Deck railing height and balustrade rules (AU and US)
In Australia a deck needs a balustrade once someone could fall 1 m or more, and it must be at least 1000 mm high with no gap that lets a 125 mm sphere through (NCC 2022 Housing Provisions Part 11.3). In the US a deck needs a guard once the walking surface is more than 30 in. above grade, and it must be at least 36 in. high with no opening that passes a 4 in. sphere (IRC R312). Along stairs the minimum drops to 865 mm in Australia and 34 in. in the US.
When do you need a balustrade on a deck?
Australia
Where a person could fall 1 m or more from the deck, stairs or landing to the surface below (NCC 11.3.3). Below 1 m the NCC doesn’t require one.
Measure from the deck surface to the lowest ground next to each open edge. On a sloping block one side can be over 1 m while another isn’t.
United States
Where the open side is more than 30 in. above the floor or grade below, measured at any point within 36 in. of the edge (IRC R312.1.1; R321.1.1 in the 2024 edition).
A deck exactly 30 in. up doesn’t need one under the model code, but ground that drops away within 3 ft of the edge can tip it over.
Plenty of people add a railing to a lower deck anyway, especially with young children around. If you do, building it to the same rules costs little extra.
How high does a deck railing need to be?
| Where | Australia | United States |
|---|---|---|
| Deck, landing or balcony | At least 1000 mm | At least 36 in. |
| Along stairs, above the nosings | At least 865 mm | At least 34 in. |
| Stair handrail | At least 865 mm | 34 to 38 in. |
Height is measured from the deck surface, or from the line of the stair nosings, to the top of the rail. Pools follow their own barrier rules (in Australia, NCC H7D2 and AS 1926.1), and some states and cities amend these figures, so check with your certifier or building department.
What is the maximum baluster spacing?
The rule is about a sphere, not a baluster gap as such. No opening anywhere in the railing, whether between balusters, under the bottom rail or between wires, may let a 125 mm sphere through in Australia or a 4 in. sphere in the US.
- The gap under the bottom rail counts. Deckulator draws its bottom rail 75 mm (3 in.) off the deck to stay well inside the limit.
- In Australia, a barrier fixed to the edge of the deck must sit no more than 40 mm from it (NCC 11.3.4).
- In the US, the open side of stairs may use a 4⅜ in. sphere, and the triangle formed by the tread, riser and bottom rail a 6 in. sphere.
Aim a little under the limit, because timber moves and parts get knocked. Deckulator spaces balusters so no clear gap is more than 120 mm in Australia or about 3.8 in. in the US.
Worked example: how many balusters between two posts?
Australia
- 90 mm posts at 1800 mm centres leave 1710 mm clear.
- With 42 mm balusters and gaps of up to 120 mm: (1710 − 120) ÷ (42 + 120) = 9.8, so 10 balusters.
- Gaps: (1710 − 10 × 42) ÷ 11 = 117 mm, under 125 mm.
United States
- 4×4 posts (3½ in. actual) at 6 ft centres leave 68½ in. clear.
- With 2×2 balusters (1½ in. actual) and gaps up to 3.8 in.: (68.5 − 3.8) ÷ (1.5 + 3.8) = 12.2, so 13 balusters.
- Gaps: (68.5 − 13 × 1.5) ÷ 14 = 3.5 in., under 4 in.
The post spacing here is Deckulator’s default layout, not a code figure. The posts, rails and their fixings must carry the barrier loads: in Australia those come from AS/NZS 1170.1, and in the US a 4×4 guard post must not be notched where it is bolted on (IRC R507.10.2), with the joist behind it tied to its neighbours so it can’t roll (R507.10.1).
Can I use wire or horizontal rails?
Australia
Wire balustrades are deemed to comply when built to the tables in NCC 11.3.6, which tie wire size (2.5 to 4.0 mm), wire spacing (60 to 100 mm) and post spacing (600 to 2500 mm) to the wire tension. Deckulator uses wires 80 mm apart on posts 1200 mm apart.
Where the fall is more than 4 m, nothing a child could climb (horizontal or near-horizontal parts) is allowed between 150 and 760 mm above the deck. That rules out horizontal wires and rails on high decks; use vertical balusters, slats or glass.
United States
The IRC has no climbing rule, so horizontal cable is common. Cables still have to stay inside the 4 in. sphere rule when pushed apart. Deckulator follows the usual practice of cables at 3 in. centres on posts no more than 4 ft apart; use the cable maker’s spacing if it differs.
What about glass balustrades?
Use toughened safety glass in a tested spigot or channel system, installed to the maker’s instructions. The sphere rule still applies to the gaps under and between panels.
Stair handrails and balustrades
A handrail is a different thing from a balustrade. In Australia stairs need a handrail on at least one side once they rise 1 m or more; in the US, once a flight has four or more risers. The deck stairs guide covers rise, going and handrails in detail.
Common questions
How high does a deck balustrade need to be in Australia?
Under NCC 2022 Housing Provisions Part 11.3, at least 1000 mm above the deck wherever someone could fall 1 m or more. Along stairs it must be at least 865 mm above the nosings.
How high does a deck railing need to be in the US?
The IRC requires guards at least 36 in. high where the deck is more than 30 in. above grade, and at least 34 in. above the nosings along stairs. Stair handrails go 34 to 38 in. high.
What is the maximum gap between balusters?
No opening may let a 125 mm sphere through in Australia, or a 4 in. sphere in the US. That includes the gap under the bottom rail, so aim for clear gaps a little under those sizes.
Do I need a balustrade on a deck under 1 m high?
The NCC only requires one where someone could fall 1 m or more, measured to the ground below each edge. Check the lowest point next to the deck, and check with your council, as local rules and pool barriers differ.
Can I use horizontal wires on a high deck in Australia?
Not where the fall is more than 4 m. The NCC then bans climbable horizontal or near-horizontal parts between 150 and 760 mm above the deck, so use vertical balusters, slats or glass.
This guide explains common rules in plain words. It isn’t engineering advice: span tables, footings and barriers depend on your timber grade, soil, wind and local rules. Have a builder, certifier or building department check your design before you build.