The Stair Opening Triangle: Where Guard Compliance Quietly Fails

There is one opening on an open stair that gets missed more than any other: the triangular gap formed by the top of a tread, the face of the riser above it, and the underside of the bottom rail of the guard. On a raking guard, that wedge repeats at every single step. Get the geometry slightly wrong and you do not have one deficiency — you have as many deficiencies as there are treads, all of them visible, all of them measured with a gauge by an inspector who has done this before.

Where the triangle comes from

A guard on a stair follows the rake of the flight as a straight line. The stair itself is a staircase profile — a series of right angles stepping down. Those two geometries do not coincide. Where the raking bottom rail crosses the stepped nosing line, a triangular void opens up at each step, widest at the back of the tread and closing to nothing at the nosing.

The size of that triangle is set by three things: the tread depth, the riser height, and how close the bottom rail sits to the nosing line. Steeper stairs with tall risers produce larger triangles. Shallow monumental stairs with deep treads produce smaller ones. And a bottom rail set 50 mm above the nosing line produces a much bigger triangle than one set tight against it — which is the lever a detailer actually controls.

The sphere rule, stated plainly

In Canadian practice, guard openings are governed by the 100 mm sphere rule: openings must be sized so a 100 mm sphere cannot pass through. It applies to the whole guard, and the standard Canadian reading applies it to the stair triangle as well as to the vertical gaps between pickets. There is no widely applicable Canadian relaxation that lets a larger sphere through at the bottom of a stair guard the way some people expect. If you are designing to the National Building Code, assume 100 mm at the triangle and detail accordingly.

This matters because a great deal of published railing detailing, catalogue content and design reference material originates in the United States, where the model codes have historically permitted a larger sphere — commonly cited as 6 inches, roughly 150 mm — in exactly that triangular region formed by the tread, riser and bottom rail. A picket layout drawn to that allowance will not pass in Ottawa. We work across Canada and into the northeastern US, and this is one of the few genuine detail differences that has to be resolved per project rather than standardised. Confirm the adopted code and edition for the jurisdiction, and where the answer is not obvious, confirm it with the authority having jurisdiction before fabrication rather than after.

The non-climbable zone rakes with the flight

The second thing that goes wrong on stair guards is the non-climbable zone, and it goes wrong because of a persistent confusion between two different numbers. The non-climbable zone is roughly 140 mm to 900 mm above the walking surface — that is the band in which climbable elements such as horizontal rails, wide ledges and projecting members are restricted. It is not 100 mm. The 100 mm figure is the sphere rule, which is a different requirement about opening size. Mixing the two is the most common single error we see on railing submittals, and it produces guards that satisfy one rule while breaching the other.

On a stair, the walking surface is the nosing line, so the non-climbable band tilts with the flight. A horizontal mid-rail drawn parallel to the floor across a raking guard will pass in and out of the restricted zone as it crosses the flight — compliant at one end, a ladder rung at the other. Any horizontal element on a stair guard needs to be checked against the raked band along its full length, not at one convenient point.

One hundred millimetres is the sphere. One hundred and forty to nine hundred is the climb zone. They are different rules, and confusing them is the most common error on railing submittals.

Open risers are a related but separate problem

Open-riser stairs are a staple of feature stair design and they carry their own opening limits. The gap between treads is an opening in the same sense as a gap in a guard, and it is subject to restriction so that a small child cannot pass through it. Requirements vary by code and occupancy, and some occupancies restrict open risers outright, so this is a design-stage question rather than a shop drawing one.

Where open risers are permitted, they interact with the guard triangle. A guard bottom rail set tight to the nosing line closes the triangle but does nothing about the gap behind the tread. If the design intent is a fully open, floating-tread look, the guard detailing and the tread spacing have to be solved together, and the answer is often a partial riser plate or a tighter tread spacing that nobody wanted but the code requires.

Four ways to make the triangle disappear

The problem is entirely solvable at detail stage. In rough order of how often we use them:

• Glass infill. A continuous glass panel following the rake, with the bottom edge close to the nosing line, eliminates the triangle completely because there is no opening anywhere in the guard. This is why glass is the default on feature stairs where the geometry is awkward.
• A raking bottom rail set tight to the nosing line. Bringing the bottom rail down so it sits close to the nosing reduces the triangle to a size the sphere cannot pass. It costs nothing if it is drawn that way from the start.
• A stepped or scalloped bottom rail that follows the stair profile rather than a straight rake. More fabrication work, but it closes the triangle exactly and can look deliberate on the right design.
• Additional short pickets or a closer picket module in the lower zone of the guard. Effective, but it changes the rhythm of the picket layout and is visibly a fix rather than a design.

The cheapest of these is the second one, and it costs nothing at all if the bottom rail height is set correctly on the first shop drawing. Every other option costs money. That is the argument for reviewing this specific detail early rather than assuming the fabricator will handle it.

What it costs to get it wrong

Building inspectors carry sphere gauges and they use them on stairs, because stairs are where the failures are. A guard that fails at the triangle fails at every step of every flight, which means the rework is the entire stair guard, not a corrective piece. Depending on the system, that can mean refabricating raking sections, drilling and adding pickets in a finished coating, or replacing glass panels cut to the wrong bottom edge dimension.

None of that is expensive relative to the building, but all of it happens at the worst possible moment — after the stair is finished, in front of the client, with occupancy pending. Our approach is to resolve the triangle geometry explicitly on the stamped shop drawings, with the sphere check shown at the governing step, so that the reviewer sees the compliance argument before anything is cut. When the geometry is genuinely marginal, a physical mock-up of two treads and a guard section is a cheap way to settle it in advance.