic stairway falls. Consequently, building codes enforce a strict maximum variance of 3/8 inch between the tallest and shortest risers in an entire flight.

Calculating stringer geometry begins by determining the exact total rise—the vertical measurement from the finished lower floor level to the finished upper floor level. Builders divide this total rise by 7.5 inches (the standard target riser height) to calculate the required number of risers. Dividing the total rise by the whole number of steps yields the exact riser height to the nearest sixteenth of an inch. Next, applying the time-tested carpenter formula—where one riser plus one tread should equal approximately 17 to 18 inches—determines the exact horizontal tread run, which is then mapped onto the stringer lumber using a framing square and brass stair gauges.

Review the statutory building code specifications governing stair stringer layout, riser heights, and tread depths.

Stair Geometry Math, IRC Building Codes, and Throat Depth Integrity

Throat depth represents the single most critical structural dimension when cutting open saw-tooth stringers. The throat is the unbroken diagonal ribbon of solid timber that remains between the internal apex of the tread-riser notch cuts and the bottom diagonal edge of the 2x12 lumber. Under residential building engineering standards, the throat depth must measure a minimum of 3-1/2 inches of continuous, uncut solid wood. If an inexperienced carpenter overcuts the notches with a circular saw past the pencil line intersections, they severely notch the throat, dramatically weakening the board load capacity and risking sudden structural collapse under heavy furniture moving loads.

Stringer spacing and quantity depend heavily on the chosen tread material and stairway width. For standard 36-inch-wide interior stairs with 1-inch-thick solid hardwood treads, building codes permit three stringers spaced approximately 16 inches on center (one on each outer perimeter and one center support). However, on exterior decks utilizing modern wood-plastic composite decking boards, manufacturers require stringers to be spaced no more than 12 inches on center—requiring four stringers across a 36-inch stairway—because composite boards possess higher flexibility and will sag under summer heat without closer support.

Securing stair stringers at their upper and lower connection points demands specialized structural metal hardware rather than simple end-nailing. The upper ends of stringers should be suspended from floor joist headers using engineered metal stringer hangers (such as Simpson Strong-Tie LSC or VPA connectors) fastened with structural screws. At the bottom landing, stringers must bear firmly against a solid treated thrust board anchored into the concrete foundation slab or subfloor framing, preventing the bottom of the staircase from kicking outward over decades of dynamic foot traffic.