Mono-stringer and central-spine staircases
Written by Scott Jones, The Stair Guys, independent staircase measuring and sourcing specialists·Last updated
A mono-stringer, or central-spine, staircase is the clean, modern look you see in open-plan homes: the treads appear to float, with no bulky strings up the sides. The trick is a single beam running up the centre of the stair, doing the job that two side strings do on a traditional flight. It is a different kind of stair to design and build, not just a different look.
How it works
On a traditional stair the treads are housed into a string on each side. On a mono-stringer, they sit on top of, and cantilever out from, one central spine. That spine is most often a steel section, which is slim for its strength, or a substantial timber beam where a warmer, all-wood look is wanted. Because everything hangs off that one beam, the treads themselves are usually thicker and are fixed to the spine with engineered brackets or plates, rather than the housed-and-wedged joints of a traditional stair.
Why it is an engineered stair
This is the important part. A central-spine stair is a structural design, not a standard joinery build. The beam has to be sized to span the stair and carry the load without excessive bounce, and each tread connection has to be designed to take the weight and the leverage of someone standing on the outer edge. That is why these stairs are engineered and made to measure rather than pulled from a standard range, and why the fixings matter as much as the timber. It is also why they cost more than a like-for-like traditional flight.
What it pairs with, and the rules that still apply
Central-spine stairs are usually paired with the rest of the modern kit: open risers (no board between the treads), and a glass balustrade or slim metal spindles, to keep the open, floating feel. None of that changes the safety rules. The same guarding applies, gaps a 100mm sphere cannot pass and a handrail at the right height, covered on the guarding page, and open risers have their own overlap rule in the regulations. A floating look still has to be a safe stair.
Frequently asked
What is a mono-stringer staircase?+
A mono-stringer staircase carries its treads on a single central beam, or stringer, instead of the two side strings of a traditional stair. The treads cantilever out to each side, which gives the open, floating look. The central beam is usually a steel section or a thick timber beam, engineered to carry the load.
What is a central spine staircase?+
It is another name for a mono-stringer staircase: the treads run off one central spine beam rather than two side strings. The spine can be steel or a substantial timber beam, and the treads are fixed to it with engineered brackets, giving the modern, floating appearance.
Are floating or central-spine staircases strong and safe?+
Yes, when they are properly engineered, which is the key point. Because the whole stair hangs off one central beam, the beam has to be correctly sized and each tread fixing designed for the load, so it is a structural, made-to-measure design rather than a standard joinery build. Done right it is as safe as any stair, and it still has to meet the same guarding and handrail rules.
Related guides
- Winder staircases: what they are and the rules that govern themA winder staircase turns a corner on tapered treads instead of a flat landing. Here is how winders work, the two rules every winder has to keep, why four winders is a harder job than three, and when a landing is the better answer.
- Types of staircase: the main shapes, and how to chooseThe main staircase types are straight, quarter-turn, half-turn, winder, spiral, helical and space-saver, with open or closed strings on each. The right one is driven by the space you have, the look you want and the budget.
- Space-saver (alternating tread) stairs: what they are and when you can have oneWhat a space-saver or alternating tread staircase is, when building regulations actually allow one, the trade-offs, and why it is a space and budget compromise rather than a premium upgrade.
- Cut string vs closed string staircase: the difference, and how to chooseA closed string staircase has a solid raking board on the outer edge that hides the ends of the treads, while a cut or open string has that edge cut to the step profile so the tread ends show. Closed is simpler and cheaper, cut is a feature with more joinery.
- Cantilever (floating) staircasesA cantilever or floating staircase fixes each tread at one end only, usually built into the wall, so the treads appear to float with no support on the open side. The wall and a steel support carry the load, so it is a structural, engineered stair rather than standard joinery.
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