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Open riser stairs look modern, but they pass code only when the gap between treads blocks a 4-inch sphere. Here is the exact IRC rule, how to test every step the way an inspector does, and where open risers are banned.
Open riser stairs are the floating-tread look you see in modern homes: each tread sits on its stringers with no vertical board between steps, so light and sightlines pass straight through the stair. The style is popular, but it always raises the same first question: is it legal? Under the International Residential Code, the answer is yes, with one simple condition. The gap between treads must not let a 4-inch sphere pass through. That single rule, often called the 4-inch sphere test, is the line between a passing inspection and a failed one. This guide explains exactly what the code says, how to measure the opening the way an inspector does, where open risers are not allowed, and how to fix a stair that fails the test.
IRC Section R311.7.5.1.1 is short and plain: open risers are permitted provided that the opening between treads does not permit the passage of a 4-inch-diameter (102 mm) sphere. The rest of the stair rules stay in force around it. Riser height still may not exceed 7-3/4 inches, and the greatest riser height in a flight may not exceed the smallest by more than 3/8 inch (R311.7.5.1). In practice, that means you plan the whole flight first, keeping every riser even, and then keep every gap between treads under the 4-inch limit. Running the numbers through a stair calculator before you cut anything is the fastest way to get even riser heights and a legal gap on the first try.
There is one useful exception. When the total rise of the stair is 30 inches or less, the opening between adjacent treads is not limited at all, so a short porch or deck access stair can use wider gaps without failing. The rule exists to keep a small child from slipping through the gap, which is why the sphere size matches the 4-inch sphere rule used for guard openings (R311.7.5.1.1 and R312.4.1). There is a related guard rule worth knowing: the triangular opening formed by a tread, a riser, and the bottom rail on the open side of a stair must not permit the passage of a 6-inch sphere.
The test is simple, but inspectors fail stairs over the details. Work through it in this order.
Measure the right gap. The tested opening is the clear vertical distance between the top surface of one tread and the underside of the tread above it, not the riser height. Nosing projection works in your favor here: the nosing of the upper tread extends 3/4 to 1-1/4 inches past the riser line (R311.7.5.3), which shrinks the tested opening. Just do not count on nosing that is not finished and fixed yet. The nosing calculator page walks through how nosing projection interacts with tread depth if you want the full picture.
Test every step, not just one. Treads are never perfectly identical. A board that cupped as it dried or a tread that was planed thinner during installation can make one gap wider than all the rest, and the inspector only needs one failure.
Use a true 4-inch sphere. A tennis ball is only about 2.7 inches across and a softball is about 3.8 inches, so neither one proves compliance. Buy or make a rigid 4-inch ball or a 4-inch ring template. Push it gently from the most open side, usually the open stringer side.
Test at finished conditions. If carpet, pad, or a finish floor is going on the treads, test after it is installed. Added thickness shrinks the gap, while sanding a tread down widens it.
If the sphere passes through at any step, the stair fails, and the fix belongs in the next section.
Open risers are a residential permission, not a universal one. Keep them out of these situations.
Accessible stairs. The ADA Standards for Accessible Design, Section 504.2, require closed risers on accessible stairways. Any stair on an accessible route, including the required egress path in many commercial buildings, cannot use open risers.
Commercial egress stairs. The Inte
rnational Building Code is written around solid risers for means-of-egress stairs, and most commercial plan reviewers expect closed risers as a matter of course. If the stair serves a business, an apartment common area, or any commercial occupancy, assume open risers will be rejected and confirm with the authority having jurisdiction before you build.
Local amendments. Some jurisdictions delete the open-riser permission or shrink the allowed gap. A quick call to the building department before ordering treads is far cheaper than rebuilding a finished stair.
Rental properties. Even where the code allows open risers, many landlords close them anyway. The liability picture with small children and pets is different from an owner-occupied home, and a closed riser removes the argument entirely.
Four practical fixes, from simplest to most design-forward:
1. Add riser boards. The straightforward fix. A 3/4-inch board glued and screwed between the treads closes the gap completely and adds real stiffness to the stair.
2. Use slatted or routed infill. If you want to keep some openness and light, a set of closely spaced slats or a routed panel keeps every individual opening under 4 inches while the stair still reads as open.
3. Fit thicker treads. Replacing thin treads with 2-inch or thicker stock shrinks the clear gap and stiffens the stair at the same time, which solves two problems with one change.
4. Install tempered glass or acrylic riser panels. These keep the floating look while blocking the sphere. Panels must be thick enough to take a kick and fastened so they cannot be pushed out of their grooves.
One structural caution before you open up an existing closed stair: riser boards do real work. They tie adjacent treads together and help the whole flight resist racking, so a stair designed with closed risers can feel bouncy if the risers are simply removed. If you are converting, add blocking between the stringers or glue-and-screw the treads, then recheck that every riser height still falls within the 3/8-inch uniformity limit after the change. The same 4-inch logic also governs the balusters and guard openings around the stair, as covered in the guide to baluster spacing and code checks.
Light and space. Open risers make small rooms feel larger and let daylight reach the area under the stair.
Child and pet safety. Even a code-passing gap under 4 inches is a spot where small toys roll through, pet paws explore, and little fingers reach. Families with toddlers often close the risers regardless of what the code allows.
Comfort at height. Some people find open stairs unsettling to look down through, especially on a tall flight. A solid handrail and a closed stringer on at least one side usually settles the feeling without giving up the look.
Cleaning. Dust falls through open gaps, which keeps the treads cleaner but means the floor under the stair collects everything. Plan for easy access underneath.
Yes, under the IRC, as long as no opening between treads permits the passage of a 4-inch sphere (R311.7.5.1.1). Stairs with a total rise of 30 inches or less have no opening limit at all.
In most jurisdictions, yes. Removing risers alters both the structural behavior and the code compliance of the stair, so it counts as an alteration that needs a permit and an inspection. Ask before you start pulling boards.
That is a fail. The code says the opening must not permit the passage of a 4-inch sphere, so a 4-inch gap lets the sphere through. Aim for 3-3/4 inches or less to leave a margin for wood movement and finish changes.
Yes, under the IRC, with the same 4-inch sphere rule, and the 30-inch total-rise exception often covers short deck stairs entirely. Outdoor treads shrink and swell with the weather, so leave extra margin and recheck the gaps seasonally.
They can. Riser boards brace the treads, so open-riser stairs need solid stringers and well-fastened treads to avoid a springy feel. If the stair bounces, check the tread fastening and the stringer connections before changing anything else.
Open riser stairs are a legitimate, code-approved design choice, not a gray area. The entire rule comes down to one measurement: nothing between the treads may let a 4-inch sphere through. Measure every gap at finished conditions, remember the 30-inch exception on short stairs, and close the risers wherever accessibility rules or a commercial occupancy apply. Do that, and the floating look passes inspection the first time.
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