Mono-stringer Stairs from Arden Architectural Stairs

In stair construction terminology, the stringer describes the beam or member that plays the structural role of supporting the treads.

A mono-stringer or centre-stringer is an open form of staircase that has a single supporting beam supporting the treads (stringer) which is usually placed in the centre of the treads but can be offset. Similar to other forms of open stairs, mono-stringers typically support timber treads, but may conceivably support treads in utilizing a wide variety of materials, such as stone, steel, or glass.

Comparisons with traditional stairs

Mono-stringers stair designs tend to be more open since they are not enclosed by supports on either side of the treads. The stringer itself tends to be less apparent since it is located centrally and underneath the treads themselves. The ability of mono-stringer to promote a sense of lightness and openness is regarded as a desirable property. When compared to traditional closed or enclosed stairs, mono-stringers demand the use of relatively stronger materials in order to be structurally sound. This may explain why mono-stringer stair designs have seen widespread adoption in the later 20th century, as improvements in material technology has lead to widespread availability of components with the required tensile strength. The use of modern engineering methods and materials allow for relatively long spans to be achieved by a mono-stringer stair. Also in comparison to traditional stairs, mono-stringers tend to rely more heavily on the use of fabricated structural components, such as square-hollow-section (SHS) mild steel members, laminated veneer lumber (LVL), laser-cut folded metal plates, etc.

Fabrication approaches

Saw-tooth type mono-stringers are possible, where space to locate the treads is cut-away from the stringer profile. Alternatively, the stringer may present a continuous top-line, and is therefore located below the bottom line of the treads so as not to interfere. In this case, some form of support fixing is required to connect the treads to the stringer. Common methods of attachment of the treads to the mono-stringer include a plate-and-gusset methodology, the folded tread plate method. Saw-tooth mono-stringers are commonly fabricated from structural timber, while continuous or straight mono-stringers are commonly created from steel. Steel mono-stringers are often based on a SHS or rectangular-hollow-section (RHS) main member. This approach is the simplest, and usually the most efficient way to construct a mono-stringer stair. In timber, the mono-stringer may consist of a laminated veneer lumber (LVL) core, with feature-timber veneer cladding. Increasing in complexity, the mono-stringer main member may itself be fabricated from multiple components, such as twin plate-steel laser-cut members, connected by a web of bridging plates. The laser-cut members may be straight, or themselves be cut to create a zigzag pattern. More elaborate mono-stringer designs are possible. For example, Arden Architectural manufactures a stainless steel clad zigzag mono-stringer that utilizes composite steel and ply construction.