Wind classification for houses, N1 to C4

A roof is fixed down to suit a wind class. These are the letters, who sets them, and why they show up in a re-roof quote.

Figures checked

Rules 3 min read Published

AS 4055 gives a house site a wind class of N1 to N6, C1 to C4. The class is worked out from the site's region, topography and shielding. Cyclonic areas are wind regions C and D. The class matters on a re-roof because fixing details are written for it: the QBCC repair guide works to C2 (61 m/s).

What a site's wind class is worked out from On the left, a house alone on a hilltop with long wind arrows reaching it, marked as exposed topography. On the right, a house in a row of neighbouring houses on flat ground with short arrows, marked as shielded. Above both, a note that the wind region sets the starting point and that regions C and D are the cyclonic ones. Wind region sets the starting point. Cyclonic: wind regions C and D. exposed hilltop shielded by neighbours
AS 4055 looks at three things: the wind region, the lie of the land and how sheltered the house is. Two houses in one street can end up in different classes.

What do the N and C classes mean?

N classes cover non-cyclonic areas and C classes cover cyclonic ones. The Housing Industry Association describes the AS 4055:2021 range as N1 to N6, C1 to C4, a set of site wind speed classes that designers, builders and product manufacturers specify against. Within a series, the classes step up with the wind the site has to handle.

Reading across the two series by number alone does not work, and we are not going to print a comparison table that implies it does. If you need the design wind speeds behind each class, they are in the standard itself.

How does a site get its class?

The HIA lists three inputs, and all three are about your particular block rather than your suburb.

  • Region. Where the site sits on the national wind region map.
  • Topography. The lie of the land, so a hilltop and a valley floor differ.
  • Shielding. Whether the house is protected, for instance by surrounding houses.

Two houses in one street can end up in different classes, and knocking down the house next door can change the shielding picture. Ask your builder, certifier or the approved plans for the class that applies to your site instead of reading it off a map.

Which parts of Australia are cyclonic?

In the National Construction Code, cyclonic areas are wind regions C and D. The region that covers a specific address is a question for your certifier or engineer, and it is worth having their answer in writing before a roofer prices the fixings.

Why does the class matter when re-roofing in Queensland?

The QBCC guide to repairing sheet metal roofs in cyclonic areas sets out roof sheeting fixings in line with manufacturer specifications for C2 (61 m/s). It does not stop at the sheets: it covers roof battens, roof framing and trusses, uplift forces on the wall studs, pre-drilling timber, what to do about split, decayed or insect-damaged timber, and washer sizes. Every one of those is a line a cheap quote can skip without you noticing until a storm.

What should a roofing quote say about wind?

  • The wind class the roof is being fixed for, in writing.
  • That fixings follow the manufacturer's specification for that class.
  • Who confirmed the class: the certifier, the engineer or the original plans.
  • What happens if battens or framing are found to need work once the old roof is off.

Our quote checklist covers the licence, contract and deposit side of the same document.

The wind classes and the factors behind them come from a named industry source, the Housing Industry Association. The cyclonic regions and the C2 repair detail come from official pages, linked below.