Australian Timber Sizing Reference Guide
Timber grades, section sizes and questions for professional review. No allowable span values are provided; consult official sources and a qualified designer.
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Important — indicative reference only, not engineering advice
This page is educational and does not reproduce the official AS 1684 span tables, which are copyright Standards Australia. It shows which timber sizes and grades are commonly used for each member, but every actual allowable span must be read from the published AS 1684 tables for your wind classification, roof type, spacing and loading category. Allowable spans depend on specific loading, roof pitch, tie-down requirements and local building codes. This is not engineering or certification advice — always confirm with the published AS 1684 tables and a qualified structural engineer before specifying or building.
Roof framing anatomy: members, span and spacing
Before reading any table, it helps to know what each member does and what "span" and "spacing" mean. The diagram shows a typical pitched roof cross-section — the ridge beam at the apex, rafters carrying the roof load down to the wall plates, and the ceiling joist tying the walls together.
70 AU reference guides — pick a structural member, then the timber grade. These pages do not provide allowable spans.
Machine Graded Pine - Standard structural grade for general framing
| Timber Size (mm) | @ 450mm spacing | @ 600mm spacing | @ 900mm spacing |
|---|---|---|---|
| 70 x 35 | Verify against AS 1684.2 | Verify against AS 1684.2 | Verify against AS 1684.2 |
| 90 x 35 | Verify against AS 1684.2 | Verify against AS 1684.2 | Verify against AS 1684.2 |
| 90 x 45 | Verify against AS 1684.2 | Verify against AS 1684.2 | Verify against AS 1684.2 |
| 120 x 35 | Verify against AS 1684.2 | Verify against AS 1684.2 | Verify against AS 1684.2 |
| 120 x 45 | Verify against AS 1684.2 | Verify against AS 1684.2 | Verify against AS 1684.2 |
| 140 x 45 | Verify against AS 1684.2 | Verify against AS 1684.2 | Verify against AS 1684.2 |
| 190 x 45 | Verify against AS 1684.2 | Verify against AS 1684.2 | Verify against AS 1684.2 |
Timber Size: Width x depth in millimeters. The depth (second number) is the dimension aligned with the load direction.
Spacing: The center-to-center distance between parallel members. Closer spacing allows longer spans for the same timber size.
Maximum Span: The maximum unsupported length between bearing points. Always design with some margin below the maximum.
Roof Pitch: Steeper pitches may allow slightly longer rafter spans due to reduced horizontal thrust.
Roof Load: Heavy roof tiles require shorter spans than lightweight metal or shingle roofing.
Wind Zone: High wind areas require additional tie-down connections and may affect member sizing.
Understanding Timber Grades
MGP (Machine Graded Pine)
- MGP10: Standard structural grade (MOE 10,000 MPa)
- MGP12: Higher strength grade (MOE 12,000 MPa)
- MGP15: Premium structural grade (MOE 15,000 MPa)
F-Grades (Visual Stress Grades)
- F5: Low-stress applications only
- F7: Common structural grade for softwoods
- F14/F17: Higher-strength hardwood grades
MGP10 reference guidance for roof rafters and ceiling joists
MGP10 is the workhorse grade of Australian and New Zealand residential roof framing — most pre-graded structural pine (radiata, hoop and slash) sold at merchants is stamped MGP10. With a modulus of elasticity of about 10,000 MPa, it covers the majority of suburban rafter and ceiling joist spans without upsizing to a stronger grade.
As a rule of thumb, deeper sections (140 × 45 mm and 190 × 45 mm) span further than shallower ones, and ceiling joists reach further than rafters of the same size because they carry no roof load above. We deliberately don't publish the exact metre figures here — the allowable span changes with wind classification, roof type and loading category, so look up your size in the published AS 1684.2 rafter and ceiling-joist tables, which assume sheet roofing with no additional ceiling load.
For tile roofs, raked ceilings, or ceilings carrying additional load (such as storage areas), step up to MGP12 or step down in spacing.
MGP12 reference guidance
MGP12 is a grade designation, not an allowable span. Obtain product-specific properties and current pricing from the supplier.
Do not substitute a timber grade or remove a load-bearing wall based on this guide. Have a qualified designer assess the complete structure.
MGP15 reference guidance
MGP15 does not establish a permitted application or span by itself. Request grading evidence and applicable design documentation for the product.
Availability, cost and suitability need project-specific confirmation.
F7 hardwood reference guidance
F7 is a stress-grade designation. It does not independently establish durability, preservative treatment requirements or suitability for exposed use.
F17 hardwood: high-strength rafters, lintels and ridge beams
F17 is a stress-grade designation. This guide does not establish a universal strength, span or cost advantage over other grades.
Have a qualified designer select the product and design the member, supports and connections for the actual load case.
Stress grade and durability are different properties. Confirm exposure suitability separately using authoritative product documentation.
Ceiling joist references: what's different
Ceiling joists and rafters have different roles. Actual loads and structural functions must be assessed; do not transfer a span from one application to another.
Strength, deflection, connections and additional loads require design checks. This guide does not recommend a percentage of a published span limit.
If the ceiling space will be used for storage or supports a water tank, AS 1684 requires a higher live-load assumption, and the standard ceiling joist tables no longer apply — design as a floor joist instead.
Consult the applicable official standard
This guide does not publish span values or claim to reproduce a conservative subset of official tables. Ask a qualified designer to identify the applicable AS 1684 provisions, current edition and project-specific requirements.
New Zealand projects require assessment against applicable New Zealand requirements. Do not substitute Australian reference material for NZS 3604 or project-specific engineering. A reference guide is not an approved design.