AS 1684 Timber Framing for Residential Builds in Australia

What it is

AS 1684 is the Australian Standard for residential timber-framed construction. It is the deemed-to-satisfy reference in the National Construction Code (NCC) Housing Provisions for sizing and fixing timber framing in Class 1 and Class 10 buildings. Builders working on detached houses, townhouses, garages and carports in AU follow this code when the design fits inside its scope. If the design pushes outside scope (large openings, tall walls, unusual roof shapes) a structural engineer must design the framing under AS 1720.1.

The code is split into parts. Part 1 sets the design criteria. Part 2 covers non-cyclonic wind areas. Part 3 covers cyclonic wind areas. Part 4 covers simplified design for non-cyclonic areas. Most residential builders use Part 2 or Part 3 plus the matching span table supplements for the relevant stress grade and wind classification.

Cyclonic versus non-cyclonic

The split between Parts 2 and 3 is the most important branch in the code. It is driven by wind region under AS/NZS 1170.2 and wind class under AS 4055.

Non-cyclonic areas (AS 1684.2)

AS 1684.2 applies where the wind class is N1 to N4. This covers most of Australia south of the Tropic of Capricorn including all of Victoria, Tasmania, the ACT, most of New South Wales, southern Queensland, southern Western Australia and South Australia. Bracing requirements use kN per metre values pulled from the bracing tables for the relevant wind class. Connection requirements (tie-down, fixings) are lighter than in cyclonic areas.

Cyclonic areas (AS 1684.3)

AS 1684.3 applies in wind classes C1 to C3 (some references include C4 by engineered design). Cyclonic areas cover coastal northern Queensland, the Top End of the Northern Territory, the Kimberley and Pilbara coasts of Western Australia, and parts of coastal New South Wales subject to cyclone modelling. The code requires heavier tie-down at every load path (roof to wall, wall to floor, floor to footing), additional bracing capacity, and proprietary cyclone-rated connectors at hold-down points.

Wind classes and how they flow through the code

AS 4055 classifies wind for housing in six non-cyclonic classes (N1 to N6) and four cyclonic classes (C1 to C4). The class drives almost every structural decision in AS 1684. Higher wind class means deeper rafters, larger lintels, more bracing and stronger tie-downs.

Bracing rules in summary

Bracing is the part of AS 1684 that catches builders out most often. Each storey of the building needs enough bracing capacity to resist racking loads in two directions. The required bracing is read from a table based on:

• Wind class (from AS 4055)
• Roof type (sheet roof or tile roof)
• Building height and width
• Storey under consideration

Bracing is provided by braced walls. Common bracing types in residential framing include:

• Plywood or structural panel sheet bracing fixed to studs and plates
• Diagonal metal angle or strap bracing in let-in or face-fixed configurations
• Speed brace or similar proprietary tensioned brace systems

The capacity of each bracing unit (in kN) is given by the manufacturer or by the AS 1684 supplement. The total installed capacity must equal or exceed the required bracing in each direction at each level. Bracing must be distributed across the plan, not concentrated at one end of the building, with a minimum number of braced walls in each external wall line.

How AS 1684 sits under the NCC

The NCC Housing Provisions (Volume Two reference) call up AS 1684 as the acceptable construction practice for timber framing in Class 1 and 10 buildings. If a builder follows AS 1684 within its scope, the framing is deemed to comply with the NCC performance requirements for structural reliability. Stepping outside the scope (taller storey heights, wider spans, posted-up floors over 1800 mm, or unusual loads) means engineered design under AS 1720.1 takes over.

Certifiers and private building surveyors typically ask for framing layouts, span tables used, bracing schedules and tie-down schedules at frame inspection. If the wind class on the engineering documents does not match the bracing supplied on site, the frame will fail inspection.

Common framing failures at inspection

The framing defects that show up most often at residential frame inspection are:

• Insufficient bracing capacity for the wind class
• Bracing concentrated on one side of the building
• Missing tie-down at the top plate to stud connection
• Lintels too small for the opening and load width
• Wall studs notched or drilled beyond the limits in AS 1684

Each of these traces back to a misread of the wind class or a span table used outside its stated conditions (stress grade, spacing, supported load width).