Wind region and wind specification for housing — AS 4055

The problem

Every house needs a wind classification before framing, bracing, tie-down and roof fixings can be specified, and that classification is determined under AS 4055. The first question in it is the site's wind region. That is a map question, and near a region boundary or a smoothed coastline a misread can move a site between a cyclonic and a non-cyclonic area.

Early in a job, a builder or engineer often wants a quick, sourced answer to that question from the address, and a first draft of the wind specification for the class they have determined.

How it works

The wind tool sits inside SiteIQ, the site assessment tool, on its Wind Detail tab. It is for houses only: AS 4055 is the wind standard for housing.

1. Run a site assessment from the address. SiteIQ supplies the state, latitude and longitude, reads the wind region and sub-region from Geoscience Australia's digitised map of the AS/NZS 1170.2 regions, and says whether the site is a cyclonic area under the NCC definition (regions B2, C and D).
2. Open the Wind Detail tab and enter the terrain category you determined for the site, TC1, TC2, TC2.5, TC3 or TC4, and the building height, ground to the highest point of the roof. Both are recorded as your inputs; nothing is calculated from them.
3. Select Assess Wind Region. The tool returns the region, sub-region and cyclonic status, with a warning wherever the region was not read at the site itself. It gives no design wind speed and no wind class. It says where to determine them: the design wind speed under AS/NZS 1170.2 (Sections 2–4), or the wind classification under AS 4055 for a house, both available from Standards Australia.
4. For a draft, choose the class you determined, any of N1 to N6 or C1 to C4, enter the design wind speed you determined if you want it stated, and select Generate Specification for Supplied Class. The AI draft states that you supplied the class, states your design wind speed as yours or says none was given, and lists the items you must specify, each pointing to the document it is taken from. Fixing sizes, spacings, grades and speeds that a pattern check finds in the model's text become [ENGINEER TO SPECIFY], and the step 3 warnings are carried into the text and the PDF.

Steps 1 to 3 are a fixed lookup. Only step 4 is written by AI.

Worked example

Illustrative inputs, not a real site:

• location: a suburban lot in Victoria (VIC), latitude −38.0, longitude 145.1, supplied by the site assessment
• terrain category: TC3
• building height: 6 m, a single-storey house

For these inputs the tool returns Region A, sub-region A5, not a cyclonic area, with no warnings. No design wind speed and no wind class are given. A test in the product's code runs these inputs through the tool and checks every value in this paragraph.

What you would do next: look at the site. The tool knows nothing about shielding from neighbouring houses or any nearby hill, ridge or escarpment. Determine the wind classification under AS 4055 yourself, or the design wind speed under AS/NZS 1170.2, and have it checked by a qualified engineer before it goes on the drawings.

What it doesn't do

• It gives no design wind speed and no wind class. The tool holds no regional wind speed, multiplier or class table. The class and the speed in a specification are always the ones you supplied.
• Its wind region map is an interpretation. Geoscience Australia's map digitises the Standard's figure with a smoothed coastline and is not for design. Within about 2 km of a more severe region, or between two mapped regions, the tool uses the more severe one and says so. Confirm the region against AS/NZS 1170.2 Figure 3.1.
• It checks nothing about the building. The terrain category and height you enter are recorded as yours. Whether the house is within the scope of AS 4055 is yours to confirm.
• The specification is a draft. It is AI-generated and may contain errors. The tool holds no fixing values, and the pattern check that replaces the model's can miss some, so read the whole text for any remaining size, spacing, grade or class. A qualified engineer must complete and check it.

The full text of AS 4055 and AS/NZS 1170.2 is available from Standards Australia.