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    AS 1684 reference · Australia

    MGP12 Ceiling Joists Sizing Reference Guide (Australia)

    Timber grades, section sizes and questions for professional review. No allowable span values are provided; consult official sources and a qualified designer.

    Verify against AS 1684.2

    All numeric span values for ceiling joists must be confirmed against the official AS 1684.2 (non-cyclonic) or AS 1684.3 (cyclonic) table for your wind classification and load condition. The information on this page is a sizing starting point for builders, owner-builders and homeowners — it is not a substitute for the standard or for engineer sign-off on permitted work.

    General roof terminology

    The diagram below illustrates a pitched roof, not an application-specific design. It shows where roof members sit in a typical pitched roof, and what "span" and "spacing" mean when you read a sizing table.

    Cross-section of a pitched timber roofA triangular roof frame showing the ridge beam at the apex, two rafters sloping down to the wall plates on top of the walls, a horizontal ceiling joist tying the walls together, the clear span measured across the building, and the rafter spacing measured between repeated rafters.Ridge beamRafterClear spanCeiling joistmeasured clear between the supporting wallsSpacingWall plate
    Roof framing anatomy — how rafters, the ceiling joist, span and spacing relate. Spans shown here are illustrative only; read every allowable span from the published tables.

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    About MGP12 Pine

    A grade label alone does not establish suitability. Confirm the actual product, grading evidence and intended use with a qualified designer.

    Common species: Radiata Pine, Slash Pine.

    About ceiling joists as a structural member

    Ceiling joists carry plasterboard, insulation and light fittings — no roof load. Lighter loading allows 30–40% longer spans than the same section as a rafter.

    Common MGP12 ceiling joist sizes used in Australia

    Section sizes most commonly stocked and specified for MGP12 ceiling joists in Australian residential construction. Maximum allowable spans for each section must be confirmed against the AS 1684.2 ceiling joist table.

    • 90×45
      Typical spacing (mm)
      450
      Max single span (m)
      verify against AS 1684.2 Table
    • 90×45
      Typical spacing (mm)
      600
      Max single span (m)
      verify against AS 1684.2 Table
    • 120×45
      Typical spacing (mm)
      450
      Max single span (m)
      verify against AS 1684.2 Table
    • 120×45
      Typical spacing (mm)
      600
      Max single span (m)
      verify against AS 1684.2 Table
    • 140×45
      Typical spacing (mm)
      450
      Max single span (m)
      verify against AS 1684.2 Table
    • 140×45
      Typical spacing (mm)
      600
      Max single span (m)
      verify against AS 1684.2 Table
    • 190×45
      Typical spacing (mm)
      450
      Max single span (m)
      verify against AS 1684.2 Table
    • 190×45
      Typical spacing (mm)
      600
      Max single span (m)
      verify against AS 1684.2 Table
    • 240×45
      Typical spacing (mm)
      450
      Max single span (m)
      verify against AS 1684.2 Table
    • 240×45
      Typical spacing (mm)
      600
      Max single span (m)
      verify against AS 1684.2 Table

    Sizing guidance and AS 1684 compliance notes

    Project-specific sizing guidance is withheld pending qualified review. Ask a qualified designer to assess loads, supports, connections, exposure and the applicable current standard. No size, spacing or span on this reference page is an approved specification.

    How to read a MGP12 ceiling joist span table

    1. Determine your required span — the clear distance between the supports the ceiling joist will sit on.
    2. Note your spacing — typically 450 or 600 mm centres.
    3. Identify your wind classification (N1–N4 non-cyclonic, C1–C4 cyclonic) and load category.
    4. Look up the AS 1684.2 ceiling joist table for MGP12 and read off the smallest section size whose maximum span exceeds your required span.
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