Cantilever Stairs: How Floating Treads Really Work

Cantilever stairs, where each tread appears to project from a wall with no visible support on the open side, are among the most requested features in contemporary homes. They look weightless. Structurally, they are the opposite: every tread is a cantilever that concentrates load and bending at its root, and the wall behind has to resist that bending, twisting and pull-out for the life of the building.

This article explains the main ways cantilevered stairs are built, what the supporting wall has to do, the design checks an engineer makes, and the questions to ask before committing to one in a new build or a renovation.

The mechanics of a cantilevered tread

A cantilever is a member fixed at one end and free at the other. Load on the free end produces a bending moment at the fixed end equal to the load multiplied by its distance from the support. For a stair tread, a person standing near the outer edge creates a large moment at the wall relative to the size of the tread. The tread must resist that moment, and the wall must resist it too, by providing a fixing that will not rotate or pull out. Our article on cantilever structural design explains the principle in more general terms.

Deflection follows the same logic. The tip of a cantilever deflects far more than the middle of a simply supported beam of the same length and section. For stairs, perceptible bounce is often a more demanding criterion than strength, because a tread that is perfectly safe can still feel unnerving if it dips underfoot.

Common ways to build cantilever stairs

Treads fixed into a concrete wall

The most robust approach is to anchor each tread into a reinforced concrete wall. This can be done by casting steel plates or reinforcement into the wall during construction, to which timber, steel or stone treads are later fixed; or by casting the treads monolithically with the wall. Concrete walls of adequate thickness can resist the moment and torsion at each fixing, and the load path is clear. Our guide to concrete staircases covers the reinforced concrete side.

Hidden steel stringer in the wall

Where the wall is masonry or drywall, a common solution is a steel stringer, often a plate or channel, concealed within or fixed to the face of the wall, with steel arms projecting to carry each tread. The treads are then clad in timber or stone. The stringer itself must be supported at top and bottom and, usually, at intermediate points to stop it twisting. This is a structural steel element and should be designed as one.

Mechanical fixings into existing walls

Proprietary systems use bars or brackets resin-anchored into an existing wall. These can work well in solid concrete, but they depend on the wall’s thickness, strength and condition, and on correct installation. Fixing them into hollow blocks, lightweight concrete or thin brick partitions is a recipe for loosening over time.

Stone cantilevered stairs

Historic stone cantilevered stairs, built in Europe for centuries, work differently from what the name suggests. Each tread is built into the wall at one end and bears on the tread below along its front edge. The load is carried largely through torsion in the treads and compression between them, rather than by each tread acting as a free cantilever. These stairs can perform for centuries, but when a single tread cracks the whole system is affected, which is why they need specialist assessment.

The supporting wall does the real work

Clients often focus on the treads, but the critical element is the wall. A cantilevered tread delivers to the wall a vertical force, a bending moment trying to rotate the fixing outward, and, if the load is off-centre, a torsion. Near the fixing, the top of the tread pulls on the wall and the bottom pushes into it. The wall therefore needs enough thickness, strength and mass at every tread position, and any embedded plates need enough anchorage to resist pull-out.

A typical brick or block partition in a Singapore house is not designed for this. Unless the wall is reinforced concrete of adequate thickness, or a steel structure is introduced, cantilevered treads should not be fixed into it. If the wall is part of the building’s structure, adding cantilever loads to it changes how it behaves, and our article on load-bearing walls explains why that needs engineering input.

Design checks an engineer makes

Tread strength under the imposed load from the loading code, including concentrated loads near the free edge.
Deflection and vibration of each tread and of the stair as a whole, often the governing criterion for slim treads.
Fixing design: embedded plates, bolts, welds or resin anchors, checked for tension, shear and bending.
Wall capacity locally at each fixing and globally for the accumulated load and moment.
Robustness: what happens if one tread or fixing fails, and whether that could trigger progressive failure.
Fire where the stair is part of an escape route, which may affect material choice.

Safety, regulations and practicality

Cantilever stairs are often shown without a handrail on the open side. Building regulations in Singapore, as elsewhere, set requirements for balustrades and handrails on stairs above certain heights, as well as rules on riser height, tread going and openings. An open riser and an unguarded edge can conflict with those requirements, particularly where children may use the stair. Our guide to stair dimensions outlines the geometry rules. A thin glass balustrade or a discreet steel rod barrier can keep the floating appearance while meeting the rules, but it needs its own fixing design.

Practical points also matter. Stone and thick timber treads are heavy to install. Each tread must be set accurately to give consistent rises. Finishes and the wall plaster around each fixing need careful detailing so that small movements do not crack the wall at every step.

Adding cantilever stairs in a renovation

In landed house renovations, owners sometimes want to replace an existing stair with a cantilevered one. That usually involves forming or enlarging a slab opening, introducing a new concrete wall or steel stringer, and removing the existing stair, all of which affect the structure. In Singapore, such works generally require a Professional Engineer’s design and BCA approval. The engineer will also want to understand how the existing stair currently supports the floors, because removing it can take load off one element and place it on another. Getting cantilever stairs right is far easier at design stage than after the wall has been built.

Frequently asked questions

Can cantilever stairs be fixed to a brick wall?

Generally not directly. Ordinary brick or block partitions lack the strength and anchorage to resist the bending and pull-out at each tread. A reinforced concrete wall or a concealed steel stringer properly supported at top and bottom is the usual solution.

Are cantilever stairs safe?

Yes, when they are designed by an engineer, fixed into a wall or frame that can resist the loads, and fitted with guarding that meets regulations. Problems arise when treads are anchored into inadequate walls or when deflection and vibration are ignored.

Why do some floating stairs feel bouncy?

Cantilevered treads deflect more than supported ones, and slim treads or flexible fixings make the movement noticeable underfoot. Designing for stiffness, not just strength, avoids the bounce.

Do cantilever stairs need a handrail?

Building regulations generally require handrails and guarding on stairs above certain heights, so an open, unguarded side may not comply. A slim glass or steel balustrade can meet the rules while preserving the floating look.

Considering cantilever stairs for a new home or renovation? Talk to our structural engineering consultancy or get in touch.