SG6, SG8, SG10, SG12: how NZ sawmills produce and brand structural timber grades

If you’ve been in the NZ timber industry for more than a decade, you remember when structural grades had different names depending on how the timber was graded. Machine stress graded timber was branded MSG8, MSG10, MSG12. Visually graded timber was branded VSG8, VSG10. No.1 Framing existed as a separate category entirely. The customer had to understand three parallel systems to order the right product.

In 2011, that changed. The industry amalgamated the grading brands into a single SG (Stress Grade) system. MSG8 and VSG8 both became SG8. MSG10 became SG10. No.1 Framing mapped roughly to SG6. The grading methods stayed the same. The branding simplified.

This guide covers how the SG system works from the mill floor, what each grade actually means mechanically, how grading is done in practice, and how grade data should flow into your inventory and dispatch systems.

What the SG number means

The number in the grade designation is the characteristic modulus of elasticity (MoE) in gigapascals (GPa). SG8 timber has been verified to have a characteristic MoE of at least 8.0 GPa. SG10 is at least 10.0 GPa. The higher the number, the stiffer and stronger the timber.

In practical terms:

SG6 (6.0 GPa MoE): The lowest structural grade. Suitable for non-critical framing, blocking, dwangs, and light structural applications where the spans are short and loads are low. Previously branded as No.1 Framing or VSG equivalent.

SG8 (8.0 GPa MoE): The standard NZ framing grade. This is the default specification for residential wall frames, roof trusses, and floor joists in NZS 3604 timber-framed construction. The vast majority of structural radiata pine leaving NZ sawmills is graded SG8. When a builder orders “framing timber” without specifying a grade, they mean SG8.

SG10 (10.0 GPa MoE): Higher-performance structural timber used where longer spans, heavier loads, or engineering specifications demand better mechanical properties. Common in engineered applications, commercial construction, and specific residential situations where an engineer has specified it.

SG12 (12.0 GPa MoE): The highest standard stress grade. Used in demanding structural applications, often in engineered wood products (LVL, glulam) or where the engineer needs high-stiffness timber for specific load paths.

Beyond MoE, each grade also has associated characteristic values for bending strength, compression, tension, and shear. These are tabulated in NZS 3603:1993 (now superseded by AS/NZS 1720.1:2022) and are what structural engineers use when designing to a specific timber grade.

How grading works: machine vs visual

There are two methods for grading structural timber in NZ, and both produce timber that carries the same SG brand.

Machine stress grading (MSG)

A grading machine measures the stiffness of each piece of timber as it passes through. The most common machines use a bending principle: the timber is fed through rollers that apply a controlled deflection, and the resistance to bending is measured. The machine calculates the MoE from the deflection and either accepts or rejects the piece for its target grade.

Machine grading is fast (processing speeds of 100-200+ lineal metres per minute depending on the machine), consistent, and objective. It grades every piece individually based on its actual mechanical properties. A piece that passes SG8 on the machine genuinely has the stiffness to perform at that grade.

The machine operator’s role is to ensure the machine is calibrated correctly, to handle rejected pieces (downgrading them to a lower SG or rejecting them from structural use entirely), and to verify that the branding equipment is marking pieces correctly.

Visual stress grading (VSG)

A trained grader visually inspects each piece of timber and assesses it against the grading rules in NZS 3631. The grader looks for characteristics that reduce structural performance: knot size and position, slope of grain, wane, splits, distortion, and the presence of compression wood.

Visual grading is slower than machine grading and relies on the skill and consistency of the grader. It’s most commonly used by smaller mills that don’t have machine grading equipment, or for timber types and dimensions that aren’t suited to machine grading.

Under the amalgamated system, visually graded timber carries the same SG brand as machine graded timber. A piece visually graded to SG8 rules can be used interchangeably with a machine graded SG8 piece. The structural performance expectation is the same.

Verification

Both grading methods operate under a verification regime. In NZ, the Verified Timber Programme (administered by the NZTIF) provides third-party verification that a mill’s grading operation consistently produces timber that meets the claimed grade. Mills participating in the programme are audited regularly, and their graded timber can carry the “Grade Verified” or GV mark.

For sawmill operators, maintaining verification involves keeping grading records, machine calibration logs, and making audited sample pieces available for testing. The verification programme is voluntary but increasingly expected by the market, especially for export and commercial construction supply.

The grade stamp

Every piece of graded structural timber should carry a grade stamp. The stamp includes the stress grade (SG8, SG10, etc.), whether the timber is seasoned (KD for kiln dried), the grading standard reference (AS/NZS 1748 for machine graded, NZS 3631 for visually graded), and the grading organisation’s identifier.

The grade stamp is applied by ink stamp, burn brand, or sticker during the grading process. For machine grading, the branding is usually automated and applied as part of the grading line. For visual grading, the grader stamps each piece manually.

Getting the stamp right matters. A piece of timber without a grade stamp, or with an illegible stamp, cannot be sold as graded structural timber. It reverts to ungraded, which has significantly lower value and limited structural applications.

How grading connects to your inventory

Structural grade is one of the primary attributes that defines a timber product in your inventory. A pack of SG8 and a pack of SG10 in the same dimensions and treatment class are different products with different pricing and different customer applications.

At pack creation

When a pack comes off the grading line, the grade should be recorded at the pack level. If the pack contains a single grade (which it should), the pack label carries that grade. If the grading line has produced mixed-grade output within a single pack (which indicates a process problem worth fixing), the pack should be labelled with the lowest grade present.

In the yard

Grade segregation in the yard is less error-prone than treatment class segregation because grade doesn’t affect visual appearance. An SG8 pack and an SG10 pack of the same species, treatment, and dimensions look identical. The only identifier is the pack label and the grade stamps on individual pieces.

This means your barcode scanning workflow is the primary mechanism for ensuring the right grade gets dispatched to the right customer. A forklift operator pulling packs for a dispatch cannot distinguish SG8 from SG10 visually. They need a pack ID to scan and a system that confirms the pack matches the order.

At dispatch

Grade is a critical field on dispatch dockets. The customer’s order specifies a grade, and the dispatch docket must confirm the grade of every pack shipped. If a customer orders SG10 and receives SG8, the timber doesn’t meet the structural specification their engineer designed to, and the mill is liable for the replacement.

The reverse error (shipping SG10 when SG8 was ordered) is less consequential structurally since SG10 exceeds SG8 performance, but it costs the mill money. SG10 commands a premium, and giving it away at SG8 pricing erodes margin.

Douglas fir grading

Douglas fir exhibits more uniform properties within the stem than radiata pine, which means a higher proportion of Douglas fir timber achieves the higher stress grades. It’s common for Douglas fir to grade predominantly into SG8 and SG10, with less SG6 downgrade than a typical radiata run.

The grading methods and standards are the same. Douglas fir is graded under the same NZS 3631 (visual) and AS/NZS 1748 (machine) standards as radiata.

Common grading issues

Machine calibration drift. Grading machines need regular calibration checks. If the machine drifts, it may over-grade (passing SG8 pieces that should be SG6) or under-grade (rejecting good pieces). Both cost money. Over-grading is the more dangerous error because it produces non-conforming structural timber.

Mixed grades in a single pack. This happens when the grading line output isn’t properly segregated by grade before packing. The fix is mechanical: clear separation bins or conveyors for each grade, with proper signage and operator discipline.

Missing or illegible grade stamps. Often caused by worn ink rollers, incorrect stamp alignment, or stamps applied to wet timber that smears before drying. Check stamp quality regularly during a grading run. A pack of unstamped timber is worth significantly less than the same pack with clear stamps.

Not tracking grade in inventory. Some mills track species, treatment, and dimensions but don’t record grade at the pack level in their inventory system. This means every dispatch requires someone to physically check the grade stamps on individual pieces in the pack. At volume, this becomes a bottleneck that proper pack-level grade tracking eliminates. Grade errors also compound at stocktake time when ungraded packs can’t be reconciled against orders.