Timber structural grades (SG) — SG6 to SG12 explained

Structural grades tell you the strength and stiffness of the timber you’re ordering — not how it’s treated.

A structural grade (SG) classifies the strength and stiffness of a piece of timber. It’s a different thing from H-class, which tells you the preservative treatment — so a stick carries both, and you need to read both before it goes into a wall.

What the SG numbers mean

The higher the SG number, the stiffer the timber. Here’s how the common grades stack up for NZ framing:

SG6 — lower stiffness and less common. Used where the engineering allows it.
SG8 — the standard for residential framing and the most common grade you’ll handle. It’s the NZS 3604 default.
SG10 — higher stiffness for longer spans and heavy loads. Often used for ridge beams and deeper joists.
SG12 — the highest stiffness, typically reserved for engineering applications as an LVL alternative.

Other grading systems you’ll see

Not every stick on the rack uses the SG system. A couple of others turn up, especially on imported stock:

MGP10 / MGP12 — Machine Graded Pine, an Australian system sometimes seen on imported stock.
F-grades — the older AS system. As a rough guide, F4 ≈ SG6, F5 ≈ SG8, and F8 ≈ SG10. NZ moved from F to SG to align with engineering.

How to read the stamp

NZ timber is stamped on the end-grain with its grade plus treatment. So a stamp reading “SG8 H1.2 NZ001” breaks down like this:

SG8 — the structural grade.
H1.2 — the preservative treatment.
NZ001 — the mill number.

If you can’t see a stamp, reject the timber — treat it as untreated and uncertified.

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.

Do it in Toolie

Put this into practice with the material calculators · quoting with an NZ pricebook. Try Toolie free — no signup — or open this topic in the Toolie app.

Common questions

What’s the difference between SG grade and H-class?

They tell you two different things. The structural grade (SG) classifies the strength and stiffness of the timber, while H-class is the preservative treatment. A piece of timber carries both, so you read both off the stamp.

Which SG grade is standard for house framing?

SG8 is the standard for residential framing and the most common grade you’ll use. It’s the NZS 3604 default. SG6 has lower stiffness and is less common, SG10 handles longer spans and heavy loads like ridge beams and deeper joists, and SG12 is the highest stiffness, typically reserved for engineering applications as an LVL alternative.

How do old F-grades compare to SG grades?

F-grades come from the older AS system. As a rough guide, F4 is about SG6, F5 is about SG8, and F8 is about SG10. NZ moved from F to SG to align with engineering.

What if timber has no visible grade stamp?

Reject it. NZ timber is stamped on the end-grain with its grade and treatment — for example “SG8 H1.2 NZ001” is SG8 grade, H1.2 treatment, mill number NZ001. If you can’t see a stamp, treat it as untreated and uncertified.

More in Materials

All Materials topics → · Full NZ Building Code index

Quote it, comply, get paid — in one app

Toolie turns this knowledge into the job: NZS 3604 take-off, H1 & Healthy Homes, consents, retentions and invoicing — one flat NZD price.

Quote a job free →