Timber Framing Span Tables and Load-Bearing Requirements

What This Requirement Covers

This requirement covers the design and selection of timber framing members using span tables and load-bearing criteria for residential and similar low-rise buildings in Australia. It explains the purpose of span tables and prescriptive framing rules - to ensure structural stability, limit deflection, and provide safe transfer of gravity and lateral loads without requiring bespoke engineering for common domestic situations. The rules apply mainly to timber-framed roofs, floors, walls, bearers and joists in Class 1 and 10 buildings under the Deemed-to-Satisfy provisions of the National Construction Code (NCC) and the ABCB Housing Provisions.

These provisions exist so builders, designers and certifiers can rely on tested, conservative tables and referenced Australian Standards (notably AS 1684 for timber-framed residential construction) to size members and spacing without an engineer for typical spans, loads and geometries. Where conditions fall outside the Tables or the limitations of the Housing Provisions, a structural design by a qualified practitioner is required.

Key Requirements

1. Applicable documents: NCC 2022 Volume Two - Building Code of Australia (Class 1 and 10), ABCB Housing Provisions Standard 2022 Part 6.3 and related tables, and Australian Standard AS 1684 (timber framing). Cite specific provisions when using prescriptive tables (for example, as specified in NCC Volume Two, Part H and ABCB Housing Provisions 6.3).
2. Scope and limitations: The Housing Provisions apply subject to conditions listed in H1D6(1)-(6) including geometric limits and that loads are evenly distributed unless otherwise allowed; see H1D6(3)-(6) for limitations such as wind/earthquake exclusions and snow loads (NCC Volume Two, Part H1D6).
3. Span and spacing determination: Effective load widths and maximum spans for bearers, joists and rafters are determined from the Housing Provision tables (e.g., Table H1D6a, H1D6b, and Tables 6.3.3a-d in the ABCB Housing Provisions). Where single or continuous spans are used, use the corresponding table and figure: Table H1D6a/H1D6b and Figures H1D6a-H1D6e (NCC Volume Two, Table H1D6a and Figures).
4. Typical numeric limits (examples from Housing Provisions tables):
5. Bearer maximum single-span values (depend on steel section and effective load width) - see Table 6.3.3a. For steel sections shown, spans can be up to around 4.2 m for small sections under small effective widths, or greater for larger UB sections (refer to the tabulated values in Table 6.3.3a).
6. Effective load widths are given as fractions of spans: e.g., 0.5 x Span 1, 0.4 x Span 1, 0.6 x (Span 1 + Span 2) depending on member position and continuity (Figure H1D6c and Table H1D6a notes).
7. Geometric software limits for use of structural software under H1D6(7)(b): eave height ≤ 6 m, highest roof point ≤ 8.5 m, building width ≤ 16 m, building length ≤ 5 × building width, roof pitch ≤ 35°.
8. Load assumptions: Span tables in AS 1684 and Housing Provisions assume typical imposed and dead loads for residential occupancy. The Housing Provisions require all loads along a bearer to be evenly distributed unless otherwise allowed (ABCB Housing Provisions 6.3.3(c)). Point loads on strutting beams are only allowed if within the middle third of the beam span (NCC Volume Two, H1D6 explanatory info).
9. Referenced Australian Standards:
10. AS 1684 - Residential timber-framed construction (use span tables and detailing).
11. AS 1170.0 / AS 1170.2 / AS 1170.4 - structural design actions (wind, earthquake) referenced for determining if the prescriptive provisions apply.
12. Where steel members are used with timber framing, design must also consider relevant steel standards and the Housing Provisions (e.g., structural steel nominal yield strength 250 MPa requirement in 6.3.2(2)).
13. NCC/ABCB clause references:
14. NCC 2022 Volume Two - Part H (Section 2 and Part 6.3 references).
15. ABCB Housing Provisions Standard 2022 - Part 6.3 (Structural steel members) including Tables 6.3.3a-d and Figures H1D6a-e, and clauses H1D6(1)-(7).

Residential vs Commercial

1. Residential (Class 1 and 10): The prescriptive span tables and Housing Provisions are primarily targeted at Class 1 and 10 buildings. AS 1684 span tables and the Housing Provisions provide Deemed-to-Satisfy pathways for typical domestic framing without bespoke engineering, provided the project fits the stated limits (e.g., building geometry, load distributions, wind/earthquake limitations). See NCC Volume Two and ABCB Housing Provisions Part 6.3 for specific table references (e.g., Table H1D6a, Table 6.3.3a).
2. Commercial (Class 2-9): Commercial and multi-storey buildings generally fall under NCC Volume One and require higher design rigour. Span tables from AS 1684 are not intended for many Class 2-9 applications. Where timber framing is used for Class 2-9, designers must follow NCC Volume One structural provisions and relevant Australian Standards and will often need a structural design-refer to NCC Volume One clauses and the specific FRL/load requirements. The Housing Provisions do not apply to these commercial classes; point loads, unusual spans, higher imposed loads or different occupancy loadings require engineer design.

Exceptions and Exemptions

1. The Housing Provisions do not have to be complied with if alternative compliance is used under H1D6(5)
2. (a) or
3. (b) (for example, engineered designs or other acceptable methods). See ABCB Housing Provisions 6.3.1 and H1D6(5).
4. Where conditions exceed the limitations in H1D6 (e.g., snow loads, specific earthquake design requirements from AS 1170.4, roof pitch or geometry outside allowed limits), the Deemed-to-Satisfy tables cannot be used and a structural engineer must design members.
5. Use of structural software is restricted by H1D6(7); software that replaces the manual methodology in AS 1684 is acceptable for member sizing, but full structural packages must comply with the ABCB Protocol for Structural Software and geometric limits in H1D6(7)(b).
6. Point loads on bearers and strutting beams are restricted: point loads on strutting beams are permitted only if located within the middle third of the beam span (NCC Volume Two, explanatory information to H1D6).

State and Territory Variations

1. The NCC is national but each state and territory has a schedule that may modify or replace national provisions. Users must check the relevant state schedule in NCC 2022 Volume Two (Schedules 4-12) for local amendments.
2. Examples to note:
3. Queensland, New South Wales, Victoria, Western Australia and other jurisdictions may have specific requirements for cyclonic regions, local wind maps or additional conditions referenced in their state schedules; the Housing Provisions reference that design wind speeds may be available from the appropriate authority and that cyclonic regions are mapped in Part 2.2 (NCC Volume Two, H1D6 explanatory info).
4. Where a state schedule imposes different limits for appendices to AS 1170.4 or additional earthquake requirements, the prescriptive Housing Provisions may not apply. Always verify against the state schedule in NCC 2022 Volume Two (Schedules 4-12).

Practical Compliance Tips

1. Use the right table: Always select the correct Housing Provision table for single versus continuous spans (Table H1D6a for single spanning; Table H1D6b for continuous spanning) and check the associated figure for effective load width definitions.
2. Check geometric limits first: Confirm building geometry (eave height, roof height, width, length, roof pitch) is within H1D6(7)
3. (b) limits before relying on prescriptive tables; if not, obtain an engineered design.
4. Treat loads as evenly distributed: Design and layout to avoid concentrated point loads on bearers unless expressly allowed; if point loads are present, ensure they fall within the allowed middle third of a strutting beam or seek engineering confirmation (ABCB Housing Provisions 6.3.3(c)).
5. Reference AS 1684: Use AS 1684 span tables and detailing for timber framing members and bracing; if using software, ensure it follows the same methodologies permitted under H1D6(7)(c).
6. Document citations on drawings: When submitting for approval, cite the specific NCC/ABCB clause or table used (e.g., ABCB Housing Provisions Table 6.3.3a or NCC Volume Two, Table H1D6a) so certifiers can quickly verify the prescriptive pathway used.
7. Account for additional dead loads: Remember that heavier ceiling linings, insulation or finishes increase dead loads and may require up-rating member sizes or a design check; the Housing Provisions note insulation weight considerations (NCC Volume Two explanatory text to H1D6).
8. Verify state schedule variations: Before finalising design, check the relevant state or territory schedule in NCC 2022 Volume Two for any modifications or extra requirements (Schedules 4-12) that may affect allowable spans or design assumptions.

References

1. NCC 2022 Volume Two - Building Code of Australia, Part H and Figures/Tables H1D6a-H1D6e (Tables H1D6a and H1D6b cited above).
2. ABCB Housing Provisions Standard 2022, Part 6.3 Structural steel members and Tables 6.3.3a-d.
3. AS 1684 - Residential timber-framed construction (span tables and detailing).
4. AS 1170 series (structural actions) where wind, earthquake or snow loads are relevant.

Couldn’t find or include full numeric span tables in this article due to their tabulated nature; refer directly to the ABCB Housing Provisions Tables 6.3.3a-d and NCC Volume Two Table H1D6a/H1D6b for exact numeric span values and section selection when preparing designs.