NZ TIMBER EDITORIAL GUIDE
Fence Rail Sizes NZ
A rail is not selected by section size alone. Its job depends on the distance between posts, whether it is fixed on edge, the fence height, the number and weight of palings, wind exposure and how every joint is supported.
75 × 50 and 100 × 50 are product families, not span promises
The current catalogue contains H3.2 rough-sawn rail families at 75 × 50 mm and 100 × 50 mm, with set lengths including 2.4, 3.6, 4.8 and 6.0 m depending on the product. Those dimensions describe geometry. They do not, by themselves, state a permitted span for every fence.
Public NZ specifications show how the complete assembly matters. Kāinga Ora specifies 100 × 50 rails for its published timber-fence details, using two rails for a 1.2 m fence and three for a 1.8 m fence. Ministry of Education guidance for a close-boarded school fence also uses three 100 × 50 rails fixed on edge, at a maximum 900 mm rail spacing, with approximately 2.4 m post centres. These are defined systems for particular uses, not permission to transplant one member into an unrelated layout.
| Catalogue family | Relative depth on edge | Useful comparison questions |
|---|---|---|
| 75 × 50 H3.2 rail | 75 mm | Is it part of a tested or specified system? What are post centres and fence height? |
| 100 × 50 H3.2 rail | 100 mm | Does the detail require the deeper rail, three rows, halving joints or bolts? |
| Other structural section | varies | Is it graded, treated and approved for the proposed substitution? |
Orientation changes how a rectangular rail behaves
A rectangular rail is normally strongest against horizontal bending when its greater dimension is vertical—commonly described as fixed ‘on edge’. Rotating a 100 × 50 rail flat is not an equivalent arrangement. It also changes the surface available for paling fixings, water retention and the way a halving joint reduces the section.
Face-fixing a rail across the post, housing it into the post, or cutting it between posts produces different load paths and fixing requirements. Predrilling near ends can reduce splitting, but fixing size, coating and pattern should come from the chosen detail rather than a generic nail rule.
Two rails, three rails, and where movement begins
A low fence may use two rails in a published system; a 1.8 m close-boarded fence commonly uses three. The extra row reduces the unsupported paling length and helps control bowing or twist. A heavier 25 mm paling, lapped arrangement or cladding on both sides changes the demand again.
- Check the distance from each paling end to the nearest rail. Long free ends are more likely to move.
- Place the middle rail to control the central portion of a tall paling, not simply where it looks balanced from one side.
- Allow for the bottom clearance required by the selected fence detail; avoid leaving H3.2 palings sitting in soil.
- At a stepped fence, confirm that every shortened paling still connects to the required number of rails.
- Gate posts carry hinge and latch loads. A rail terminating beside a gate does not turn an ordinary line post into a suitable gate post.
Wind exposure is not solved by adding a rail in isolation. Post size, embedment or footing, post centres, cladding permeability and connections all contribute. On open or high-wind sites, get the assembly checked rather than assuming the next rail size is enough.
Convert rail rows into stock lengths
A 24 m fence with three rows contains 72 lineal metres of rail before waste. Fifteen 4.8 m lengths also total 72 m, but that arithmetic works only if the post layout and supported joints allow every length to be used efficiently. With ten 2.4 m bays, each 4.8 m piece may cover two bays in a compatible continuous detail. A gate, corner or 2.25 m bay changes the cut schedule.
| Run | Rows | Net rail | Initial stock check |
|---|---|---|---|
| 12 m | 2 | 24 LM | 5 × 4.8 m before cut allowance |
| 18 m | 3 | 54 LM | 12 × 4.8 m gives 57.6 LM |
| 24 m | 3 | 72 LM | 15 × 4.8 m if the joint layout works |
Order by the chosen SKU and length, not only by lineal metres. Retain a cut list so the supplier can identify alternatives without silently changing rail section, treatment or grade.
A rail decision that can be quoted
- Fix the fence height, cladding pattern and post centres.
- Select a published or designed rail section, orientation, row count and connection detail.
- Draw every supported joint, corner and gate termination.
- Convert each row into a length-by-length cut list.
- Confirm H3.2 identification, actual dimensions, stock lengths, current price and freight with the merchant.