Stringer

A stringer is the inclined beam that carries the treads and risers of a staircase flight. In Slovak it is called schodnica. Whether visible or hidden, the stringer decides how the flight is supported and how much of its structure a user sees.

What does a stringer do in a staircase?

A stringer runs along the inclined edge of a flight and holds the steps in position. Each tread and riser is fixed to it, and the stringer passes the load of the steps down to the floor or landing below and into the structure at the top of the flight. Most flights have one stringer on each side, so the pair forms the backbone of the stair.

A stringer is structural even when it looks light. Its job is to resist bending and keep the flight rigid, so the steps do not move under people walking on them. For how the steps themselves are proportioned, see the staircase article.

How does a stringer carry the steps?

The stringer behaves as an inclined beam, supported at its ends or at one end and along a wall. Each tread is the horizontal walking surface, and each riser is the vertical face between two treads. The stair works as one rigid inclined structure only when the stringer, treads and risers are fixed properly to one another.

The weakest point is usually where a tread meets the stringer, not the middle of the beam. A notch, groove or bracket removes material and concentrates stress at that point, so the detail has to be designed rather than left to the installer.

What are the main stringer types?

Stringers are grouped by their shape and by where the treads sit. The five types below cover nearly every residential stair you are likely to design or review.

Stringer type	How the treads are carried	What is visible	Typical material	Best suited to
Closed (housed) stringer	Treads and risers are let into grooves cut in a solid side panel	A closed side, steps hidden from the side	Timber, sometimes steel	Traditional and warm interiors, stairs with a clean side
Cut (open) stringer	Treads sit on top of a notched profile and are fixed to it	Step profile in the side, often with a decorative board outside	Timber or steel	Open stairs where the underside should show
Central spine stringer	One beam runs down the middle and the treads project to both sides	A slim spine, treads that appear to float	Steel or timber	Slim, sculptural stairs in open-plan rooms
Steel flat-bar stringer	Thin steel plates carry brackets or fixings for the treads	Light, linear edges with every fixing visible	Steel	Lightweight stairs with timber or glass treads
Hidden steel stringer	A steel element fixed to the structure behind the wall, with no visible support	Treads that seem to project from the wall	Steel within a wall	Cantilevered stairs where the wall carries the load

The closed stringer gives a solid, traditional look. The cut stringer shows the step profile and suits open stairs. A central spine keeps both sides open, but all the load passes through one member, so its connections carry much of the design. The steel flat-bar is light, though every fixing shows.

How are the treads fixed to the stringer?

The fixing detail decides the stringer's visible character and how easily the stair can be adjusted on site. Agree it with the joiner or steel fabricator before the stair is ordered, because a change at that stage costs far less on a drawing than in the workshop or on site.

Stringer type	Tread connection	Typical fixing	Adjustment after fabrication
Closed (housed)	Tread ends slide into grooves and risers are set into the stringer	Glue and screws through the stringer, with grooves cut in the workshop	Limited, so the cutting must be accurate
Cut (open)	Treads bear on the notched profile	Screws or brackets fixed from underneath each tread	Easy to adjust treads on site
Central spine	Treads bolt or weld to the spine and project to both sides	Bolted or welded connections, often hidden inside the tread	Set out with the steel fabricator
Steel flat-bar	Flat bars carry bolted or welded tread brackets	Brackets through the plate with the finish fixed to them	Depends on accurate fabrication
Hidden steel	Treads fixed to a concealed steel element	Fixings through the tread into hidden steel, with anchors in the wall	Anchors must be set before the wall is finished

A housed tread gives the cleanest finish but limits changes after fabrication. A cut stringer is easier to set out, but each notch weakens the member, so the notch geometry belongs in the structural check.

How is a stringer different from a cantilevered staircase?

A stringer staircase sends its load down the flight to the floor below and to the supports at the top. A cantilevered staircase sends its load into a wall, and its treads project from that wall with no visible support underneath. The difference lies in where the load goes, not in whether steel is used.

Many cantilevered staircases still contain a steel element, which is in effect a hidden stringer. It sits inside the wall or behind the cladding, fixed to the structure, so the treads appear to float while a concealed member carries the load. Designing that element and its anchors belongs with the structural engineer, because the anchors must be placed while the wall is built.

Which stringer type suits a Slovak family house?

The answer follows the main structure of the house. In a drevostavba (timber-frame house), a timber closed stringer or a steel flat-bar stringer is a natural fit, because both suit light construction and fix easily to timber framing. In a house of brick or masonry, a steel or timber stringer is usually carried by the walls at each end, and a hidden steel stringer is possible where a solid wall can take the anchors.

In a house with an in-situ concrete structure, the stair is often cast as a single element, so there is no separate stringer to choose. The decision there concerns the form of the flight and where it is cast.

What are the common misconceptions about stringers?

• A stringer is decoration. The board on the outer face of an open stair is a finish, but the structural stringer sits underneath or behind it and must be designed as a load-carrying member.
• Closed stringers are always stronger. The grooves for the treads remove material, so a closed stringer must be checked for its reduced section, just like an open one.
• A hidden stringer needs no engineering. A concealed steel element carries the whole flight through a few anchors, so it is the part that most needs calculation.
• The stringer can be chosen at the end. Its type affects the wall, the floor opening and the fixings, so it belongs to the early concept alongside the position of the stair.