The winds are changing

With building quality and compliance putting a significant spotlight on the industry, it has never been more important to have a detailed understanding of how the correct site wind rating impacts your building. This will ensure your building designs and construction are carried out in accordance with the NCC and relevant Australian Standards. 

The site wind rating affects the design of most building elements including:

• footing size and type
• framing and member sizing
• bracing and tie down requirements
• cladding and roofing fixing requirements
• window and door types.

Even external elements, such as eave and soffit fixing and framing, as well as external waterproofing against wind driven rain, are impacted and have a specific specification based on the site wind rating.

When the wind rating is applied to these elements, it will impact the design and may even limit the design type for certain wind classes – particularly in cyclonic areas of Australia.

The framing design for a dwelling, including member sizes and maximum spans, will be influenced because the loads will vary greatly based on the relevant site wind rating.
Additionally, the bracing design, including material types, fixings, spacings and the number of bracing units for the frame, need to be specifically designed for the house based on the wind rating. For brick buildings, columns may need to be built in to support the brickwork against the wind loads. 

Similarly, the fixing of the frame and tying down to the slab or sub-floor, as well as truss and roofing member tie downs, need to be specifically designed for a house. AS1684 Residential timber-framed construction and the NASH standard for lightweight steel framing sets out these requirements in the relevant span tables, and bracing and fixing requirements.

The wind rating can also limit the use of certain material types or products. For example, wind ratings of N3 or lower (that is, N1, N2 and N3) are included in a number of simplified design scenarios, such as those in the deemed to comply solutions in the NCC wall claddings. 

Given these differing and critical factors for the compliance and ongoing performance of the building, it highlights how important it is to ensure the building’s design and construction is carried out to suit the relevant wind rating for the site.

The wind rating for the site will determine fixing types and spacings of fixings for wall cladding

Assigning wind ratings 

Determining the wind rating for the site is not as simple as it may seem from the note on your plan or engineer’s report. Rather, the site wind rating is determined based on a range of factors including: the site location; the slope of the site and surrounding areas; building geometry (such as width, length and height of building); and any shielding from adjoining structures or landscapes or similar. 

As such, it is best to engage an appropriately skilled design professional, usually a structural engineer, to determine the site wind rating and framing design or other factors specifically for that site. 

Even for new greenfield estates the shielding factors can be quite complicated, with some assumptions being required on the surrounding structures that might be built, particularly early on in the establishment of a new housing estate.

 

Getting the correct wind rating or wind class for the site you are building on is critical

Understanding product limitations

The wind rating for the site will also determine fixing types and spacings of fixings for wall and roof cladding, which will differ based on whether the site is located in N1/N2, N3 or C1 to C4. For wall claddings, the wind rating can also limit maximum stud spacings, could exclude types of fixings from being used or require higher capacity fixings to be used, particularly in cyclonic areas. For roof cladding, the rating can similarly influence batten spacings and fixing types.

It is also typical for specifications for roof and wall claddings to prescribe different fixing requirements for the middle of the roof or wall area and with increased fixing requirements within 1.2 metres of the edges or corners. This takes into account the greater forces and pressures that act on the corners and edges of the building as opposed to the central field of the dwelling.

Therefore, it is critically important to look through the product specifications for the materials being used onsite, and in particular when scoping the use of the product, noting any listed limitations or conditions. Many products and systems have test reports or certificates that accompany the product which list out the range of limitations that apply. Approval authorities will commonly request this information as part of building inspections or issuing of approvals. 

These limitations and conditions of use, and the detailed information in the product specifications, are also directly related to warranties on products.

Wind rating and compliance

Most of the time we take wind ratings for granted, and sometimes we can: for typical sites, a common design that is used project to project will perform as expected and not have any long-term issues. 

However, it’s best to be cautious and pay extra attention because getting something wrong, even simple things, such as incorrect fixing or not fully fixing off brackets or tie downs in high wind events, could prove to be the weak point of the building. It could cause other building elements to fail and result in very costly and difficult issues to rectify afterwards. 

This article was developed with input from the Cyclone Testing Station at James Cook University. For the first instalment of this series visit www.hia.com.au/housing/in-focus/2020/on-the-right-footing.