stringer

A stringer is the angled or vertical structural member that carries the weight of steps or rungs. In a staircase, stringers are the sloped beams that run from bottom to top landing, with notches or grooves cut into them to support each tread and riser. In a ladder, the stringers are the two parallel side rails that hold the rungs at fixed spacing. Without stringers, steps would have nothing to rest on and would collapse under use.

Staircase stringers come in two main types: cut stringers, where each step is notched into a solid board (leaving a sawtooth profile), and housed stringers, where the treads and risers slide into continuous grooves routed along the inside face. Cut stringers are simpler to build and repair but remove more material, reducing strength. Housed stringers are stronger and neater but require precise routing equipment and are harder to adjust. A typical residential staircase uses two stringers on the sides; wider commercial stairs use three or more stringers, with a central one to reduce span and deflection.

Materials and loads

Stringers are usually made from solid sawn timber (2x10 or 2x12 in residential work) or engineered lumber such as LVL, chosen to span the vertical rise without sagging. A stringer supporting a flight must resist both the downward load of people and furniture and the lateral racking forces that come from uneven foot placement. Building codes specify minimum stringer thickness based on span length and expected load. For example, a 12-foot rise might require a 1.75-inch thick LVL stringer rated for 40 pounds per square foot live load.

Failure in stringers typically occurs through deflection (the middle sags, steps tilt, and connections loosen), splitting along the grain at notches, or crushing where the tread bears down. Wood stringers are prone to rot where moisture collects at the junction with the floor or at joints with risers. In ladder stringers, the main risk is that lateral force from a leaning user can cause the rungs to tear out of their sockets if they were not drilled and secured properly.

The term stringer is borrowed from the verb to string, meaning to thread or line up in a row. The stringers literally string the steps or rungs together in their proper sequence and spacing. In timber framing, stringers also appear in floor joists and roof trusses wherever a member is required to tie multiple elements in line and share their load.