AS 2047 cyclone zones: which zone your opening sits in

Three documents decide the case, and they do not overlap. AS/NZS 1170.2 names the wind region for the building. AS 4055, the wind loading code for housing, names the wind class. AS 2047 is the performance standard the window is built, tested and labelled against, and its ratings are based on either AS/NZS 1170.2 or AS 4055. A specification that starts from the product and works backwards to the zone is the one that fails at the inspection, because the inspector reads the zone first.

For anything other than housing the region is read straight from AS/NZS 1170.2. For housing the National Construction Code routes the calculation through AS 4055, and the deemed-to-satisfy provisions for windows and glazed doors only apply inside the geometric limits that standard sets. A site outside those limits is not a higher class, it is a different calculation, and it goes back to AS/NZS 1170.2 with an engineered case.

AS/NZS 1170.2 divides the country into four wind regions, and the same map is published in the NCC. Region A covers the non-cyclonic south and inland, and it is subdivided A1 to A5 by wind direction, though the regional design wind speed is the same across A1 to A5. Region B is the intermediate band along the near-coastal fringe. Region C is where tropical cyclones cross the coast. Region D is the severe tropical cyclone coast, the north-west of Western Australia.

For a window the AS 2047 cyclone zone line is the one between B and C. Regions A and B are non-cyclonic, and Regions C and D are cyclonic. Crossing it adds a requirement the frame cannot meet on its own: in the cyclonic regions the glazed area takes an impact test, and that test is the reason the glass schedule changes when the region changes.

AS 4055 works in ten wind classes rather than four regions: N1 to N6 for non-cyclonic sites and C1 to C4 for cyclonic ones. The class is not read off a map on its own. It is the wind region combined with the terrain category, the shielding the site gets from its neighbours, and the topography. A ridge, a hill or an exposed coastal block lifts a site a class or two without moving it at all.

The classes do not form a straight ladder across the country. Region A spans N1 to N5, Region B spans N1 to N6, and the cyclonic regions C and D map onto C1 to C4. In Region D the top combinations have no value in AS 4055 at all: the table says to go to AS/NZS 1170.2 instead. A site that lands on one of those cells is not a higher class, it is a different calculation.

This is where the shorthand on most schedules goes wrong. N1 to N4 is not an AS/NZS 1170.2 range, and it is not the top of the AS 4055 scale either. The correct entry is the region, A to D, or the wind class, N or C, and the two answer different questions.

AS 2047:2014, Windows and external glazed doors in buildings, incorporating Amendment No. 1 (2016) and Amendment No. 2 (2017), is the standard the unit is built, tested and labelled against. It says in its own scope notes that the window ratings are based on either AS/NZS 1170.2 or AS 4055, the wind loading code developed for housing. The 2014 edition is the one that aligned the two: wind pressures for housing, including the cyclonic regions and the corner conditions, were updated to match AS 4055, and the guide to wind pressures in Appendix A was revised to match AS 1170.2.

The scope is wider than windows. It covers external windows, sliding and swinging glazed doors including French and bi-fold doors, adjustable louvres, shopfronts and window walls with one-piece framing elements. It does not cover skylights, rooflights, windows outside the vertical plane, or frameless sliding doors. Those sit with AS 1288, and a project that mixes the two has to keep the line straight.

What the standard tests is the assembly: deflection under the serviceability load, ultimate strength, water penetration resistance, air infiltration and operating force. The NCC calls it up as the deemed-to-satisfy route for the structural and weatherproofing performance requirements on Class 1 buildings, and the installation rules sit separately in Part 8.2 of the Housing Provisions. A compliant unit fitted wrongly is still non-compliant.

A window in housing carries a permanent performance label, positioned so it can still be read once the unit is installed, and not on the glass. It carries the manufacturer identification, the wind pressures at both the serviceability and the ultimate limit state, and the water penetration resistance. Timber windows in housing, and assemblies for buildings other than housing, may carry a certificate instead.

That label is the fastest check on a delivery. The wind pressures on it are read against the wind class on the structural drawings, and the two either match or the consignment is wrong. It is a two-minute check that catches the most expensive error in the chain: a full container of units rated for a calmer region.

The installation provisions are short and specific. Structural loads must not be transferred to the window assembly. A minimum 10 mm gap is left between the top of the assembly and any loadbearing framing or masonry wall, and that gap can be increased where the opening needs room for frame settlement. Packing, where used, runs along each side and the bottom, is fixed so it stays in place, and is kept clear of flashing material.

None of that is a site preference. The gap is what stops building movement loading the frame, and the packer rules are what keep the frame square so the sash still closes and the perimeter seal still works. Both are the reason a factory-rated unit can fail on site, and both belong on the same drawing as the wind class.

• The AS 2047 cyclone zone, stated as the wind region, A to D, or the wind class, N1 to N6 and C1 to C4, read from the site and not from the postcode.
• The basis: terrain category, shielding and topography, or the engineer reference to AS/NZS 1170.2 where AS 4055 has no value.
• The AS 2047 rating, with the label reference that proves it.
• The impact reference where the region is C or D, and the glass build that carries it.
• The head gap and the packer detail, cross-referenced to the installation provisions.
• The water penetration resistance, because a cyclone site is a rain site.

• Reading the suburb or the postcode instead of the site. A region line can run through a suburb, and the line, not the label, sets the rating.
• Using the N1 to N4 shorthand. It is neither the AS/NZS 1170.2 region range nor the top of the AS 4055 scale, and it hides whether the site is cyclonic at all.
• Rating every opening on a wall to the same class. AS 4055 places corners in their own pressure zone, and walls more than 1200 mm from an external corner are in the general area.
• Treating a region upgrade as a frame problem. Moving from B to C changes the glass schedule and adds the impact test, which is a different order from a deeper section.