Aus.ToolsHow-To Guides

How to choose the right structural timber for a job

Selecting the correct structural timber is paramount for the integrity, longevity, and compliance of any build. This guide details the critical factors, Australian standards, and practical considerations for choosing the right timber species, grade, and treatment for various structural applications,

Skill level
Intermediate
Time
Varies
Trade
Carpentry & Joinery
Updated
4 November 2026

Tools you will need

  • Tape Measure
  • Pencil/Marker: For marking timber.
  • AS 1684 Residential Timber

🪚 Choosing Structural Timber: A Comprehensive Guide

Why is selecting the correct structural timber important for Australian builds?

Selecting the correct structural timber is paramount for the integrity, longevity, and compliance of any build. This guide details the critical factors, Australian standards, and practical considerations for choosing the right timber species, grade, and treatment for various structural applications, from framing to outdoor decking.

What skill level is required to choose structural timber effectively?

Intermediate — This guide assumes a foundational understanding of carpentry principles and Australian building codes.

What essential tools do chippies need for structural timber selection?

Moisture Meter: For checking timber moisture content, especially for internal applications. Tape Measure (8m): For accurate measurement of spans and lengths. Pencil/Marker: For marking timber. AS 1684 Residential Timber-Framed Construction Handbook: Essential for span tables and design requirements.

What materials are essential for accurate structural timber specification?

Timber Samples (Optional): To visually compare species, grades, and treatments. Project Plans/Drawings: To determine structural requirements, spans, and environmental exposure.

What are the critical safety requirements when handling structural timber in Australia?

⚠️ Safety: Always wear appropriate Personal Protective Equipment (PPE) when handling timber, including:

Safety Glasses (AS/NZS 1337.1): To protect against splinters and dust. Gloves: To prevent splinters and improve grip. Dust Mask (P1 or P2): When cutting or sanding, especially treated timbers. Steel-capped Boots (AS/NZS 2210.3): For foot protection on site. Be aware of manual handling risks when lifting heavy timber sections.

Step-by-Step Guide

Step 1: 1️⃣ Understand the Application and Load Requirements

Begin by thoroughly reviewing the project plans. Identify whether the timber will be used for internal framing (walls, floors, roofs), external framing (pergolas, carports), decking, or other structural elements.

 Internal Framing: Requires timber with good structural integrity, often less emphasis on weather resistance. External Framing/Decking: Demands timber with inherent durability or appropriate treatment for weather exposure, insect resistance, and fungal decay. Load-Bearing: Determine if the timber is load-bearing (e.g., floor joists, roof rafters, lintels) and calculate the anticipated loads (dead loads, live loads, wind loads). This directly influences timber size and grade. Refer to AS 1684 for span tables and design data.

Step 2: 2️⃣ Assess Environmental Exposure

The environment where the timber will be installed is a critical factor.

Internal, Dry Conditions: Timber is protected from weather and direct moisture. External, Above Ground: Exposed to rain, sun, and temperature fluctuations. Requires durability or treatment. External, In-Ground Contact: Fence posts, stumps. Requires high durability or specific treatment for ground contact. Wet Areas: Bathrooms, laundries, or areas prone to condensation. Consider moisture-resistant options or appropriate sealing. Termite Zones: Most of Australia is a termite-prone area. Untreated susceptible timbers will require chemical barriers or physical protection.

Step 3: 3️⃣ Select Timber Species and Durability Class

Based on environmental exposure, choose a timber species or treatment.

 Durability Classes (AS 5604): Class 1: Highly durable (e.g., Ironbark, Spotted Gum, Merbau). Suitable for in-ground or exposed applications. Expected life >25 years in ground. Class 2: Durable (e.g., Blackbutt, Jarrah, Cypress Pine). Suitable for exposed applications, not typically in-ground without treatment. Expected life 15-25 years in ground. Class 3: Moderately durable (e.g., Radiata Pine (untreated), Oregon). Not suitable for exposed or in-ground applications without treatment. Expected life 5-15 years in ground. Class 4: Non-durable. Not suitable for structural use without treatment. Common Structural Species: Hardwoods (e.g., Spotted Gum, Blackbutt, Ironbark): Naturally durable, high strength, excellent for decking, bearers, joists, and exposed structural elements. Softwoods (e.g., Radiata Pine, Douglas Fir/Oregon): Readily available, cost-effective. Requires treatment for external or structural applications.

Step 4: 4️⃣ Determine Treatment Level (Hazard Class)

For non-durable timbers or specific applications, chemical treatment is essential. Refer to AS 1604 for timber treatment.

 H1: Interior, protected from weather and wetting. Protection against borers. H2: Interior, protected from weather and wetting. Protection against borers and termites (south of Tropic of Capricorn). H3: External, above ground. Protection against moderate decay, borers, and termites. Common for decking joists, pergolas (e.g., 90x45 H3 Treated Pine). H4: External, in-ground contact. Protection against severe decay, borers, and termites. Common for fence posts, garden edges (e.g., 100x100 H4 Treated Pine). H5: External, in-ground contact, severe marine exposure, or freshwater immersion. Protection against very severe decay, borers, and termites. For retaining walls, jetty piles. H6: Marine piles, permanent saltwater immersion.

💡 Pro Tip: Always check the treatment stamp on the timber. It will indicate the hazard class (e.g., H3, H4) and the treatment chemical used.

Step 5: 5️⃣ Specify Timber Grade and Stress Grade

Timber is graded for structural performance.

 Visual Grading: Based on visible defects (knots, gum veins, slope of grain). Machine Stress Grading (MSG): Timber is mechanically tested to determine its strength. Stress Grades: Common grades include MGP10, MGP12, MGP15 for machine-graded pine, and F5, F7, F8, F11, F14, F17, F22, F27 for visually graded and some hardwoods. MGP10/F5/F7: Suitable for general framing, non-load-bearing walls. MGP12/F8/F11: Common for load-bearing walls, floor joists, roof rafters. MGP15/F14/F17+: For high-stress applications, long spans, or specific engineering requirements. AS 1684 specifies the minimum stress grade required for various structural elements based on span and loading.

Step 6: 6️⃣ Consider Timber Dimensions and Availability

Once species, treatment, and grade are determined, select the appropriate dimensions.

Standard Sizes: Common structural timber sizes include 90x45, 140x45, 190x45, 240x45 for framing, and various sizes for posts and beams. Span Tables: Use AS 1684 span tables to determine the minimum required depth and width of timber for joists, bearers, rafters, and studs based on span, spacing, and load. Availability: Check with local timber suppliers for stock availability and lead times for specific species, grades, and treatments. Sometimes, a slightly higher grade or different species might be more readily available and cost-effective than waiting for a specific order.

Step 7: 7️⃣ Factor in Aesthetics and Cost

While structural integrity is primary, aesthetics and budget are also considerations.

 Appearance Grade: For exposed timber elements (e.g., pergolas, feature beams), consider appearance-grade timber which has fewer defects. Cost-Benefit Analysis: Treated pine (e.g., H3 MGP10 Treated Pine) is generally more cost-effective for external framing than naturally durable hardwoods, but hardwoods offer superior natural aesthetics and longevity in some applications.

Common Mistakes to Avoid

1.❌ Under-specifying Timber Grade: Using a lower stress grade than required by AS 1684 can lead to structural failure, excessive deflection, and non-compliance.
2.❌ Incorrect Treatment for Exposure: Using untreated or inadequately treated timber in external or in-ground applications will result in premature decay, insect attack, and structural compromise.
3.❌ Ignoring Moisture Content: Using timber with high moisture content for internal framing can lead to shrinkage, twisting, and cracking as it dries out, affecting finishes and structural stability.
4.❌ Not Checking for Certification: Always ensure timber is sourced from reputable suppliers and carries appropriate certification (e.g., FSC, PEFC) and treatment stamps.
5.❌ Over-reliance on Visual Inspection Alone: While visual grading is important, for critical structural elements, machine stress-graded (MSG) timber provides a more reliable strength rating.

Pro Tips

 💡 Consult AS 1684: Keep a copy of AS 1684 Residential Timber-Framed Construction handy. It's your bible for timber sizing, grading, and span tables. 💡 Supplier Relationships: Build good relationships with your timber suppliers. They can offer valuable advice on availability, alternatives, and local conditions. 💡 Moisture Meter for Internal Work: For internal fit-outs, especially with hardwoods, use a moisture meter. Aim for 10-14% moisture content to minimise movement after installation. 💡 End Sealing: For external hardwoods, consider sealing the ends of cut timber with a wax-based end sealer to reduce end-checking and splitting. 💡 Future-Proofing: When in doubt, slightly over-specifying the timber grade or treatment can add a margin of safety and longevity to the project, often at a minimal additional cost.

Recommended Tools

AS 1684 Handbook: Essential for compliance and design data. Tramex Moisture Meter (ME5 or similar): For accurate moisture content readings, crucial for internal timber work. Makita 18V Circular Saw (DHS782Z): A reliable, powerful saw for efficient and accurate cutting of structural timbers on site. Milwaukee M18 FUEL Impact Driver (M18FID2-0): For fast and secure fastening of structural screws and bolts.

Frequently asked questions

What is involved in choose the right structural timber for a job?
Selecting the correct structural timber is paramount for the integrity, longevity, and compliance of any build. This guide details the critical factors, Australian standards, and practical considerations for choosing the right timber species, grade, and treatment for various structural applications,
What skill level do I need for choose the right structural timber for a job?
Intermediate — This guide assumes a foundational understanding of carpentry principles and Australian building codes.
What are common mistakes when choose the right structural timber for a job?
1. ❌ Under-specifying Timber Grade: Using a lower stress grade than required by AS 1684 can lead to structural failure, excessive deflection, and non-compliance. 2. ❌ Incorrect Treatment for Exposure: Using untreated or inadequately treated timber in external or in-ground applications will result in premature decay, insect attack, and structural compromise. 3. ❌ Ignoring Moisture
What tools do I need for choose the right structural timber for a job?
This guide includes a comprehensive tools list. Key carpentry & joinery tools are detailed in the Tools Required section.
What is the Australian standard for residential timber framing?
The primary Australian standard for residential timber framing is AS 1684.2:2021 Residential timber-framed construction. This standard specifies design and construction requirements for timber framing in houses and similar structures, ensuring structural integrity and safety across Australia. It covers everything from bracing to member sizing.
What is the difference between MGP10 and MGP12 timber?
MGP (Machine Graded Pine) refers to the stress grade of structural timber, with the number indicating its bending strength in megapascals (MPa). MGP12 timber has a higher bending strength (12 MPa) than MGP10 (10 MPa), meaning it can span further or carry heavier loads. Always specify the correct MGP grade as per engineering designs or AS 1684.
Why is a moisture meter important for timber selection?
A moisture meter is crucial for checking the moisture content of timber, especially for internal applications. Using timber with an inappropriate moisture content can lead to issues like shrinkage, warping, and cracking after installation. According to FWPA guidelines, timber for internal use should typically have a moisture content between 10-15%.

Written by Chippy, the Carpentry & Joinery specialist at Aus.Tools.