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    AS 1684.2 reference · Australia

    Australian Ceiling Joist 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.

    Indicative reference only, not engineering advice

    This page is educational and does not reproduce the official AS 1684.2 ceiling joist tables, which are copyright Standards Australia. The table below shows common sizes only — every allowable span must be read from the published AS 1684.2 table for your wind classification and loading, and confirmed by a structural engineer. This is not engineering or certification advice.

    Standard ceiling joist sizes in Australia

    The most common ceiling joist sizes used in Australian residential framing are 90×45, 120×45, 140×45, 190×45 and 240×45 mm — almost always supplied as MGP10 treated pine. Larger sections (190×45 and up) are typically used for long unsupported spans or where ceiling storage loads apply; smaller sections suit short spans between internal load-bearing walls.

    Ceiling joist spacing — 450 mm vs 600 mm centres

    600 mm centres is the default in most Australian residential framing because it lines up with 1200 mm wide plasterboard sheets and uses fewer members, lowering material and labour cost. 450 mm centres allow longer spans (or smaller joist sizes) and are typically specified where ceiling loads are higher — for example, when the ceiling carries heavy light fittings, tiled soffits or supports a future loft conversion.

    MGP10 ceiling joist section-size reference

    Common MGP10 ceiling joist sizes at 450 and 600 mm centres. This page does not publish allowable spans — read each from the official AS 1684.2 ceiling joist table for your loading.

    • 90 × 45
      Spacing (mm)
      450
      Max single span (m)
      Verify against AS 1684.2
      Max continuous span (m)
      Verify against AS 1684.2
    • 90 × 45
      Spacing (mm)
      600
      Max single span (m)
      Verify against AS 1684.2
      Max continuous span (m)
      Verify against AS 1684.2
    • 120 × 45
      Spacing (mm)
      450
      Max single span (m)
      Verify against AS 1684.2
      Max continuous span (m)
      Verify against AS 1684.2
    • 120 × 45
      Spacing (mm)
      600
      Max single span (m)
      Verify against AS 1684.2
      Max continuous span (m)
      Verify against AS 1684.2
    • 140 × 45
      Spacing (mm)
      450
      Max single span (m)
      Verify against AS 1684.2
      Max continuous span (m)
      Verify against AS 1684.2
    • 140 × 45
      Spacing (mm)
      600
      Max single span (m)
      Verify against AS 1684.2
      Max continuous span (m)
      Verify against AS 1684.2
    • 190 × 45
      Spacing (mm)
      450
      Max single span (m)
      Verify against AS 1684.2
      Max continuous span (m)
      Verify against AS 1684.2
    • 190 × 45
      Spacing (mm)
      600
      Max single span (m)
      Verify against AS 1684.2
      Max continuous span (m)
      Verify against AS 1684.2
    • 240 × 45
      Spacing (mm)
      450
      Max single span (m)
      Verify against AS 1684.2
      Max continuous span (m)
      Verify against AS 1684.2
    • 240 × 45
      Spacing (mm)
      600
      Max single span (m)
      Verify against AS 1684.2
      Max continuous span (m)
      Verify against AS 1684.2

    For MGP12 and F17 hardwood ceiling joists, see the MGP10 reference guide and F17 hardwood reference guide.

    How to read a ceiling joist span table

    1. Determine your required span — the clear distance between the load-bearing supports the joist will sit on.
    2. Note your joist spacing — typically 450 or 600 mm centres.
    3. Find your timber grade column (MGP10, MGP12 or F17).
    4. In the published AS 1684.2 table, read off the smallest joist size whose allowable span is greater than your required span.

    AS 1684 and ceiling joists

    Ceiling joist sizing for residential construction in Australia is governed by AS 1684.2 (Residential timber-framed construction — non-cyclonic) or AS 1684.3 in cyclonic regions. The published tables in those standards are the authoritative source — this page is an indicative educational guide only and does not reproduce them.

    Ceiling joist vs rafter — what's the difference?

    Rafters and ceiling joists have different structural roles. A designer must establish the loads and functions for the actual project. Do not convert a rafter span into a ceiling-joist span using a percentage or assume a joist carries only ceiling lining.

    Related reference guides
    MGP10 reference guide
    MGP10, MGP12, MGP15, F7 and F17 — rafters and ceiling joists.
    F17 hardwood reference guide
    Spotted gum, blackbutt and ironbark — for longer spans.
    Timber types and grades
    How MGP10, MGP12 and F17 differ in strength and use.
    Roofing timber best practices
    Installation, fixings and AS 1684 compliance tips.