How to read AS 1684 span tables: a step-by-step guide for builders and chippies
Step-by-step process for reading AS 1684.2 span tables on site. Wind class, stress grade, load width, spacing, and the roof-down sequence for residential timber.
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Wrong inputs kill span table selections more than anything else. Wind class mismatch (designing to N2 on an N3 site) and wrong stress grade (assuming MGP10 when the supplier delivers F7) produce undersized members that pass the frame inspection then fail in a storm. The span table process has a fixed sequence: confirm wind class from the site classification first, then pick the right supplement booklet, then work roof-down from battens to bearers. Each member type has its own table keyed to a specific set of inputs. Miss one input and the whole row selection is wrong.
When you do this
Any time you need to select timber member sizes for a residential timber-framed Class 1 or Class 10 building under AS 1684.2:2021 (non-cyclonic) or AS 1684.3:2021 (cyclonic). This includes new builds, additions, and alterations where the framing is affected.
Do this before ordering timber. Changing member sizes on site after timber is cut wastes a day and costs margin.
Who’s involved
- Builder / foreman: confirms site wind classification and checks the frame is built to the selected sizes
- Chippy: uses the span tables to select member sizes and cut accordingly; may prepare the span table selections for the building surveyor
- Building surveyor / certifier: checks the span table selections are from the correct supplement for the confirmed wind class, and that the tie-down schedule matches AS 1684 uplift tables
- Structural engineer: involved only when the project is outside AS 1684 scope (N5/N6 wind class, out-of-scope geometry, LVL or engineered timber)
Steps
Step 1: confirm the site wind classification
This is the non-negotiable first step. The wind classification determines which supplement booklet you open. Using the wrong wind class produces a frame that is structurally wrong in a way that a visual inspection will not catch.
The wind classification for the site must be determined under AS 4055:2021 (verified 2026-05-09, Standards Australia store). Inputs to the classification are: wind region (from AS/NZS 1170.2 maps), terrain category (TC1 to TC3), shielding (S1-S3), and topographic effects (T1-T5).
For non-cyclonic areas the classes are N1, N2, N3, and N4. N5 and N6 are outside AS 1684.2 scope and require an engineer. For cyclonic areas use AS 1684.3 (C1-C3; C4 requires an engineer).
The wind classification must be documented. It should appear on the drawings submitted for the building permit. Do not assume N2 on a coastal site. Coastal sites with open exposure commonly classify N3 or N4.
Step 2: confirm the timber species and stress grade
Before opening a span table, confirm what timber will actually be supplied. The supplement booklets are specific to stress grade, not to species name.
Common stress grades in AS 1684.2 supplement tables (verified 2026-05-09, slideserve.com AS 1684.2 span table guide):
| Timber category | Stress grades covered |
|---|---|
| Seasoned softwood | MGP10, MGP12, MGP15, F5, F7, F8 |
| Unseasoned softwood | F5, F7 |
| Seasoned hardwood | F14, F17, F27 |
| Unseasoned hardwood | F8, F11, F14, F17 |
MGP grades apply to machine-graded pine only. MGP10 is equivalent to F8 for most span table purposes; MGP12 is broadly equivalent to F11; MGP15 to F14. The supplement booklets list equivalent substitutions explicitly.
If the supplier quotes you MGP10 pine but you designed to F8 tables, verify the equivalence in the supplement before proceeding. If LVL is being specified for any member, do not use AS 1684 span tables: LVL requires the manufacturer’s certified span tables, not AS 1684 (see LVL).
Step 3: select the correct supplement booklet
AS 1684.2:2021 span tables are not in the main standard body. They are published as separate supplement booklets keyed to combinations of wind classification and stress grade. Each booklet contains 53 design tables covering all the major framing members (verified 2026-05-09, slideserve.com AS 1684.2 span table guide).
Pick the supplement that matches:
- Wind class (N1/N2 combined, N3, or N4)
- Timber type (seasoned softwood, unseasoned softwood, seasoned hardwood, unseasoned hardwood)
- Stress grade within that type
Using the wrong supplement is the most common span table error. Check the cover page of the booklet confirms the wind class and stress grade before reading any tables inside.
Step 4: work roof-down in sequence
Design sequence runs from the roof down because loads from above determine the loads on members below. Do not jump to floor joists before you have confirmed roof loads. The sequence is (verified 2026-05-09, slideserve.com AS 1684.2 span table guide):
- Roof battens
- Rafters
- Ceiling joists
- Underpurlins and ridge boards
- Wall studs and plates
- Lintels over openings
- Floor joists
- Bearers
Each member feeds load information into the member below it. Get the rafter size wrong and you risk undersizing the underpurlin that supports it at mid-span.
Step 5: identify the inputs for each member type
Every table in the supplement requires a specific set of inputs. Using only some inputs and guessing the rest will produce a wrong result. The inputs vary by member type.
Rafters
| Input | What to determine |
|---|---|
| Wind class | From Step 1 |
| Stress grade | From Step 2 |
| Roof covering type and mass | Tiles (heavy) vs sheet metal (light); changes the load assumption |
| Rafter spacing | Typically 450mm, 600mm, or 900mm centres |
| Rafter span | Clear span between supports (not overall length including overhang) |
| Roof slope | Pitch in degrees; some tables separate low-pitch from standard |
Ceiling joists
| Input | What to determine |
|---|---|
| Wind class | From Step 1 |
| Stress grade | From Step 2 |
| Ceiling type | Plasterboard (standard) or no ceiling |
| Joist spacing | 450mm or 600mm typical |
| Joist span | Clear span between supports |
| Single or continuous span | Continuous span allows longer run than single |
Floor joists
| Input | What to determine |
|---|---|
| Wind class | From Step 1 |
| Stress grade | From Step 2 |
| Roof load width (RLW) | Width of roof structure contributing load to the floor system; measured horizontally from the building perimeter |
| Floor load width (FLW) | Width of floor contributing load to the supporting bearer |
| Joist spacing | 450mm or 600mm typical |
| Joist span | Clear span between bearers |
Roof load width (RLW) and floor load width (FLW) are the inputs most often guessed incorrectly. RLW is measured horizontally from the perimeter of the building to the nearest internal support (or ridge for single-span roofs). FLW is measured as the contributory width of floor carried by each bearer.
Bearers
| Input | What to determine |
|---|---|
| Wind class | From Step 1 |
| Stress grade | From Step 2 |
| Floor load width (FLW) | Contributory floor width to the bearer |
| Roof load width (RLW) | If the bearer also carries wall and roof loads |
| Bearer span | Clear span between stumps, piers, or foundation supports |
Lintels over openings
| Input | What to determine |
|---|---|
| Wind class | From Step 1 |
| Stress grade | From Step 2 |
| Roof load width (RLW) | Width of roof above the opening contributing to lintel load |
| Floor load width (FLW) | Floor load above if applicable (upper storey) |
| Lintel span | Clear span of the opening |
| Wall type | Load-bearing or non-load-bearing; external or internal |
Step 6: read the table
Once you have all inputs for the member type, locate the correct table in the supplement booklet. The table header confirms the wind class and timber grade. The rows and columns are typically:
- Rows: member spacing (or load width for floor/roof members)
- Columns: span lengths
- Cell values: the minimum timber size that passes for that combination (e.g. 90 x 45 MGP10)
Where the table has multiple sub-sections (e.g. light vs heavy roof), make sure you are in the correct sub-section for the roof covering type.
If your span or load width falls between two values in the table, always step up to the next larger value. Do not interpolate between cells.
Step 7: check the tie-down and bracing schedules against the same wind class
Selecting the right timber size from the span tables is only part of the frame design. The tie-down connections and bracing layout must also be derived from the same wind class used in the span table selection. AS 1684.2 provides uplift forces and connector requirements keyed to wind class in separate tables. The building surveyor will check that the span table selections, tie-down schedule, and bracing plan all use the same wind class as the site classification.
Step 8: document the selections
Before the frame starts: record the supplement booklet used (wind class, stress grade, edition), the member type, inputs, and selected size for each member in the frame. This record is part of the permit documentation and is the basis for the framing inspection.
If any member selection goes outside the scope of the span tables (geometry, wind class, timber type), note this and obtain an engineer’s design for that element. Do not substitute a proprietary engineering product without the manufacturer’s certified span tables.
Try it
Common holds
| Hold | Cause | Fix |
|---|---|---|
| Wrong wind class applied | Wind classification not obtained before design | Get written wind classification from certifier or engineer before opening span tables |
| Wrong supplement booklet used | Assumed wind class or stress grade | Check the booklet cover page matches the confirmed wind class and confirmed timber supply |
| Load width guessed | RLW or FLW not calculated from drawings | Measure from dimensioned floor plan; confirm with the designer if building geometry is unusual |
| Span measured to wrong point | Overall length used instead of clear span | Measure between face of supports, not to outside of wall |
| LVL selected from AS 1684 table | AS 1684 span tables do not cover LVL | Use the LVL manufacturer’s certified span tables instead |
| Sizes selected from superseded supplement | Old 2010 edition supplement used | Confirm supplement edition matches AS 1684.2:2021; supplementary tables for 2021 are keyed to the 2021 joint group (JD5) not the 2010 (JD4) |
Documents needed
- Site wind classification (from certifier or engineer, referencing AS 4055:2021)
- AS 1684.2:2021 supplement booklets for the confirmed wind class and stress grade
- Dimensioned floor plan and roof plan (to calculate spans and load widths)
- Span table selections record (member type, table reference, inputs, selected size)
- Tie-down schedule (from AS 1684.2 uplift tables, same wind class)
- Bracing plan (bracing capacity from AS 1684.2, same wind class)
References
- Standards Australia, AS 1684.2:2021 Amd 1:2024, Residential timber-framed construction, Part 2: Non-cyclonic areas. https://store.standards.org.au/product/as-1684-2-2021-amd-1-2024 (verified 2026-05-09).
- Standards Australia, AS 4055:2021 Amd 1:2024, Wind loads for housing. https://store.standards.org.au/product/as-4055-2021-amd-1-2024 (verified 2026-05-09).
- Australian Building Codes Board, NCC 2022 Volume Two, Part H1 Structure, clause H1D6(4). https://ncc.abcb.gov.au/editions/ncc-2022/adopted/volume-two/h-class-1-and-10-buildings/part-h1-structure (verified 2026-05-09).
- Forest and Wood Products Australia / WoodSolutions, Using AS 1684.2 Span Tables (Teaching Guide). https://www.slideserve.com/angus/timber-framing-using-as-1684-2-span-tables (verified 2026-05-09).
Related
- AS 1684 timber framing standard: builder’s guide, the reference overview of all four parts of AS 1684, wind class coverage, and NCC compliance pathway
- Span tables, what span tables are and how the AS 1684 table system is organised
- Stress grade, timber stress grading systems (MGP, F-grades) and what they mean for span table selection
- Wind classification, how wind classes N1-N4 and C1-C3 are determined under AS 4055
- Load width, roof load width (RLW) and floor load width (FLW) as used in AS 1684 span table inputs
- Lintel, sizing lintels over openings from AS 1684 span tables
- LVL, why AS 1684 span tables do not apply to LVL and what to use instead
- Tie-down, the connection schedule that must match the same wind class as the span table selections
See also
- Noggings, horizontal blocking members with spacing governed by AS 1684
- Batten, roof battens as the first member in the top-down span table design sequence
- Purlin, horizontal roof support members between rafters and wall frames
- Engineers’ details, the alternative design path when the project is outside AS 1684 scope
- MGP10, the most common machine-graded pine stress grade used in AS 1684.2 span tables
- AS 4055 wind loads for housing, the wind classification standard referenced in Step 1
Last updated: 2026-05-09. Verified: 2026-05-09. Quarterly review for currency: confirm AS 1684.2 and AS 4055 edition status at Standards Australia.