125 mm sphere test
The 125 mm sphere test is the NCC opening rule for barriers: no opening may pass a 125 mm sphere (clause 11.3.4(4)). Where it applies and the 300 mm exception.
Ask Chalkline about this →The 125 mm sphere test is the dimensional rule the NCC sets for openings in a residential barrier: no opening may permit a 125 mm sphere to pass through (NCC 2022 Volume Two, Housing Provisions clause 11.3.4(4), verified 2026-05-29 via NCC stairs and balustrades). It is the rule that drives baluster spacing on stairs, decks, balconies, and any other barrier protecting against a fall of 1 m or more.
Where it is measured
On a flat barrier (deck, balcony, mezzanine), the test is measured anywhere openings exist in the barrier face: between vertical balusters, between rails, around penetrations.
On a stair barrier, the test is measured above the nosing line of the treads, not from the floor. The triangular opening between the riser, the going, and a low barrier rail can pass a 125 mm sphere even when the vertical baluster spacing is fine; the sphere has to fail to pass everywhere along the barrier line.
The 300 mm exception
For barriers serving non-habitable rooms (storerooms, garages, attics), the NCC relaxes the opening rule to a 300 mm sphere under clause 11.3.4(7). This concession does not apply to any barrier where a child would reasonably have access; the 125 mm sphere rule applies by default in any space normally used or accessed by occupants.
Where the rule fails on site
The recurring defect at PCI is baluster spacing set by eye or with pre-cut spacers rather than physically tested with the sphere. A nominal 100 mm spacing in the design can drift to 130 mm under field tolerances, and the inspector’s sphere passes through. Two practical points:
- Test with the actual sphere. A 125 mm sphere (or any 125 mm circular template) is the only authoritative check.
- Mind the climbability zone. Where the fall exceeds 4 m, horizontal elements between 150 mm and 760 mm above the floor are restricted by clause 11.3.4(8) regardless of the sphere test.
Also known as: 125 mm sphere rule, baluster opening test.
Category: Compliance / barriers
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Last updated: 2026-05-29. Verified: 2026-05-29. Quarterly review for currency.