Building: Class 5 office, new internal spiral stair serving Level 6
Location: Inner Melbourne
NCC edition: NCC 2022, Volume One
Area of NCC requirements: D1P2 safe movement; D3D14 stair treads and risers (Table D3D14)
Raised by: The relevant building surveyor, after site measurement of the installed stair
Year: 2026
The challenge
The shop drawings showed a 250 mm going. Measured on site at the D3D14(2) line, 270 mm in from each side, the installed tapered treads were about 245 mm. That is roughly 5 mm under the 250 mm minimum for a public stair. Risers, the 2R + G formula and tread to tread variation all complied. Replacing a fabricated spiral stair would have been costly and disruptive.
The performance solution
Expert judgement, supported by comparison with the Deemed to Satisfy Provisions, assessed the shortfall against D1P2. The report was frank about the risks: most stair falls happen on descent, the going narrows further toward the inner string, and an office stair carries peak time traffic and visitors who do not know its geometry. It then credited the measures already in place:
going measured and quantified on site, not estimated
high contrast nosing strips on every tread
solid treads and risers, no open risers
risers and goings kept within the D3D14 variation limits
a continuous handrail to D3D22, and adequate lighting
Because an office stair gets heavier and less predictable use than a residential one, the report recommended further measures: a tread and nosing finish with better than minimum wet slip resistance, a second handrail on the inner string, six monthly inspection of nosing wear, a caution sign at the top landing for visitors, and photoluminescent nosings for after hours use.
Outcome
Report issued in August 2026 and successfully accepted by the relevant building surveyor.
Lesson for builders
Check stair goings against the D3D14(2) measurement line at shop drawing stage, not after installation. A few millimetres of fabrication tolerance can turn a compliant stair into a departure.
Related: Performance solution report | What is a performance solution?
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