The Balustrade Detail That Fails Inspection Even When the Spacing Looks Right

A balustrade can be built to a good standard — evenly spaced balusters, solid fixings, a barrier that looks the part — and still fail final inspection. The reason usually isn't the obvious stuff. It's a triangle most crews never measure.

What the NCC actually requires

Under NCC 2022 Housing Provisions Part 11.3, barriers to stairs, landings, balconies and decks have four separate numeric requirements, and a builder needs to get all four right.

Barrier height (HP 11.3.4(2)). The barrier must be at least 865mm above the nosings of the stair treads. Away from the stairs — on a landing, floor, balcony, deck or verandah — it needs to be at least 1 metre. A transition zone is allowed where the height steps up from 865mm to 1m at the landing.

The 125mm sphere test (HP 11.3.4(4)). No opening in the barrier — including decorative screening — can let a 125mm sphere pass through. For a stairway, this is measured above the nosing line of the stair treads. That last detail is the one that catches people out, and we'll come back to it.

The 40mm edge gap (HP 11.3.4(5)-(6)). Where a barrier is fixed to the vertical face at the edge of a landing, balcony, deck or stairway, the gap between the barrier and that face can't exceed 40mm, measured horizontally from the edge of the trafficable surface to the nearest internal face of the barrier.

No climbable rails on tall falls (HP 11.3.4(8)). Where the fall is more than 4 metres, no horizontal element in the barrier between 150mm and 760mm above the floor can facilitate climbing. That band is exactly where a child's foot finds purchase.

There's also an exception for barriers to non-habitable rooms like attics or storerooms not used daily (HP 11.3.4(7)) — those can use a 300mm sphere instead, or a top-and-intermediate-rail arrangement with openings up to 460mm.

Where it actually goes wrong

Most crews check the sphere test against the regular baluster spacing up the run of the stair — and that part's usually fine, because it's easy to jig and repeat.

What gets missed is the bottom of the flight. Where the bottom rail of the balustrade meets the newel post, the stringer, and the first tread's nosing, there's often a triangular void — not a rectangular gap between two balusters, but an irregular shape formed by three different building elements meeting at once. Because the code measures the sphere test above the nosing line for stairs, that triangle is squarely inside the zone that gets checked. And because it's not a repeating, jig-built opening like the rest of the balustrade, it's the one most likely to be oversized.

A 125mm steel ball — or a good approximation of one — held against that triangle before the certifier arrives will tell you in seconds whether you've got a problem. It's a five-minute check that avoids a call-back.

When the answer's buried in your own plans and specs, AskThePlans finds it and cites the drawing and page. You keep the judgement — it does the finding.

Sources: NCC 2022 · HP 11.3.4(2), NCC 2022 · HP 11.3.4(4), NCC 2022 · HP 11.3.4(5)-(6), NCC 2022 · HP 11.3.4(8). NCC editions vary by state — always check the edition and variations adopted in your state.