Floating Concrete Steps: How They Are Built

The two kinds of floating step

Recessed-base steps. Each step is a concrete slab sitting on a smaller base, a plinth or a short wall, set back from the front and sides. The overhang throws a shadow and the step seems to float above the ground. Structurally, every step is still supported from below. Only the overhanging lip is a cantilever, and it is short.

Cantilevered treads. Each tread is fixed at one end into a wall or a central spine and is unsupported along its length. A US patent for an open staircase describes the type plainly: a tread "may be cantilevered from a single stringer or a wall. These are known as floating staircases." In poured concrete they are an engineered job, for reasons that follow.

Most outdoor "floating concrete steps" in garden and entry photos are the first kind. If yours is, everything in how to build concrete steps still applies, with the changes below.

The one rule that changes: steel goes on top

A normal slab on the ground, or a beam supported at both ends, sags in the middle and cracks on the bottom, so the steel belongs near the bottom. A cantilever does the opposite. Princeton's civil engineering teaching material puts it simply: in a cantilever "the top face is in tension, in contrast to the simply supported beam where the bottom face is in tension." Princeton CASCE uses sponges to show it, and the rule it teaches is that the steel goes wherever the concrete is in tension.

So the overhanging lip of a floating step wants its bars near the top surface, running back over the base far enough to anchor. Bars that sag to the bottom of the slab during the pour do nothing in the overhang. Hong Kong's Buildings Department, which regulates a lot of cantilevered concrete, makes two points in its practice note on cantilevered slabs that apply at any scale: every effort should be made to keep bars from being "displaced or depressed," and the main bars must be properly anchored into the supporting structure. Tie the steel to chairs and check it before the concrete goes in.

Cover still applies. IRC R403.1.3.5.3 requires 1-1/2 inches of concrete over No. 5 and smaller bars exposed to weather and 3 inches where concrete is cast against earth, per the ICC code text. On a thin tread, 1-1/2 inches of top cover plus the bar plus some concrete below it sets a practical minimum thickness. No standard publishes a minimum thickness for a floating tread. The one manufacturer figure we found is Wausau Tile's 4 inch minimum for its self-supporting precast treads, and those span between stringers rather than cantilevering.

Building recessed-base floating steps

There are two practical ways to get the recess, and the second is easier for most people.

• One pour with a foam block-out. Form the full step, then fix rigid foam inside the form where the recess will be, so concrete fills around it. EPS foam needs a release agent made for it. Nox-Crete, for example, makes a foam form release specifically for expanded polystyrene block-outs, because ordinary form oil is not meant for foam.
• Two pours. Pour the bases first, as small walls or blocks set back from where the step edges will be. Then form and pour each tread slab on top, with dowels or bars coming up out of the base to tie it down. The underside of the overhang is formed with a board that comes out after the concrete has gained strength.

Either way, the formwork under the overhang is carrying load, which puts it in a different category from the side forms. OSHA's concrete rule, 29 CFR 1926.703(e)(1), says forms and shores other than those for slabs on grade stay in place until the concrete "has gained sufficient strength to support its weight and superimposed loads." The riser faces on ordinary steps come off in hours so they can be finished. The support under an overhang should stay for days. For comparison, ACI 347 gives about four days for a short one-way slab, counted in time above 50 degrees F.

Cure it well. Thin overhangs dry from the top, the edge and the underside at once. ACI's guide to floor and slab construction, as quoted by the American Society of Concrete Contractors, recommends curing for 7 days above 40 degrees F. Cure time covers the methods.

The base still needs to be on solid ground. For steps attached to a house in a frost climate, that means a footing below the frost line under IRC R403.1.4.1, which requires "foundation walls, piers and other permanent supports" to be protected from frost, typically by extending below the local frost depth. The footing calculator sizes it.

True cantilevered treads need an engineer

The residential code has no prescriptive recipe for cantilevered concrete stairs. When a house contains structural elements beyond what the code prescribes, IRC R301.1.3 says they must be "designed in accordance with accepted engineering practice," which in practice means a stamped design. Seattle's 2021 residential code chapter 3 reproduces the section.

The loads are not trivial either. IRC Table R301.5 sets 40 pounds per square foot for stairs, and a footnote requires each individual tread to carry a 300 pound concentrated load over 4 square inches, whichever is worse. A tread cantilevered 4 feet out from a wall has to carry that 300 pounds at its far end, and all of that moment lands where the bar enters the wall. That is the calculation an engineer is checking, and it depends on the wall as much as the tread.

Open risers have their own rule. Under IRC R311.7.5.1, open risers are allowed only if openings more than 30 inches above the floor or grade below "do not permit the passage of a 4" diameter sphere," per Placer County's stair handout. A tall flight of floating treads often needs a riser plate or a lower gap to pass.

The dimensions are the same as any other step

A floating look does not change the stair rules for steps that serve a door: risers no taller than 7-3/4 inches and within 3/8 inch of each other, treads at least 10 inches, a 36 inch landing at the door, a handrail at four risers, and treads sloped no more than 2 percent. The stairs calculator has the full set and works out the concrete volume. For a recessed-base design, run the calculator for the full step shape, then subtract the volume of the recess.

Floating steps also tend to be designed with wider treads and shallower risers than code minimums, because the look depends on long horizontal lines. That works in your favor. A 6 inch rise and a 14 inch tread is well inside the rules, and it fits the US Forest Service's outdoor rule that twice the riser plus the tread should come to 25 to 27 inches.