# Rafter Spans and Roof Members: Geometry Is Only the Start Separate roof geometry from structural member selection, including loading, support arrangements and the approved roof design. - Canonical HTML: https://www.chippy.tools/building-codes/au/framing/rafter-spans-and-roof-members/ - Publisher: Chippy Tools - Updated: 2026-09-06 - Market: au - Code edition / reference: NCC 2022 reference pathway - References: AS 1684.2; NCC22 H1D6 - Scope: Australian housing framing: reading plans, references and product details. No permitted member spans are supplied. - Source text checked: 2026-09-06 - Review scope: Public source text checked for this explanatory guide. No licensed span-table values or project design are verified. - Worked arithmetic checked: 2026-09-06 ## Separate rafter length from rafter span Roof geometry gives you lengths and angles. A structural check selects members that can carry the loads. Knowing the sloping length of a rafter does not tell you its permitted span or timber size. ## Identify the member first | Member | What to find on the roof plan | |---------------------|---------------------------------------------------------------------| | Rafter | Its spacing, supports and any overhang | | Underpurlin | Where it supports rafters and where its own struts or supports land | | Ridge or hip member | Its role, joints and support detail | | Roof truss | The truss mark, layout and supplier’s design details | A roof member’s name does not prove how it is supported. Read the plan detail with it. A ridge board and a load-bearing ridge beam do different jobs. A rafter table cannot stand in for a truss design. [NCC 2022 H1D6(4)](https://ncc.abcb.gov.au/editions/ncc-2022/adopted/volume-two/h-class-1-and-10-buildings/part-h1-structure) lists timber framing routes and a separate design standard for nailplated timber roof trusses. See the [AS 1684 guide](https://www.chippy.tools/building-codes/au/framing/as-1684-span-tables-explained/) for that split. ## Match the roof and the loads Record the roof covering, ceiling, member spacing and support layout. Use the load width or area required by the chosen table. Do not assume every member takes the whole roof width. Check the [site wind report](https://www.chippy.tools/building-codes/au/framing/wind-classification-australia/) too. [WoodSolutions Guide 40](https://www.woodsolutions.com.au/system/files/WS%20TDG%2040%20Building%20Timber-framed%20Houses%20to%20Resist%20Wind%2011-20.pdf), sections 5.4 and 5.5, treats roof members and their connections together. A sound member still needs the specified joints and tie-downs. The guide dates from 2016; use it for background, not as a current table of permitted sizes. ![Rafter geometry example: a horizontal run of 3 m and rise of 2 m give a sloping line of about 3.606 m.](https://www.chippy.tools/building-codes/au/framing/rafter-spans-and-roof-members/workflow.svg) **Diagram labels:** RAFTER GEOMETRY EXAMPLE; CHIPPY TOOLS; RAFTER GEOMETRY EXAMPLE; Horizontal run: 3 m; Rise; 2 m; √13 ≈ 3.606 m; Assumed geometry • No permitted span shown *Original Chippy Tools geometry example: √(3² + 2²) ≈ 3.606 m. These are assumed dimensions, not a permitted structural span.* ## A length example with clear limits Assume a straight sloping line has a 3 m horizontal run and 2 m rise. Its length is: `√(3² + 2²) = √13 ≈ 3.606 m` That is the line between those two points. It excludes any extra eave length, ridge allowance and cut details. It also says nothing about timber capacity. Use the [rafter length guide](https://www.chippy.tools/building-codes/concrete/rafter-length/) to follow the geometry and keep the structural detail beside it. ## Check a change before cutting Suppose a tiled roof changes to sheet roofing. Tell the designer; do not assume the change only makes the job easier. Roof weight and wind uplift must both be addressed. Confirm the member list and fixing details before cutting or ordering. Record the plan version, each member mark, grade, size, length and support detail. Keep the [Chippy Tools code-check sheet](https://www.chippy.tools/building-codes/printables/#code-check) with the roof take-off. Use it to flag any detail that is missing or unclear. ## Sources - [NCC 2022 H1D6(4)](https://ncc.abcb.gov.au/editions/ncc-2022/adopted/volume-two/h-class-1-and-10-buildings/part-h1-structure) - [WoodSolutions Guide 40 (2016), sections 5.4–5.5: roof members and connections](https://www.woodsolutions.com.au/system/files/WS%20TDG%2040%20Building%20Timber-framed%20Houses%20to%20Resist%20Wind%2011-20.pdf) Publisher links identify the standards. This review did not inspect or reproduce their licensed span tables. - [AS 1684.2:2021 publisher catalogue](https://store.standards.org.au/product/as-1684-2-2021) Content adapted from the National Construction Code 2022, provided by the Australian Building Codes Board under the CC BY 4.0 licence. This page is general information for tradespeople and is not compliance, engineering or legal advice. Requirements are subject to state and territory variations and adoption timing. Always confirm requirements with your building certifier and relevant state authority. ## Frequently asked questions ### Does a rafter length calculator give a permitted span? No. It works out geometry from the rise, run or pitch. Timber size, grade, loads, supports and connections need their own design check. ### Can I use a rafter span table for roof trusses? No. Use the truss supplier’s layout and design details for that system. NCC 2022 H1D6(4) lists a separate design standard for nailplated timber roof trusses. ### What does the 3.606 m example include? Only the sloping line for an assumed 3 m horizontal run and 2 m rise. It excludes eaves, ridge allowances and cut details, and does not approve a timber size. ### Does a lighter roof covering remove the need for a design check? No. Ask the designer to check the changed roof and fixing details. Wind uplift and connections still matter. ## Source-check references - [NCC 2022 H1D6(4)](https://ncc.abcb.gov.au/editions/ncc-2022/adopted/volume-two/h-class-1-and-10-buildings/part-h1-structure) - [WoodSolutions Guide 40 (2016), sections 5.4–5.5: roof members and connections](https://www.woodsolutions.com.au/system/files/WS%20TDG%2040%20Building%20Timber-framed%20Houses%20to%20Resist%20Wind%2011-20.pdf)