NZ Timber Grades Explained: How to Choose the Right Wood for Your Project

When you’re planning a deck, pergola, garden beds or a new build, the timber jargon can get confusing fast:

SG8? H3.2? Structural vs appearance grade? 

If you’re not a builder, it can feel like another language.

At TNT Milling, we want our Marlborough customers to feel confident about what timber to choose, where to use it, and why it matters. This guide breaks down the main New Zealand timber grades in simple terms, and how to match them to real-world projects.

What does “timber grading” actually mean?

In New Zealand, timber is graded so builders (and councils) know how strong it is and where it can safely be used.

The Building Code and standards such as NZS 3604 Timber-Framed Buildings allow only verified structural grades, such as SG6, SG8, and SG10, for building frames and load-bearing structures.

MBIE’s guidance also explains there are three main ways timber is graded:

• Machine stress graded (MSG) – a machine tests the stiffness and sorts timber by structural performance

• Visual stress-graded (VSG) – a trained grader checks knots, grain, cracks, etc.

• Visually graded, unverified – used where structural strength isn’t critical (e.g. some non-structural use).

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Who is MBIE and Why Do They Matter When Choosing Timber?

You’ll often see MBIE mentioned in building and timber documents across New Zealand. MBIE stands for the Ministry of Business, Innovation and Employment, a New Zealand Government department responsible for overseeing the Building Code and construction standards.

In simple terms, MBIE helps make sure that the materials used in New Zealand buildings, including timber, are safe, fit for purpose, and used correctly.

MBIE doesn’t mill timber or sell building products. Instead, they:

• Oversee how the New Zealand Building Code is applied

• Provide official guidance on how standards like NZS 3604 (timber-framed buildings) should be interpreted

• Help councils, builders, and designers make consistent decisions about structural strength, durability, and compliance

• Clarify what types of timber grades and treatments are suitable for different uses

When a timber guide or building document references MBIE, it means the information is aligned with government-approved building practices, not just industry opinion.

For homeowners and DIYers, this matters because choosing timber that aligns with

MBIE guidance helps ensure:

• Your project is safe and structurally sound

• It meets council and consent requirements

• Your timber is appropriate for New Zealand’s climate and conditions

• You avoid costly mistakes or rework down the track

In short, MBIE helps set the rules that make sure the timber you use is the right timber for the job, protecting your build, your investment, and the people who use it.

You can read more here:

• MBIE guide to non-structural vs structural timber: https://www.building.govt.nz/building-code-compliance/b-stability/b1-structure/non-structural-timber-issues/

  
  

• NZS 3604 overview: https://c.pool.nzffa.org.nz/specialty-timber-market/information-resources/the-new-zealand-building-code/nzs-3604-timber-framed-buildings/

  
  

Structural grades: SG6, SG8, SG10 (what they mean in real life)

When you see SG6, SG8 or SG10, think “how strong and stiff is this piece of timber?”

Verified Timber and other industry groups explain:

• SG6 – entry-level structural grade. SG6 is considered a lower-strength structural timber grade, best suited to areas where weight loads are minimal and spans are short. In New Zealand homes, it is commonly used for certain structural components that don’t carry heavy loads, rather than major load-bearing elements.

  
  

• SG8 – SG8 is the most widely used structural timber grade in New Zealand residential construction. It’s commonly specified for framing elements such as walls, floor joists, rafters and roof trusses, as it offers reliable, consistent strength verified through machine stress grading. SG8 meets the requirements of NZS 3604 for standard home builds and is typically kiln-dried for stability. It is also available with treatment options, such as H3.2, to protect against moisture and pests, with alternative variants suitable for certain outdoor or exposed applications.

  
  

• SG10 – SG10 is a higher-performance structural timber grade used when a build requires greater strength or stiffness than standard grades like SG8 can provide. It’s often specified for heavier loads, longer spans, or more demanding design situations, offering a practical middle ground between standard framing timber and higher-cost engineered products such as LVL.

  
  

GradeRight’s info sheet also notes that SG6, SG8 and SG10 can be supplied with different treatment levels (H1.2, H3.2, H4, etc.), so you get both the right strength and durability for the job.

In plain English:

• Building a house frame or shed? You’re probably looking at SG8.

• Need heavier beams or high-load areas? Your designer/engineer may spec SG10.

• Doing non-structural cladding, trims, or landscaping features? You may not need a structural grade at all.

Hazard classes: H1.2, H3.2, H4, H5, H6 (how “treated” the timber is)

Alongside the structural grade, you’ll also see H-numbers on treated timber – this tells you how well it’s protected from rot, insects and moisture.

The EPA and NZ timber treatment guides explain that hazard classes run from H1.2 up to H6, with higher numbers designed for harsher conditions.

In simple terms:

• H1.2 – Indoor, dry areas

  • Use for: Interior wall framing, dry subfloors, roof trusses inside the building envelope.

    
    

• H3.2 – Exterior, above ground

  • Use for: Deck joists and bearers, fence rails, pergolas, exterior timbers not in contact with the soil.

    
    

• H4 – Exterior, in ground

  • Use for: Fence posts, garden beds, landscaping sleepers, low retaining walls touching soil.

    
    

• H5 – High-hazard ground contact

  • Use for: House piles, major retaining walls, important structural posts.

    
    

• H6 – Marine contact

  • Use for: Sea walls, piles in salt water, harsh coastal/marine environments.

Appearance grades: when looks matter more than strength

Not all timber is graded for structural strength. Some is graded for how it looks, fewer knots, straighter grain, fewer defects.

NZS 3631 (New Zealand Timber Grading Rules) describes four categories:

1. Appearance grades – for visible finishes: furniture, panelling, feature walls, bench tops.

2. Cutting grades – for short, clear pieces.

3. Structural grades – for strength/stiffness (SG grades).

4. Box grades – for packaging and lower-grade uses.

More detail here:

• https://codehub.building.govt.nz/resources/36311988nzs

• https://www.specialtytimbers.nz/brand-grades/

This is where TNT Milling’s macrocarpa flitch slabs and feature timber shine, they’re chosen for character and appearance, not for holding up your house.

Matching timber grades to real projects (NZ examples)

Here’s how this all comes together when you’re planning a job and talking to your timber supplier.

🏡 1. Framing a house, shed or sleepout

• Look for: SG8, H1.2

• Why: Strong enough for NZS 3604-type timber framing; treated to handle dry interior conditions and insect risk.

  
  

🌞 2. Building a deck (Marlborough sun, NZ conditions)

• Substructure (bearers/joists):

  • Structural grade: SG8

  • Treatment: H3.2 (above ground, exposed to weather)

• Posts in ground:

  • H5 (or H4 for lower-risk landscaping, depending on design and council requirements).

• Decking boards:

  • H3.2 treated pine, or naturally durable species where allowed.

    
    

🪵 3. Garden beds & landscaping sleepers

• Options:

• H4 treated pine sleepers; or

• Naturally durable macrocarpa sleepers (popular for a more natural look – always check local council/engineer advice for retaining walls).
  

Why...Constant soil contact, moisture and rot risk.

🌿 4. Pergolas, outdoor furniture & feature posts

• Use: SG8 or appearance grade + H3.2 (above-ground external), or a naturally durable species.

• Bonus: This is where TNT’s macrocarpa and flitch slabs can create stunning outdoor tables, benches and bar leaners.

🪚 5. Indoor feature walls, benches and furniture

• Use: Appearance-grade or select-grade timber

• Treatment: Often untreated or low-level treatment depending on use and location

• Think: Macrocarpa slabs, dressed pine, or other specialty timbers.

Why choosing the right timber grade really matters

Picking the right timber grade isn’t just a box-ticking exercise:

• ✅ Safety: Structural grades ensure your deck, frame or pergola can safely handle loads and NZ wind/snow conditions.

  
  

   ✅ Durability: Using the correct H-class means posts don’t rot out in a couple of years.

  
  

  ✅ Compliance: Councils and builders rely on NZS 3604, NZS 3602 and associated standards – using the wrong grade can cause consent headaches later.

  
  

   ✅ Value for money: Right grade + right treatment = fewer replacements and repairs.

Need help choosing? Talk to TNT Milling.

You don’t need to remember every code – that’s our job.

At TNT Milling, we can help you pick:

• The right structural grade 

• The right treatment level 

• The right species and appearance grade for your project

…whether you’re a DIY legend, a builder, or just starting to plan a deck or outdoor project in Marlborough.

🪵 Ready to start your next build or landscaping project?Get in touch with TNT Milling for local advice and quality NZ-graded timber, cut and supplied for the job.