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1Balustrade runs
Run
Length
Ends
Posts
Balusters
No runs yet. Add one below.
Maximum post spacingCentre to centre, from the post and rail design. 1800 is a common maximum for timber; steel systems varymm
Post sizeFace width along the railmm
Baluster widthFace width along the rail: 42 for a 42 × 42 timber, 12 to 16 for a steel rodmm
Maximum clear gap125 mm: an opening in a barrier must not let a 125 mm sphere throughmm
–Total balustrade length
–Posts, all runs
–Balusters, all runs
Bays = (length − one post) ÷ maximum post spacing, rounded up. The posts are then set out evenly so every bay is the same, and a run finishing on a wall swaps the end post for a wall fixing. Balusters per bay = (clear bay − gap) ÷ (baluster + gap), rounded up, which gives the fewest balusters that keep every gap at or under the limit; the actual gap is then spread evenly so the set-out is symmetrical in each bay.
2Bay set-out
Show the set-out for
Balusters per bay––per bay
Clear gap between balusters––mm
Baluster positions in the bay, from the face of the first post
No.
Leading edge
Centre
Trailing edge
3Heights, rails & timber
Barrier height1000 mm minimum above the finished deck or floor where a fall is possible; 865 mm on the pitch line of stairsmm
Top rail depthVertical depth of the top rail or handrail the balusters stop undermm
Bottom rail depth0 if balusters fix straight to the deck or a platemm
Gap under the bottom railAlso limited to 125 mm, and where the fall is more than 4 m, nothing climbable between 150 and 760 mmmm
Post lengthAbove the deck plus the fixing below: bolted to a joist or bearer, or set in a stirrupmm
Wastage on rails and balusters%
Baluster length––mm
Rails, top and bottom––lm
Item
Basis
Quantity
Unit
Baluster length = height − top rail − bottom rail − gap under. Rails run the clear bay between posts, top and bottom, so a bottom rail depth of 0 drops the bottom rail line. Fixings, post brackets or stirrups and the handrail on stairs are not counted. The NCC sets the 1000 mm height, the 125 mm gap and the 4 m climbable-zone rule for barriers; a balustrade over a fall must also take the loads in AS 1170.1, which is a post and fixing design question, not a spacing one.
Set-out and estimating guide only. Barrier heights, openings and loads are set by the NCC and AS 1170.1 for the fall height and the building class; post spacing depends on the post, rail and fixing design. Confirm the system with its supplier or the engineer before fabrication.
Calculate spacing for flat run balusters that are face-fixed (attached) to the rail. Accounts for baluster width and attachment method to give compliant 125mm maximum gaps.
Frequently Asked Questions
What is the difference between attached and between-post balusters?
Attached balusters are face-fixed to the top and bottom rails. Between-post balusters sit inside a channel or groove. Attached balusters are easier to replace individually but may require more precise spacing to meet the 125mm gap rule.
Do attached balusters need to meet the same spacing rules?
Yes. The 125mm maximum gap rule applies regardless of attachment method. Whether balusters are attached, slotted, or welded, the clear gap must not allow a 125mm sphere to pass through.
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