Timber merchants throughout Australia quote products across completely different units. Hardwood sawmills talk in cubic metres ($/m³), retail trade yards quote in lineal metres ($/Lm), and imported architectural timber specs list board feet. Knowing how to convert between these units and verify timber stress grades is essential to protect your trade profit margins.
Sizing Structural Members to AS 1684?
1. How to Convert Lineal Metres to m³ & Board Feet
Timber volume is calculated by finding the cross-sectional area of the profile in square metres and multiplying by the total lineal length:
Volume (m³) = [Thickness (mm) × Width (mm) ÷ 1,000,000] × Total Lineal Metres (m)
For example, if you are ordering 60 sticks of 90×45mm structural framing timber at 4.2 metres long:
- Total Lineal Metres:
60 × 4.2m = 252 Lm - Cross-section area:
(90 × 45) ÷ 1,000,000 = 0.00405 m² - Total Net Volume:
252 Lm × 0.00405 = 1.021 m³
To convert cubic metres to Imperial Board Feet (FBM), multiply the cubic metres by 423.776:
1.021 m³ × 423.776 = 432.7 Board Feet
Conversely, if an architectural drawing specifies 1,000 board feet of custom hardwood screen battens, divide by 423.776 to get 2.36 m³, which your local sawmill can quote immediately.
2. Comparing $/Lm vs $/m³ Merchant Pricing
When comparing a wholesale quotation against a local trade desk (like Bowens, NHS, or Dahlsens), you will often find one quoting $3,600/m³ and the other quoting $14.50/Lm. Here is the mathematical bridge to compare them like-for-like:
| Conversion Direction | Formula | Worked Example (190×45mm Bearer) |
|---|---|---|
| Convert $/m³ to $/Lm | Rate ($/m³) × [Thickness (mm) × Width (mm) ÷ 1,000,000] | $2,200/m³ × 0.00855 = $18.81 / Lm |
| Convert $/Lm to $/m³ | Rate ($/Lm) ÷ [Thickness (mm) × Width (mm) ÷ 1,000,000] | $16.50/Lm ÷ 0.00855 = $1,930 / m³ |
| Convert $/Lm to $/Board Foot | Rate ($/Lm) ÷ [Cross Section (m²) × 423.776] | $16.50/Lm ÷ (0.00855 × 423.776) = $4.55 / Bft |
3. Nominal vs Actual Dressed (DAR) Timber Dimensions
A frequent source of on-site ordering errors for apprentices is the difference between nominal rough-sawn sizes and finished planed sizes:
- Nominal Size: The green, rough-sawn dimension fresh from the sawmill blade before timber seasoning and planing (e.g. 100×50mm or historic 4×2 inches).
- Dressed All Round (DAR): The timber is kiln-dried and run through a high-speed planer on all four faces to create smooth, uniform parallel edges. A nominal 100×50mm rough stick finishes at 90×45mm. Similarly, a 150×50mm nominal member finishes at 140×45mm, and 200×50mm finishes at 190×45mm.
AS 1684 Compliance Rule
4. Australian Timber Stress Grading: MGP vs F-Grades vs LVL
Structural timber in Australia is categorized by structural stiffness and strength properties. Understanding the difference prevents costly site rejections by building certifiers:
| Grade Category | Common Grades | Testing Method | Modulus of Elasticity (Stiffness) | Typical Job Applications |
|---|---|---|---|---|
| MGP (Machine Graded Pine) | MGP10, MGP12, MGP15 | Mechanical roller deflection testing | 10,000 to 15,200 MPa | Domestic residential wall studs, floor joists, truss chords, top/bottom plates. |
| F-Grades (Softwood & Hardwood) | F5, F7, F8, F14, F17, F27 | Visual inspection of knots, grain slope & splits | 6,900 to 18,500 MPa | F7 for outdoor treated pine pergolas; F17 seasoned hardwood for heavy bearers and lintels. |
| LVL (Laminated Veneer Lumber) | E13, E14, E16 (e.g. SmartFrame, hySPAN) | Engineered rotary-peeled veneer billets | 13,200 to 16,000 MPa | Long-span garage lintels, roof ridge beams, hanging beams where solid timber would sag. |
MGP10 vs F7: Under AS 1684 span tables, MGP10 and F7 pine perform almost identically in bending capacity. However, MGP10 is machine-verified for stiffness, making it the industry standard for lightweight stud framing, while F7 is commonly specified for external preservative-treated pergolas and verandahs.
5. AS 1604 Hazard Classes: H1 to H6 Guide
Preservative timber treatment in Australia is governed by AS 1604. Using the wrong hazard class is an immediate defect notice during building inspections:
| Hazard Class | Exposure Environment | Biological Hazard | Approved Timber Uses |
|---|---|---|---|
| H1 | Completely dry interior, well-ventilated | Lyctid borers (powder post beetle) | Internal joinery, furniture, mouldings. |
| H2 / H2F | Interior dry, protected from wetting | Subterranean termites & borers | Blue-dyed roof trusses and wall framing studs south of Tropic of Capricorn. |
| H3 | Outside, above ground, exposed to rain | Moderate fungal decay, rot & termites | Decking boards, joists, bearers, pergola beams, fascia, weatherboards. |
| H4 | Outside, in-ground direct soil contact | Severe fungal decay, continuous dampness | Fence posts, landscaping sleepers, low retaining walls (≤1m), pergola post stumps. |
| H5 | In-ground contact with critical loads | Extreme fungal rot, waterlogged soil | Structural house stumps, retaining walls over 1m, building foundation poles. |
| H6 | Marine waters / saltwater immersion | Marine wood-borers and decay | Boat jetties, marine piles, slipways. |
Critical Trade Trap: H3 in Ground
6. Stock Lengths & Ordering to Minimise Offcuts
Australian structural softwoods and hardwoods are stocked in standardised increments of 300mm or 600mm:
2.4m | 2.7m | 3.0m | 3.6m | 4.2m | 4.8m | 5.4m | 6.0m
If your building plans require 2.8-metre ceiling joists and you order 3.0m sticks, you will discard 200mm off every single member. But if you order 6.0m sticks and cut two 2.8m joists from each, you generate only 400mm total offcut per pair, reducing timber waste from 7% down to 3%.
Trade Pack vs Broken Pack: When purchasing large orders, order in full unbroken mill packs (typically 48 to 108 pieces depending on section). Most timber yards charge a 5% to 15% broken pack handling fee if yard workers have to snip steel banding to pull individual sticks.
7. Species Density & Ute Payload Weight Check
Many tradies underestimate the weight of their timber order and inadvertently risk overloaded vehicle fines. Timber weight is heavily dependent on moisture content and species density:
| Timber Species & Condition | Average Density (kg/m³) | Approx Weight of 90×45 Stick (4.8m) | Max Lm for 1-Tonne Ute (950kg Tray) |
|---|---|---|---|
| Radiata Pine / MGP10 (Kiln-Dried) | ~550 kg/m³ | 10.7 kg | ~425 Lm (approx 88 pieces) |
| Treated Pine H3/H4 (Fresh/Wet Treatment) | ~850 kg/m³ | 16.5 kg | ~275 Lm (approx 57 pieces) |
| Merbau Hardwood (Kiln-Dried) | ~850 kg/m³ | 16.5 kg | ~275 Lm (approx 57 pieces) |
| Spotted Gum / Ironbark (Dense Hardwood) | ~1,100 kg/m³ | 21.4 kg | ~210 Lm (approx 44 pieces) |
| LVL (Laminated Veneer Lumber) | ~600 kg/m³ | 11.7 kg | ~390 Lm (approx 81 pieces) |
Notice that freshly treated wet pine weighs over 50% more than kiln-dried pine. If your timber was treated recently and still damp from the autoclave cylinder, a single pack can easily crush your ute's rear suspension.
8. 2026 Australian Timber Price Benchmarks
Trade account pricing across major Australian suppliers (Bowens, Bunnings Trade, NHS, Dahlsens) generally falls within these ranges (exclusive of GST):
- 70×35 MGP10 Treated Pine: $3.80 – $5.20 / Lm
- 90×45 MGP10 Structural Pine: $5.50 – $7.80 / Lm
- 140×45 MGP10 / F7 Joist: $9.50 – $13.50 / Lm
- 190×45 MGP10 / F7 Bearer: $14.50 – $19.50 / Lm
- 240×45 MGP10 / F7 Heavy Beam: $21.00 – $28.00 / Lm
- 90×90 F7 H4 Treated Pine Post: $18.00 – $25.00 / Lm
- 100×100 F7 H4 Treated Pine Post: $24.00 – $32.00 / Lm
- 86×19 Merbau Hardwood Decking: $7.50 – $11.00 / Lm
- 135×19 Spotted Gum Hardwood Decking: $16.00 – $24.00 / Lm
9. Site Delivery, Moisture & Storage Rules
How timber is handled once it lands on site directly affects whether wall frames bow or decking boards cup:
- Support Bearers Every 1.5m: Never drop structural framing packs directly onto damp grass or uneven soil. Lay scrap timber bearers flat across the ground at 1.5m intervals to keep the pack dead level and minimum 150mm off the ground.
- Ventilated Tarping: Wrap a weatherproof tarp over the top of the stack, but leave the bottom sides open. Sealing plastic completely around timber traps ground moisture, creating an oven effect under the sun that causes extreme bowing, twisting, and black mold.
- Equilibrium Moisture Content (EMC): Allow finish timbers (flooring, decking, architraves) to sit on site for at least 7 to 14 days before installation so their moisture content equalizes with local ambient humidity (typically 10% to 14% in coastal Australia).
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