NZ timber sizes: nominal vs actual

Why you order “4x2” but it turns up at 90×45 — and which size does what on a NZ build.

Every NZ builder orders timber by its nominal call size — “4x2” or “100x50” — but it arrives at the actual dressed size, “90x45”. Both names point at the exact same stick, so it pays to know which is which before you quote or order. These standard sizes follow MiTek NZ and NZS 3604.

Why there are two sizes for the same stick

When timber is rough-sawn it’s cut to the nominal size — say 100×50mm. Once it’s dressed (planed smooth on all four sides) it loses about 10mm, so it finishes at 90×45mm. NZ merchants stock the dressed size but list it under the nominal call size on quotes and orders, which is why the two numbers travel together.

Common sizes and what they’re used for

Here’s how the everyday sizes line up — imperial call, nominal, actual dressed size, and the jobs they usually cover:

3x2 — 75x50 nom / 70x45 actual: studs, noggings, dwangs, battens.
4x2 — 100x50 nom / 90x45 actual: wall framing, studs, plates, light joists.
6x2 — 150x50 nom / 140x45 actual: joists, rafters, lintels, deck joists.
7x2 — 175x50 nom / 170x45 actual: joists, rafters. Less common — prefer 6x2 or 8x2.
8x2 — 200x50 nom / 190x45 actual: joists, rafters, bearers (doubled), beams.
10x2 — 250x50 nom / 240x45 actual: large joists, bearers (doubled), heavy lintels.
12x2 — 300x50 nom / 290x45 actual: large joists, heavy bearers (doubled), beams.

The thicker 90mm stock gets used less often, and usually has a doubled-up alternative:

4x4 — 100x100 nom / 90x90 actual: posts (above ground), light beams, light pergola. Or use H5 in-ground.
6x4 — 150x100 nom / 140x90 actual: heavy posts, solid deck bearers, H4 ground-contact. Or double up 6x2 nailed (NZS 3604 cl. 2.4.4.7).
8x4 — 200x100 nom / 190x90 actual: heavy bearers, large posts. Or double up 8x2 nailed.
10x4 — 250x100 nom / 240x90 actual: max-span solid bearers, heavy beams. Or double up 10x2 nailed.

NZS 3604 cl. 2.4.4.7 lets you nail up two 45mm members as a substitute for the solid 90mm section — same load capacity, half the cost.

Common pairings on a NZ deck or build

Most builders talk in imperial on site. Here’s how the everyday jobs pair up with size, treatment and grade:

Studs / wall framing: 4x2 (100×50 nom / 90×45 actual) H1.2 SG8 at 600mm centres.
Wet area framing: 4x2 (90×45) H3.2 SG8 — bathrooms, laundries, exterior walls.
Floor joists (residential): 8x2 (200×50 nom / 190×45 actual) H1.2 SG8 at 400mm centres typical.
Deck joists: 6x2 (150×50 nom / 140×45 actual) H3.2 SG8 at 450mm centres for 32mm decking.
Deck bearers (preferred): doubled 8x2 (190×45) H3.2 nailed up — NZS 3604 cl. 2.4.4.7 — half the cost.
Heavy single-piece bearer: 10x4 (250×100 nom / 240×90 actual) H3.2 — for max 2.7m spans only.
Posts (in-ground): 4x4 (100×100 nom / 90×90 actual) H5 — pergola, deck pile, fence.
Heavy posts: 6x4 (150×100 nom / 140×90 actual) H5 — engineered loads only.
Pergola purlins: 3x2 (75×50 nom / 70×45 actual) H3.2 — NZS 3604 min for clearlite.
Deck boards: 6x1¼ (140×32) H3.2 pine standard, or 6x¾ (140×19) for a narrow profile.

A note on 135×45

You’ll sometimes see 135×45 called for in older specs, but it’s not a real NZ stock size — the yard will give you 140×45 (6x2) instead. Toolie auto-converts 135×45 to 140×45 so your order matches what actually turns up.

Plain-English guide, not advice. This page helps you understand and navigate the rules — it is general information, not design, engineering or consent advice, and it does not reproduce the copyrighted tables of NZS 3604 or any Standard. Always check the current Standard or Acceptable Solution and your BCA, and use a suitably qualified LBP, engineer or QS where it matters.

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Common questions

Why does 4x2 timber arrive at 90x45?

Because "4x2" (100x50) is the nominal, rough-sawn size. Once the timber is dressed — planed smooth on all four sides — it loses about 10mm and finishes at 90x45mm. NZ merchants stock the dressed size but list it under the nominal call size on quotes and orders, so both names refer to the same product.

Can I use two nailed-together members instead of one solid bearer?

Yes. NZS 3604 cl. 2.4.4.7 lets you nail up two 45mm members as a substitute for the solid section — same load capacity, at about half the cost. For example, a doubled 8x2 (190x45) H3.2 nailed up is the preferred deck bearer, and doubling 6x2, 8x2 or 10x2 replaces the equivalent 90mm-thick stock.

What size timber do I use for deck joists and bearers?

For deck joists, 6x2 (150x50 nom / 140x45 actual) H3.2 SG8 at 450mm centres suits 32mm decking. For bearers, the preferred option is a doubled 8x2 (190x45) H3.2 nailed up under NZS 3604 cl. 2.4.4.7. A heavy single-piece bearer of 10x4 (240x90) H3.2 is for max 2.7m spans only.

Is 135x45 a real NZ timber size?

No. 135x45 is sometimes called for in older specs, but it isn't a real NZ stock size — the yard will give you 140x45 (6x2) instead. Toolie auto-converts 135x45 to 140x45 so your order matches what's actually stocked.

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