string

A string is the diagonal structural member of a staircase that supports the treads and risers. It runs at an angle from the lower landing to the upper landing, and the treads (horizontal steps) and risers (vertical faces) are either cut into it, mounted on top of it, or attached to its side, depending on the stair design. In a typical domestic staircase, you will see two strings, one on each side of the flight, running parallel to one another.

Strings come in two main configurations. An open string (or cut string) has the profile of the treads cut directly into its upper edge, so the steps are notched into the board itself; the tread end sits on this notch and the riser sits in the space below. A closed string has a straight upper edge, and the treads and risers are either housed into grooves routed into the face, or simply mounted on top. The closed string is stronger and more common in commercial work because the wood is not weakened by notching.

Material choice depends on load and aesthetics. Domestic strings are typically 38 to 50 mm thick softwood (pine or spruce), sometimes hardwood for visible stairs. The grain should run along the length of the string to resist the bending stress created by the cantilevered treads. Defects like large knots or cross-grain sections will cause deflection and eventual cracking. In heavy-use commercial stairs, strings may be 50 mm or thicker, and are sometimes doubled or reinforced with steel plates at the notches.

The angle of the string is determined by the rise and run of the stairs: steeper stairs (higher rise per tread) result in a steeper string angle. A typical domestic stair with a 190 mm rise and 250 mm tread (run) will have a string angle around 37 degrees from horizontal. Strings must be accurately cut and fitted because they anchor the entire staircase; misalignment here cascades into wobble, creaking, and accelerated wear on the treads themselves.

The term comes from the appearance of the edge profile: when treads are notched into an open string, the outline of the cuts resembles the strings of a musical instrument stretched tight across the frame. In older carpentry texts, strings are sometimes called stringers or stringboards, particularly in North American usage. They are distinct from the carriages or rough strings that may sit beneath them as secondary structural support in wider or higher stairs.