Cantilever staircases, carried by the wall

What a cantilever staircase is

On an ordinary stair the treads sit between two stringers. On a staircase cantilever design the wall does that work instead. Each tread has a steel core, a plate or a welded box section, that is fixed into or on to the wall at one end and left free at the other. With open risers and no balustrade posts on the floor, the flight reads as a row of separate steps.

There are two common ways to build it. The treads can be bolted or resin-anchored one by one into solid masonry or concrete. Or a hidden steel stringer is fixed along the wall first, the treads are welded to it, and the wall is plastered or boarded over so only the treads show. The second suits a new build or a refurbishment where the wall is open anyway.

The wall has to be able to carry it

Every tread acts like a short beam fixed at one end, so the load from someone standing on the free edge turns into a twisting force at the wall. Solid brick, blockwork or reinforced concrete of a decent thickness can usually take that. A timber stud partition, a cavity wall with a thin inner leaf, or old brickwork in poor condition usually cannot without help.

Where the wall is not strong enough on its own, a steel post or a concealed stringer built into the wall line gives the treads something to fix to. Either way, cantilever stairs need structural calculations, and Building Control will ask for them. We check the wall at the survey and tell you straight away if it will not work, and if it will not, a central spine staircase gives a similar open look without relying on the wall.

Treads, finishes and the open side

The steel core is usually clad. Oak, ash or walnut boxed around a steel plate is the most popular finish, then stone, porcelain or a plain powder coated steel tread for a more industrial look. The core is sized so the tread does not bounce or dip at the free end.

A cantilever stair still has to meet Approved Document K: the same rise and going on every step, at least 16mm of overlap between treads where the risers are open, and no gap that a 100mm sphere could pass through on a stair children might use. The open side needs guarding wherever the drop is more than 600mm, so most cantilever stairs have a frameless glass balustrade fixed to the tread ends or the floor, or slim steel bars. A wall handrail on the fixed side completes it.

Where cantilever stairs suit, and where they do not

They suit a stair that runs along one solid wall in an open hallway, a living space or a barn conversion, where the point is to keep the room light and the stair simple. A straight flight is the easiest. A quarter turn is possible where two solid walls meet in a corner.

They do not suit a stair that has to stand away from the walls, a long run of very wide treads, or a building where the only wall available is a partition. Outside, a cantilever stair is possible off a solid masonry or concrete wall, galvanised and powder coated, but most external staircases are better and cheaper on stringers.

Survey, drawings and fitting

Send photos, the floor to floor height and the length of wall the stair would run along, and you will have a written estimate within the working day. A free survey follows, where we look at what the wall is built of and how thick it is, take the heights, and check the opening above.

You then get a drawing to approve, the structural calculations for Building Control, and a fixed quotation that only changes if the design does. The steel cores are made in the workshop, then the cladding, balustrade and handrail follow once the treads are in and level.

Straight answers on cantilever staircases