Floating stairs are staircases built without visible stringers under the treads, so each step appears to project from a wall or from a single concealed spine with nothing beneath it. The visual effect is produced by moving the structure out of sight rather than removing it, which makes the supporting wall, the embedment detail and the deflection criteria the three things that decide whether the staircase works.
Floating, cantilevered, open riser: three terms that are not synonyms
The three words are used interchangeably in marketing and they describe different things.
Open riser is a geometry. There is no vertical board closing the gap between one tread and the next. A conventional stair on two visible stringers can have open risers and look nothing like a floating stair.
Cantilevered is a structural condition. A cantilever is a beam anchored at one end only, so a genuinely cantilevered tread is held at the wall and free at its outer edge.
Floating is a visual outcome, and reference works on the subject file the whole family under cantilevered stairs regardless of how the load is actually carried. It covers any stair where the support has been concealed, whether that support is a true cantilever, a hidden spine, twin stringers buried inside the tread depth, or rods in tension from the floor above.
The distinction matters because the three carry very different costs. Open risers are nearly free. A hidden spine is a fabrication question. A true cantilever is a building question, and the answer often involves rebuilding a wall.
A structural idea roughly 450 years old
Floating stairs read as a contemporary invention. They are not.
Andrea Palladio built what is usually recorded as the first cantilevered stone stair at the Convento della Carità in Venice around 1570, stacking solid stone treads on one another with one end secured into the wall, leaving the centre of the stairwell void. In I quattro libri dell'architettura he praised staircases that are empty in the middle, because they take light from above and because anyone already climbing them is visible.
Inigo Jones saw that stair, sketched it, and adapted the idea for the Queen's House at Greenwich. Work began in 1616 and the building was completed around 1635. The Tulip Stairs are generally described as the first centrally unsupported geometric staircase in Britain. The mason Nicholas Stone added something Palladio had not used: a rebate, a grooved interlock worked along the underside of each tread, so that the steps did not depend on simple overlap alone.
Christopher Wren's Geometric Staircase at St Paul's Cathedral, built by William Kempster and completed in 1710, took the method further. Its 88 stone treads rise from the south west tower to the triforium with no central column and no other support. Each tread is set only about 150 mm into the wall.
That last figure is the point. A 150 mm embedment cannot cantilever a stone tread on its own, and the system does not ask it to. Each step also rests on the one below, so load and torsion are shared down the flight into the floor. Historians of the technique note that calling these stairs cantilevered is not strictly accurate. The same ambiguity runs through modern practice, where a great many stairs sold as cantilevered are in fact partially supported systems doing a convincing impression.
The method spread. In Scotland it became common enough in the eighteenth and nineteenth centuries to acquire its own name, the pencheck stair, used in Edinburgh townhouses and tenements and in country houses by Robert Adam.
The four ways to support a floating stair
Everything built today falls into one of four families, or a hybrid of two.
Wall anchored cantilever. Steel arms, flat bars, square tube or embedded plates reach back into a structural wall, and each tread cantilevers out from that anchor. It gives the purest floating appearance and it is the most demanding option, because the wall has to resist the bending moment generated at every tread.
Single central spine, or mono stringer. One structural beam runs up the middle of the flight, usually rectangular tube, square tube, round tube, I beam or a tri frame section, with treads cantilevering sideways from it. The load path is predictable and the building is left alone, which is why this system dominates retrofits.
Twin concealed stringers. Two steel stringers are hidden inside the depth of the treads or behind closed risers. The stair reads as floating from the side while behaving structurally much like a conventional stair.
Suspension. Treads are hung from the structure above by rods or cables, with or without a stringer below. Visually light, and it moves the problem to the ceiling, which then has to carry the whole flight.
The wall decides the project, not the drawing
On a true cantilever the bending moment at the wall is the whole engineering problem. A person standing at the outer edge of a tread applies a force at the end of a lever, and that force arrives at the anchor as a rotation. The longer the tread, the larger the moment, which is why cantilevered stairs use heavier steel than they appear to need and why the embedded end has to be gripped hard and deep.
What can hold it is a short list. Reinforced concrete, solid masonry, or a purpose built steel frame inside the wall. A conventional wood stud wall cannot, and making it able to usually means adding steel or engineered lumber inside the cavity, which is structural work in its own right. The loads also continue downward, so the foundation under the supporting wall has to accept the additional load and the overturning that comes with it.
Three consequences follow, and they are where projects go wrong.
Confirming wall capacity with a structural engineer is a first step, not a late one. Retrofitting reinforcement into a finished wall is far more expensive than building it in. And in renovation, a mono stringer is often the honest answer: it delivers most of the visual effect without asking an existing building to do something it was never designed for.
Deflection, bounce and the criterion no code sets
A floating stair can satisfy every dimensional rule in the code and still feel wrong underfoot.
Concentrating the load path into a few concealed connections makes these systems far more sensitive to deflection and vibration than a stair sitting on two visible stringers. Movement under load is deflection. The more subtle issue is vibration: a flight that bounces or resonates as someone walks it reads as unsafe even when it is nowhere near its structural limit.
Designers control both by increasing section depth, by using stiffer materials, by composite tread construction, by hidden bracing, and in demanding cases by neoprene bearings or dampers at the connections. In commercial settings, where a lobby stair may carry continuous traffic, vibration performance is often the governing design criterion rather than strength.
My recommendation, after looking at how these stairs are specified: the bounce test at handover is the single most useful quality check available to a client, and it is worth writing a deflection limit into the contract rather than discovering the answer after installation.
What the code requires
Residential work in the United States follows the International Residential Code, in its 2021 edition in most jurisdictions. Commercial work follows the International Building Code, which is stricter on almost every line. Contrary to a common assumption on design forums, neither code treats a floating stair as a special category: it is an ordinary stair that happens to hide its structure.
The rule that governs floating stairs specifically is the open riser provision. Under IRC R311.7.5.1, open risers are permitted, but where an opening between treads is more than 30 inches above the floor or grade below, it must not pass a 4 inch diameter sphere. Below that 30 inch threshold the gap is not limited. The commercial position starts from the opposite premise: IBC 1011.5.5.3 requires risers to be solid, then lists exceptions, including stairs that are not serving as exits where the opening between treads will not pass a 4 inch sphere.
Two practical notes. The 3/8 inch uniformity tolerance is the most commonly failed inspection item on any stair, and it is harder to hold on a floating stair because the treads are positioned individually rather than cut from a common stringer. And the 4 inch sphere rule is a child safety provision, not a structural one, which is why it applies to the gap and not to the tread.
Tread materials, compared on the numbers
The support system comes first. Material is the second decision, and it changes dead load, thickness and the amount of steel the structure needs.
Weight per tread is the number that quietly reshapes a project. A stair in concrete and a stair in hardwood can be geometrically identical and require entirely different walls.
Cast glass sits at the thick end of the range because its span capacity comes from section rather than from reinforcement. As a reference point, a fused tread 1 1/2 inches thick and 36 inches wide can carry up to 1,800 pounds while meeting Quebec building code requirements, and thicknesses are specified from 1 1/2 inches to 4 inches depending on span. Projects combining materials are common, and mixed flights pairing wood and glass, or steel and stone, are routine in the glass stair tread and flooring work documented across North American installations.
What actually drives the budget
Four variables account for most of the spread between quotations.
Support type comes first, because a wall anchored cantilever carries engineering, reinforcement and sometimes foundation work that a mono stringer does not. Geometry comes second: straight flights are cheapest, and quarter turns, switchbacks, helical runs and floating landings each add fabrication and setting out. Material choice is third, through both the material itself and the structure its weight demands. Site access is fourth and the most frequently forgotten, since a single steel spine for a full storey arrives as one long object that has to physically reach the stairwell.
One further cost sits outside all four. Every true cantilever requires structural engineering sign off, and in a renovation that review can change the project before a single tread is fabricated.
Summary table: floating stairs
Frequently asked questions about floating stairs
1. What is the difference between floating stairs and cantilevered stairs?
Cantilevered describes the structure, floating describes the appearance. A cantilever is a beam anchored at one end only, so a cantilevered tread is held at the wall and free at its outer edge. A floating stair is any stair whose support has been hidden, which includes cantilevers but also single spine stairs, twin concealed stringers and suspended flights. Most stairs marketed as cantilevered are in fact partially supported systems. The historic stone stairs usually described as cantilevered are a good illustration, since each tread also rests on the one below.
2. Are floating stairs safe?
They are as safe as the engineering behind them, which is a different statement from saying they are inherently safe or inherently risky. Under both the IRC and the IBC, stair treads are designed for a uniform live load of 100 psf or a concentrated load of 300 pounds on 4 square inches, and guards and handrails for 50 pounds per linear foot plus a 200 pound concentrated load in any direction. A correctly designed floating stair meets the same numbers as any other stair. The risks specific to the type are concentrated connections and inadequate wall capacity, both of which are engineering questions resolved before fabrication.
3. Are floating stairs allowed by code?
Yes, with conditions that differ between residential and commercial work. IRC R311.7.5.1 permits open risers in dwellings, but an opening more than 30 inches above the floor below must not pass a 4 inch sphere. IBC 1011.5.5.3 starts from the requirement that risers be solid, then allows exceptions, including stairways not serving as exits where the gap between treads will not pass a 4 inch sphere. Riser height, tread depth, the 3/8 inch uniformity tolerance, headroom, guards and handrails all apply exactly as they would to a conventional stair.
4. Do floating stairs need a special wall?
A true wall anchored cantilever does. The wall has to resist the bending moment delivered by every tread, which in practice means reinforced concrete, solid masonry, or a steel frame built into the wall. A wood stud wall can sometimes be upgraded with steel or engineered lumber inside the cavity, but that is structural work and it is far cheaper to build in than to retrofit. Single spine and twin stringer systems avoid the question entirely, since they carry load to the floor and landing rather than into a wall.
5. Can floating stairs be installed in an existing house?
Often, but the support system usually has to change. A mono stringer stair delivers most of the visual effect while leaving the building fabric alone, which is why it dominates renovation work. A true cantilever in an existing house generally means opening the wall, adding reinforcement, and verifying that the foundation below can take the additional load and the overturning. Confirming wall capacity with a structural engineer before the design is fixed avoids the most expensive version of this problem.
6. Why do some floating stairs feel bouncy?
Because the load path runs through very few concealed connections, these systems are more sensitive to deflection and vibration than a stair on two visible stringers. A flight can meet every strength requirement and still move noticeably, which feels unsafe even when it is not. The remedies are structural rather than cosmetic: deeper sections, stiffer stringer geometry, composite tread construction, concealed bracing, and in some cases dampers or neoprene bearings at the connections. Neither code sets a comfort limit, so the deflection criterion is worth agreeing in writing.
7. What materials can be used for floating stair treads?
Hardwood, folded steel plate, reinforced concrete, natural stone and cast glass are all in common use, and hybrid builds combining a steel core with a finish material are standard practice. The decision changes dead load more than anything else: hardwood cladding on a steel core is light, while reinforced concrete treads of 80 to 110 mm can approach 150 kg each and push the wall and foundation requirements upward. Cast glass is specified from 1 1/2 to 4 inches thick because its span capacity comes from section depth, and cast glass treads for floating and cantilevered builds are produced in that range.
8. Do floating stairs need a railing?
In almost every case, yes. The IRC requires a handrail on at least one side of any flight with four or more risers, and guards at open sides. Guard height is 36 inches residential and 42 inches commercial, with openings that will not pass a 4 inch sphere. The guard is also a structural element, since it has to resist 50 pounds per linear foot and a 200 pound concentrated load in any direction, with the connection detailed to transfer those forces back into the building. On a floating stair that transfer is less obvious than on a framed stair and needs designing rather than assuming.
9. Can floating stairs be used in commercial buildings?
Regularly, and they are common in retail, hospitality, galleries and office atria. Three things tighten compared with a house. The IBC geometry is stricter, with a 7 inch maximum riser and an 11 inch minimum tread. Solid risers are the default position, so the open riser exception has to be established for the specific stairway. And vibration performance becomes a governing criterion rather than a comfort preference, because a lobby stair carries far more continuous traffic than a residential flight.
10. How thick do floating stair treads need to be?
Thickness follows from span, material and support system rather than from a single rule. A tread cantilevering from one end carries a bending moment that rises with its projection, so a longer tread requires more section, more reinforcement, or a second support. Hardwood treads are typically 2 to 3 inches, concrete 80 to 110 mm, and cast glass 1 1/2 to 4 inches. For reference on what section alone can carry, a fused glass tread 1 1/2 inches thick and 36 inches wide supports up to 1,800 pounds while meeting Quebec building code requirements. The useful question for a supplier is not the thickness of the tread but the clear span it has been engineered for.