Balustrades and F4 barriers: the rules that catch builders out

NZBC F4/AS1 — Safety from Falling — is the clause that decides where you need a barrier and how it has to perform. The short version: whenever someone could fall 1m or more, a barrier is required, and the rules cover height, climbability, gap size and structural loading. Deck balustrades, stair balustrades and Juliet balconies are all governed here, so getting these numbers right keeps the job through inspection.

How high the barrier has to be

Height is measured vertically from the finished floor (or the step nosing on stairs) to the top of the rail.

• 1.0m minimum on housing decks, balconies and landings.
• 1.1m minimum for commercial and communal areas, such as an apartment common deck or a hotel.
• A stair handrail sits at 900mm — and that’s the handrail, not the balustrade. The handrail is a separate, lower element.

The 100mm sphere rule

In housing and areas children use, no opening in a barrier shall allow a 100mm sphere to pass through, over the full height of the barrier. That single rule drives most of your detailing.

• Vertical pickets need a clear gap of 90mm or less — with an allowance for tolerance, they’re often spec’d at 80–85mm centres.
• Horizontal rails are not allowed in this zone because they’re climbable — see the no-toehold rule below.
• A glass balustrade passes automatically, because a solid panel has no opening.
• A cable balustrade needs careful spacing and tension, because cables splay over time.

Stair triangle exception: in the triangular opening formed by the tread, the riser and the bottom of the stair barrier, the sphere can be up to 150mm. This is the only relaxation — it recognises the practical geometry of stair barriers.

The no-toehold (climb zone) rule

Kids climb barriers, so F4 bans footholds in the danger band.

• No footholds or toeholds between 150mm and 760mm above the finished floor.
• Horizontal rails are allowed below 150mm (a low rail) and above 760mm (the top rail) — just not in the climb zone.
• Bars and pickets must be vertical, not horizontal, through the climb zone.
• Common fail: a ‘ranchslider’ style horizontal-rail balustrade — illegal in housing.

Structural loading

The barrier has to actually stop a person, so it’s designed to carry a load — and the connection back to the deck must transfer it. That connection is a common failure point.

• 0.35 kN/m horizontal point load at the top for a residential balustrade.
• 0.6 kN/m for commercial or public barriers.
• 3 kN/m² crowd loading on barriers for commercial areas with crowds.

Common balustrade types

• Timber pickets: 90×42 vertical pickets at 100mm centres (90mm gap or less). Hot-dip galv or stainless fixings.
• Steel rod / tube: 12–16mm rods welded to top and bottom rails. Galv or powder-coated.
• Glass (frameless): 12mm toughened or 13.5mm laminated, on stainless spigots through the deck. Premium look.
• Cable balustrade: 3–4mm stainless cable with tensioners. Verify the sphere can’t pass, because cables splay.
• Aluminium picket panels: pre-fab panels (Yokey, ScarboroFair). Easier install.