AS 4055 Wind Loading for Residential Homes in Australia

What it is

AS 4055 is the Australian Standard that sets simplified wind classifications for housing. It is the everyday tool a builder, draftsperson or building surveyor uses to assign a single wind class to a Class 1 or Class 10 building (a house, townhouse, garage or carport). That single class then feeds into AS 1684 for timber framing, AS 4440 for roof tiling, AS 1562 for sheet roofing fixings, and AS 4773 for masonry small buildings. Without a wind class, none of those downstream codes can produce a span table or fixing schedule.

The full engineering wind code is AS/NZS 1170.2, which is general purpose and complex. AS 4055 takes the typical assumptions for a low rise house (mean roof height under 8.5 m, simple rectangular plan, basic terrain) and pre-calculates the design gust wind speed for each combination of region, terrain, shielding and topography. The output is a single letter and number, for example N2 or C2.

How wind classes break down

AS 4055 splits classes into two groups based on wind region under AS/NZS 1170.2.

Non-cyclonic classes (N1 to N6)

Non-cyclonic classes apply in wind regions A and B under AS/NZS 1170.2. These regions cover most of southern Australia, including Victoria, Tasmania, the ACT, most of New South Wales, southern Queensland, South Australia and southern Western Australia.

Class	Design gust wind speed (serviceability)	Design gust wind speed (ultimate)
N1	26 m/s	34 m/s
N2	26 m/s	40 m/s
N3	32 m/s	50 m/s
N4	39 m/s	61 m/s
N5	47 m/s	74 m/s
N6	55 m/s	86 m/s

N1 and N2 are the most common classes for inland suburbs in protected terrain. N3 is common on the urban fringe of coastal cities. N4 to N6 turn up on exposed ridgelines, large cleared rural blocks, and headland sites.

Cyclonic classes (C1 to C4)

Cyclonic classes apply in wind regions C and D under AS/NZS 1170.2. These cover the tropical coast: northern Queensland from roughly Bundaberg upward, the coast and offshore islands of the Northern Territory, the Kimberley and Pilbara coasts of Western Australia, and offshore islands such as Norfolk Island. Cyclonic classes have the same ultimate gust speeds as the matching non-cyclonic class but include cyclic loading effects (fatigue from repeated gust cycles) which drive heavier tie-down and stronger cladding fixings.

Class	Ultimate design gust wind speed
C1	50 m/s
C2	61 m/s
C3	74 m/s
C4	86 m/s

The four inputs that set the class

AS 4055 sets the class from four site factors.

Region

Read from the AS/NZS 1170.2 region map. Region A is most of southern Australia. Region B is parts of the central NSW and south-east Queensland coast. Regions C and D cover the cyclone-prone north.

Terrain category

A measure of the upwind ground roughness. TC1 is open water. TC2 is open country with scattered obstructions. TC2.5 is grassland with some buildings. TC3 is dense suburban or wooded. Most suburban housing sites land in TC2.5 or TC3.

Shielding

A measure of how much the surrounding houses or trees block wind on the subject site. Full shielding (FS), partial shielding (PS) or no shielding (NS). A bare paddock with the new house first in is NS. A typical infill block on a developed street is FS.

Topography

A measure of how the site sits on the land. Flat (T0) through hill, ridge or escarpment classes (T1 to T5). A house at the base of a ridge is T0. A house on top of a ridge can jump to T3 or higher.

Where it flows downstream

Once the wind class is set on the building approval drawings, every other structural code reads it directly. Truss manufacturers price and fabricate to the stated class. Frame fabricators size studs, lintels and bracing from AS 1684 supplements tagged with that class. Roofers select tile clip patterns or sheet fixing centres for the same class. If the class on the engineering drawings is wrong, the whole package is wrong.

Common errors that bite at certification

The most common issues that come up at building approval and frame inspection are:

• Wind class set in an office without a site inspection, missing local topography
• Shielding assumed as Full when the block is the first one developed on a new estate
• Region B assumed on the basis of the postcode when a small carve-out moves the site into Region A or vice versa
• Class reduced on the engineering plans to make framing cheaper without the site supporting it

The wind class belongs on the engineering plans, the truss layout, and the bracing schedule. Mismatches between these documents almost always trigger a hold on the build at frame inspection.