What exactly is timber and why is it such a valuable material?

Timber is a renewable, energy efficient, recyclable, and biodegradable resource. In Australia, we're fortunate to have exceptionally well-managed forests that provide this sustainable construction material. Interestingly, timber is one of the rarest elements in the universe—while there are planets made of gold, trees are remarkably rare on a cosmic scale, making us lucky to have them here on Earth.

What's the difference between hardwood and softwood?

The distinction isn't actually about hardness, despite the names. Hardwoods are flowering plants with broad leaves, while softwoods are cone-bearing plants with needles for leaves.

While hardwoods are generally denser and harder (like Australian species such as spotted gum and blackbutt), there are exceptions—balsa wood is technically a hardwood despite being one of the lightest woods available. Similarly, while softwoods like radiata pine and hoop pine are typically softer and less dense, some softwood species like larch can be quite dense and hard.

Generally speaking, hardwoods tend to be more expensive because they take longer to grow than softwoods and are typically darker in colour.

Tree Structure and Wood Characteristics

How does a tree's structure affect the timber we use?

Think of timber like a bunch of straws—trees have vessels running up and down that carry water and nutrients from the roots through the trunk to the crown. These vessels are surrounded by fibres that give the wood its strength.

A tree consists of several key parts:

• Outer bark: The protective layer
• Cambium layers: Where nutrients travel up and down
• Sapwood: The outer, living wood that's lighter in colour and carries nutrients
• Heartwood: The inner, darker wood that's actually the dead part of the timber, containing more resin and gum
• Pith: The centre core

In hardwoods, the heartwood section is generally larger compared to softwoods. The sapwood is the living part of the tree, while the heartwood, though darker and denser, is actually dead tissue.

What is sapwood versus heartwood?

Sapwood is the outer layer of living cells that carry nutrients in the growing tree. It's usually lighter in colour and thinner in hardwoods.

Heartwood (also called truewood) consists of dead cells plus extractives (gums, resins, etc.) and is typically darker. Importantly, only sapwood can be effectively treated with preservatives, while heartwood provides natural durability.

 

Why is sapwood important for treatment?

Only sapwood can be effectively penetrated by preservative treatments, regardless of whether it's hardwood or softwood. Even the most durable Class 1 hardwood has sapwood that's considered non-durable (Class 4). Australian standards limit non-durable sapwood to 20% of the cross-section maximum in structural applications.

Moisture Content and Wood Movement

What is moisture content and why does it matter?

When a tree is alive and growing, it contains significant amounts of water—all those vessel "straws" are full of water and nutrients. This is called unseasoned timber. After the tree is cut down, the moisture content changes as the timber dries:

• Unseasoned timber: Above 15% moisture content (freshly cut timber)
• Partially seasoned timber: Around 25% moisture content
• Seasoned timber: Below 15% moisture content (typically 10-15%)

The critical threshold to remember is 20%—once timber moisture content exceeds 20% for extended periods, there's a risk of decay and mold growth. Brief exposure to rain isn't a problem if the water drains away quickly, but timber sitting in water will absorb moisture and become vulnerable to decay.

How does timber shrink as it dries?

Timber doesn't shrink much longitudinally (along the length of those vertical vessels), but it does shrink significantly tangentially (around the growth rings) and radially (across the rays). This is why how timber is cut from the log matters significantly.

When timber is cut along the growth rings (quarter sawn), you get relatively flat, stable pieces. However, if you cut across the rings (back sawn), the timber will tend to cup as it dries. Quarter sawn timber is more stable but considerably more expensive and potentially harder to source.

What is equilibrium moisture content?

Equilibrium moisture content (EMC) is the stable moisture content timber reaches under specific temperature and humidity conditions. This changes daily and seasonally—during wet weather, timber will absorb moisture and swell slightly, while hot, dry, windy days cause it to dry out and shrink.

This is particularly important for decking and internal flooring. Before installation, timber should be placed in the environment where it will be installed for a few weeks to acclimatize and reach equilibrium with that space's moisture content. The timing matters too—don't acclimatize timber during unusually wet or dry periods. 

Always check moisture content before installation, regardless of acclimatization time.

Timber Products and Applications

What are the main types of sawn timber products?

• Round timber is simply debarked, treated logs used for telephone poles, bridges, and pole houses.
• Sawn timber can be cut in different ways:
• Quarter sawn: Cut along growth rings for maximum stability
• Back sawn: The most common cutting method, though more prone to cupping

What engineered timber products are available?

Modern construction uses several engineered timber products:

• LVL (Laminated Veneer Lumber): Made by peeling logs into 2.5-4mm thick veneers on a lathe, then gluing them together with all veneers running the same direction. Sometimes one layer is oriented differently for added strength. LVL is increasingly common—it's straight, stable, and now even used for wall studs. The gluing technology has become very reliable with water-resistant adhesives.
• Glulam (Glue Laminated Timber): Larger pieces of timber (like 90x35mm or 70x35mm) glued together to create substantial beams. These can be pre-cambered—if you install the beam with the camber facing up before loading, it will straighten out under load, reducing deflection. This is particularly valuable for long spans.
• I-Joists: Feature top and bottom timber flanges with OSB or plywood webs forming an "I" section. Common in floor systems and some roof applications. Important note: never cut the flanges, and only put holes through the webs within size and location limits.
• CLT (Cross Laminated Timber): Think of it as giant plywood—layers of timber (typically 90x35mm) alternating direction to build up slabs 100-300mm thick, up to 3 meters wide and 12+ meters long. These massive timber panels are lifted by crane for floors and walls. You can see CLT construction in buildings around Brisbane, like 25 King Street in the Valley, though it's often hidden behind plasterboard for fire protection.

What about wood panel products?

Several panel products serve different purposes:

• Plywood: Veneers staggered at 90 degrees, very common for various applications
• Particle board: Softwood pulp pressed together, commonly used for flooring
• MDF: Medium density fiberboard for various applications
• Hardboard: Hardwood pulp pressed together, less common now but still used for claddings like Weathertex
• OSB (Oriented Strand Board): Made from larger wood chips (bigger than wheat bix!), heavily used in the USA for sheathing entire buildings and increasingly seen in Australian bracing applications
• Veneers: Thin layers providing a timber appearance over other materials

How is timber used in frame and truss construction?

Roof and floor trusses were traditionally made from hardwood years ago, but are now primarily constructed with pine timber. LVL can be incorporated for high-tension areas. This solid timber approach remains the most common method for truss construction.

Sizing and Ordering

How is timber sized and ordered?

Timber can be ordered by nominal size or actual size, depending on the type:

• Green (unseasoned) timber, particularly hardwoods, is generally ordered by nominal size (e.g., 100x50mm or 200x50mm), with allowance for shrinkage.
• Dry timber is typically ordered by actual finished size.

How are timber dimensions specified?

In Australia, we specify depth first, then breadth:

• Studs and joists: 90x35mm, 90x45mm (depth x width)
• Wall plates and battens (used flat): 45x70mm
• Joists (standing up): 190x35mm

Note: North America uses a different convention, but in Australia, always specify depth before breadth.

What are common timber sizes?

Dry Pine common sizes:

• 70x35mm, 70x45mm
• 90x35mm, 90x45mm
• 140x35mm, 140x45mm
• 190x35mm, 190x45mm
• Some suppliers offer 120x35mm/45mm and 170mm depths

Seasoned Hardwood similar sizing:

• 90x35mm, 90x45mm
• 140x35mm, 140x45mm
• 190x35mm, 190x45mm
• 170mm is less common

Unseasoned Hardwood (overall dimensions):

• 50x50mm
• Common joist sizes: 125x50mm, 150x50mm
• Traditional measurements: 4x2" or 6x2"

Timber Grading

How is structural timber graded?

Timber is graded using two main methods:

• F-Grade (Stress Grade): Visual grading system where inspectors assess the timber's characteristics
• MGP (Machine Graded Pine): Timber passes through rollers under pressure, and deflection measurements determine strength

The rule of thumb: lower numbers indicate weaker timber; higher numbers indicate stronger timber. This applies across all grading systems.

What are F grades and how are they determined?

F grades (F4 to F43) indicate structural strength in megapascals. They're determined by combining:

1. Strength Group: Species classification (S1-S7 unseasoned, SD1-SD8 seasoned)
2. Structural Grade: Based on size and frequency of natural characteristics (Grades 1-4)

For example:

·Seasoned Spotted Gum: Typically, SD2 + Structural Grade 2 = F22. 

Merbau (Kwila): Typically, SD2 + Structural Grade 2 = F22. 

Cypress Pine: Typically, SD6 + Structural Grade 2 = F7. 

Blackwood (Acacia melanoxylon): Typically, SD5 + Structural Grade 2 = F14.

 

What are MGP grades?

MGP (Machine Graded Pine) grades (10, 12, 15) are applied to radiata, slash, Caribbean, and some imported softwoods. Timber passes through a machine that measures deflection under stress. MGP 10 typically contains heart material, while MGP 12 and 15 are usually free of heart.

 

What's the difference between MGP 10 and MGP 12?

MGP 10 (F5 equivalent) typically includes heart material and is classified as Joint Group JD5. MGP 12 (F7 equivalent) is usually free of heart material and classified as Joint Group JD4, making it less prone to checking and splitting in connections.

What about appearance grades?

For exposed timber applications, appearance grades specify visual quality:

• Select: Minimal knots and imperfections
• Standard: Moderate features
• Other: May include more knots and character

This is subjective—some people love seeing wavy grain and large knots as natural timber character, while others prefer cleaner appearances. You can specify the appearance grade that matches your aesthetic preference.

What is visual stress grading?

Visual grading involves trained graders inspecting timber for over 20 different natural characteristics including knots, checks, splits, slope of grain, gum veins, and distortion. The grading rules define specific limits for each characteristic in each structural grade. This traditional method is still widely used for seasoned and unseasoned hardwoods.

 

Where can I find strength group information?

Strength groups are listed in:

• AS 2082 (hardwood visual grading)
• AS 2878 (timber classification into strength groups)
• AS 1720 (timber engineering standard)
• AS 1684 (residential timber framing)
• Construction Timbers in Queensland Books 1 & 2 (free download, most comprehensive)

How should structural timber be marked?

Machine-graded timber must be marked with ink stamps, tags, or stickers showing the grade. Never rely solely on mill colour marks. If timber has multiple grade marks, always use the lowest grade. If grade markings are absent or timber has been damaged or re-sawn, it must be re-graded or considered non-structural.

 

Does timber lose its grade over time?

The grade applies at the time of grading. If timber is damaged by forklifts, re-sawn, or significantly altered, it loses its stress grade completely. For heritage buildings over 100 years old, consider dropping down one F grade to account for age-related strength reduction, even if visual grading indicates higher.

 

What characteristics affect structural grading?

Over 20 characteristics are assessed including: knots, holes (insect damage), slope of grain, heart and heart shakes, gum veins, ring shakes, included bark, gum/latex/resin pockets, checks, splits, wane, bow, spring, twist, cup, termite galleries, and primary rot. Each has specific size and frequency limits for each structural grade.

 

What are the hardwood appearance grades?
Hardwood is graded by how much natural character it shows:

• Select Grade: Very clean look, only tiny knots (up to 15mm or ¼ of the board’s width) and pinholes (up to 2mm).
• Medium Feature: Shows some natural markings and knots, but still neat.
• High Feature: Full of natural character — knots, streaks, and other features are part of the look.

What are the softwood appearance grades?
Softwood grading is similar, but with slightly different names:

• Clear: Almost flawless, no visible imperfections.
• Appearance: Very few defects, smooth finish.
• Select: Some natural features allowed.
• Standard: More knots and markings visible.
• Utility: Lots of natural character — the most rustic look.

Is F17 grade suitable for appearance applications?
No. F17 is a strength grade, not an appearance grade. A piece marked F17 may have large knots, holes, or cracks that don’t meet appearance standards. If you need timber that looks good and is strong, always specify both the structural grade and the appearance grade.

What about “Feature Grade” or “Colonial Grade”?
These terms don’t mean anything on their own. Only official grades (like Select, Medium Feature, High Feature for hardwood, or Clear, Select, Standard for softwood) have clear definitions. If someone offers timber under a non‑standard name, ask for a written description of the grade so you know exactly what you’re getting.

Where can I access grading standards?
You can find most interim grading standards for free on the Wood Solutions website. Full standards are available for purchase from SAI Global. If you’re a Timber Queensland member, you also get access to technical data sheets and grading summaries.

How are engineered products graded?

• Glulam (glued laminated timber): Graded as GL‑8, GL‑15, GL‑17, etc.
• LVL (laminated veneer lumber) and I‑Beams: These don’t have standard grades in Australian design codes. Each manufacturer sets their own specifications. Some may market products as “LVL 13,” “LVL 14,” or “F17 equivalent,” but you should never swap one brand for another without checking the manufacturer’s data sheet to confirm the properties match your design needs.

Environmental Considerations

What about the glues used in engineered products?

• LVL, structural plywood, and exterior products use phenol formaldehyde adhesives (similar to the old Bakelite plastic). These are very stable, safe, and long‑lasting.
• Interior plywood may use urea formaldehyde, which is less stable and can release small amounts of formaldehyde gas.

When products are made to Australian standards, the adhesives used in structural timber are designed to be safe, durable, and environmentally stable for the life of the product.

What about recycled timber grades?
Recycled timber has its own grading system to make sure it’s structurally sound. This allows older timber to be safely reused while still meeting performance requirements.

Species Identification

How can you identify different timber species?

Some species are easier to identify than others:

• Distinctive species: Cypress has a characteristic smell that's unmistakable if you've worked with it. Spotted gum and blackbutt have recognizable colours.
• Difficult species: Different pines (radiata, Caribbean, slash pine) can be extremely difficult to distinguish from each other, even for experienced professionals.

How do you definitively identify a timber species?

To be 100% certain of a species, you need laboratory testing. Labs examine samples under microscopes to measure pore sizes and other features, and perform splinter burn tests—burning a small sample and analysing the smoke colour and ash characteristics.




This testing costs approximately $400 per sample. Cut a sample using a hole saw (about 300mm diameter) and send it to the lab, where they'll compare it against thousands of reference samples.




It's surprisingly common to send multiple samples from the same site thinking they're all the same species, only to receive results showing three, four, or even five different species. Unless you saw the tree being cut down yourself, professional testing is the only way to be certain.




Are there thousands of timber species?

Yes! There are thousands of different tree species including ironbarks, bloodwoods, Southern ash, Tasmanian oak, and countless others. Visual identification without specialized knowledge and testing equipment is unreliable for definitive species confirmation.