How to build for New Zealand wind zones

If you're a soon-to-be homeowner building a new property, learning about wind zone requirements is very important - these could ultimately change your house design, which may have a knock-on effect on budget, scope, schedule, and whether you get council building compliance.

While wind zone calculations are best left to your builder or architect, homeowners can help make the process easier on themselves by understanding the basics. So, in this article we explore New Zealand wind zones, how to determine wind speed, and how to manage it.

'Wind zone' is a way of calculating the design load of building materials needed for construction in a given location.

What is a wind zone?

In New Zealand, 'wind zone' is a way of calculating the design load of building materials needed for construction in a given location, so that said materials can withstand local wind speeds. It also helps engineers and designers understand how to use those materials, and how to maintain them.

You may hear the term 'bracing demand' used by your builder, which refers to wind and earthquake forces on a home's timber bracing. The building can then be designed to suit these forces ('bracing capacity').

So what are the wind zones?

1. Low: Wind speeds below 32 metres per second.
2. Medium: 37 metres per second
3. High: 44 metres per second
4. Very high: 50 metres per second
5. Extra high: 55 metres per second
6. Specific design (SD): Over 55 metres per second

When your land is being classified, it will be given a wind zone fitting one of those six categories. These categories are assumptions based on a number of factors, and as such we recommend that you let a professional do a proper calculation rather than attempt it yourself. This is required for any site that is in the Extra High or SD zones.

Wellington is, perhaps to no surprise, considered a very windy region for wind zoning.

How to understand a site's wind speed

There are six steps to determining wind speed in New Zealand's building standards. These steps take into account the following factors:

1. Wind region: NZ has two 'wind regions', known as A and W. These are national averages based on MetService data.
2. Lee zones: Some parts of the country are in lee zones, which have higher wind speeds than either regions A or W.
3. Ground roughness: NZ homes are either in 'urban' or 'open' spaces, based on how built-up the area is. If there are more than 10 obstructions over 3 metres high (e.g. homes and trees) in a hectare, it will be classed as urban. The opposite applies to open areas. Therefore, even forested regions are technically urban when it comes to wind zoning.
4. Site exposure: Similar to roughness, exposure determines whether an area has nearby permanent shelter that is of similar size to the home being built. So, homes near beaches, open fields and similar will be classed as exposed.
5. Topographic class: Topographic class determines the topography of your site. It factors local geographical features (i.e. hills or escarpments), their steepness and where the build site is in respect to them.

The sixth step would then be to calculate the final wind zone, and use these measurements to find the bracing requirement. A professional will take care of this tricky part for you, but you can use these steps to get a feel for what the wind zone might be, and to start thinking about home designs to suit. If you aren't sure, just ask your builder or architect.

Your local council may also have wind zone information on file for your site, though a general site visit can tell you much about the wind even without the mathematics. If there is little-developed or stunted vegetation, vegetation that appears to have been shaped by wind, or local neighbours who have wind break fences, all of these indicate high wind speeds.

The first step to managing wind speed is taking wind zoning into account early on in the design process

Designing for wind zones

The first step to managing wind speed on your property is taking wind zoning into account early on in the design process. Indeed, it's needlessly expensive to submit plans to the council only to have them declined.

Additionally to timber bracing, wind also impacts other elements of design, which will have to be accounted for. This includes window and door placement, weathertightness, and of course general outdoor living enjoyment - building a home near the beach is great for taking advantage of the gorgeous NZ coast, but those hot days may also bring with them afternoon winds (hot air rises on land after a sunny day, to be replaced by cold air from the water). Thus, you may choose not to place an outdoor seating area on the beach side of the property without shelter.

The following are architectural changes or additions you can make that will mitigate wind:

1. House orientation

With some forethought, you can orientate your home to minimise strong, cold winds while still harnessing slight breezes to cool you during summer. In order to determine house orientation, you must consider:

1. Which direction prevailing winds typically arrive from (i.e. the coast).
2. If wind changes in direction or strength with the season.
3. If wind changes in direction or strength throughout the day (like our beach winds example above).
4. Site exposure.
5. Site roughness.

Orientation can play a major role in mitigating the effects of wind on a home.

2. House height

Single-storey houses have less surface area exposed to wind, so are typically better in windier areas.

3. Windows and doors

Windows and doors must be able to withstand winds - even hurricane-force or straight-line winds, if that's a risk of your area - without breaking, leaking or letting in drafts. If you live in a higher wind zone, look for glass that will withstand the pressure, and frames designed to account for horizontal forces (like rain coming in sideways).

Some practical tips:

1. Put doorways in sheltered recesses so they can be opened even during windy days.
2. Install operable windows on your less-windy sides so you can open them without wind blasting in.
3. Use sliding/stacking doors or sliding windows on windy sides so they can't slam shut. Sliding doors will also whistle less than regular doors, and because they are typically better sealed, they let less water into the house, too.

Our building performance research team is capable of testing to #wind and #wet conditions helping industry to provide better buildings for New Zealanders. https://t.co/x3Fe0kbl1E pic.twitter.com/N9WVljSoOd

— BRANZ (@BRANZlive) December 12, 2017

4. Landscaping

When mitigating wind speed, small landscaping additions can make a big difference. Have you thought about any of these for your future home comfort?

1. Dense evergreen trees are great windbreaks as they have low crowns, but are also tall enough to deflect wind for higher houses.
2. Earthen berms can lift wind away from your garden, courtyard or single-storey home.
3. Windbreak fences, densely-planted vegetation and walls are also good at deflecting ground-level wind.

Choose Altus Window Systems for your windows and doors

Altus Window Systems is one of New Zealand's leading windows and doors designers, with a variety of products to suit all aesthetic tastes, thermal requirements and - importantly - wind zones. Check out our products page to see what we have available, or contact us today to find a local fabricator near you.