Cantilever Staircases: How Floating Stairs Work

What cantilever actually means

A cantilever staircase looks like it should not stand up. Treads emerge from a wall with nothing beneath them and nothing obvious holding them in place, and the flight reads as a sequence of solid objects floating in air. It is the most requested staircase form in contemporary architecture, and the most commonly misunderstood.

The word is used loosely in the market. Some stairs sold as cantilevered are genuinely cantilevered. Others are centrally supported stairs with the structure hidden. Both can be excellent. They are not the same thing, they cost different amounts, and they make very different demands on the building around them.

A true cantilever is a beam fixed rigidly at one end and unsupported at the other. In a staircase, each tread is that beam. The load of a person standing on the outer edge is carried back through the tread into a fixing, and from that fixing into the structure of the wall. Nothing under the tread resists the load. Nothing at the outer edge does either.

This is why a cantilever staircase is a structural decision before it is an aesthetic one. The wall is doing the work. The tread is a lever, and the further it projects, the harder it pulls on whatever it is anchored to. The consequence is direct: the wall has to be designed for the staircase, usually before it is built. A cantilever stair is not something that can be added later to a wall that was not expecting it.

What the treads are fixed to

There are three common approaches, and which one suits a project depends almost entirely on the wall.

Embedded steel stringers. A steel plate or box section is built into the wall and projects out to form the spine of each tread. The timber or stone tread is then dressed around it. This is the most widely used method in new build, because the steel can be positioned and grouted in as the wall goes up.

A concealed steel backbone. A continuous steel plate runs vertically inside the wall build-up, with individual tread arms welded to it. The whole assembly is fixed to the primary structure rather than to the masonry itself. This is the usual answer where the wall alone cannot take the load, and it is common in timber frame and steel frame buildings.

Structural masonry pockets. Treads are built directly into a solid wall of sufficient depth and density. Traditional, handsome, and increasingly rare, because few modern walls are built heavy enough to do it.

In every case the visible tread is the smallest part of the engineering. What you are buying is what happens inside the wall.

The deflection question

Ask any manufacturer about cantilever stairs and the conversation arrives quickly at deflection: how much the free end of a tread moves under load.

Some movement is normal and unavoidable. A cantilevered tread is a lever, and levers flex. What matters is that the movement is small enough to be imperceptible underfoot and well within the limits set by the structural design. A stair that visibly bounces has not been engineered badly so much as engineered optimistically.

Deflection is governed by tread thickness, the depth of the steel inside it, the projection, and the rigidity of the fixing. Increase the projection and every other variable has to work harder. This is the single biggest reason two cantilever staircases that look identical can differ substantially in cost.

Balustrade, or the absence of one

The visual appeal of a floating stair is the clear space around it, and a heavy balustrade undoes that immediately. The usual answers are structural glass fixed into the side of the treads or into the floor, a slim vertical bar system, or, where the stair is enclosed on both sides, no balustrade at all.

Building regulations govern this, and the requirements differ between a private dwelling and a building the public uses. Worth saying plainly at design stage: a cantilever stair with a structural glass balustrade is a different product from one without. The glass is not decoration bolted on at the end. Where it fixes into the tread, the tread has to be designed around it.

Where a cantilever stair earns its place

It is not the right answer everywhere, and a good manufacturer will say so.

It works best against a long, solid, well-supported wall, in a space with enough height and daylight for the openness to register. A floating flight in a tight, enclosed stairwell gains very little, because the openness that justifies it is not visible. It works particularly well where the stair sits in a double-height volume, where light falls across the flight from the side, or where the underside of the stair is seen from a living space.

It is a poor fit where the wall is a lightweight partition with no capacity, where the stair needs to turn tightly in a small footprint, or where the budget has not accounted for the structural work in the wall as well as the stair itself.

Cantilever, floating and central spine: the difference

These three terms get used interchangeably and they should not be.

A cantilever stair is supported from one side only, from within the wall. A floating stair is a description of an appearance, not a structure: any stair with open risers and concealed support reads as floating, and while some floating stairs are cantilevered, many are not. A central spine stair carries a single beam beneath the centre line of the treads, usually steel, with the treads cantilevering a short distance either side of that spine. It achieves a similar visual lightness without needing a structural wall.

If your heart is set on a floating stair and the wall cannot take it, a central spine stair is almost always where the conversation goes next.

What to bring to the first conversation

Three things will get you a useful answer quickly: the floor-to-floor height, a section or elevation showing what the stair is fixed to, and a sense of what the underside of the flight will be seen against. Everything else can be developed.

The earlier that conversation happens, the better. A cantilever staircase specified while the walls are still on paper is straightforward. The same staircase specified after the walls are up is a different and considerably more expensive project.