Environmental zones

Wind, earthquake load and other environmental factors will determine your home’s design and maintenance requirements.

Every property is classified into wind, earthquake, snow load and corrosion zones.

These zones determine how strong any building on the property needs to be, what materials should be used, and how it should be maintained.

Why do zones matter?

When you build, renovate or maintain your home, knowing something about the zones your house or section is in will help you or your designer to choose the best materials.

It will also help you to design your home to take advantage of the site’s natural assets and to ensure it is strong enough to withstand the elements.

Using more durable materials may mean that less maintenance is required or that the materials will not need to be replaced as soon.
The zone of the property may also affect how often you need to carry out normal maintenance.

For example, a house built on the sea front will need washing more regularly than a house built in the middle of the island because of the effects of sea spray.

How do you know what zones you’re in?

The various calculations required to identify the zones your house or section is in will usually be worked out by the designer or engineer after a site visit. The local council may also be able to identify some of these issues for you.

The following zones are from New Zealand Standard 3604:1999 Timber Framed Buildings. This standard is an acceptable solution to the Building Code and guides much of the construction of timber-framed buildings in New Zealand. You can purchase a copy of this standard from the Standards New Zealand website.

Wind

Wind forces, like earthquakes, create horizontal forces on buildings. Any building needs to be sufficiently strong to withstand these forces.

There are five things to consider when looking at the effects of wind on your site:

• Wind region.
• Lee zone.
• Ground roughness.
• Site exposure.
• Topographic class.

Wind regions and lee zones map

This map is used to determine which wind region you are in and whether or not you are in a lee zone.

Topographic class and ground roughness

Land formations change wind flow:

• Wind speed increases as it passes over or between hills and through valleys. This is the wind tunnel effect you feel in cities with lots of tall buildings.
• Wind speed slows as it goes over rough terrain, caused by a drag effect.

The shape of the hills and valleys (the topography) and the amount of shelter upwind will also have an effect on the wind speed over your site. At least 500m of rougher ground is needed to affect the wind profile of your site.

Site exposure

The amount of protection from other buildings or natural obstructions affects whether your site is classed as ‘exposed’ or ‘sheltered’. Most suburban developments on flat or gently sloping sites would be considered ‘sheltered’.

Bracing requirements

The bracing design of your home will limit the amount of sway in your house during frequent wind storms and earthquakes. It will also stop the house collapsing due to more severe earthquakes or wind storms.

NZS 3604 can help you to determine your wind region, whether you are in a lee zone or if you need to consider rough terrain and wind drag.

The calculations for this determine the house bracing demand. The overall bracing demand for your site for wind will be classified as low, medium, high, very high or ‘specific design’. A house in a low wind zone will require less bracing than a house built in a very high wind zone.

In some cases an engineer will need to work out bracing requirements if the wind zone is higher than very high – this is called specific design

Earthquake

The earthquake zone your site is in will affect its bracing demand. As with wind, bracing demand is the horizontal forces the building, or parts of the building, can resist.

The cladding materials on the roof and walls of your house also affect bracing demand, as does the height of your house.

Snow loading

Snow loading will be assessed by your engineer or designer and may affect the design of lintels, rafters, ridge beams and verandah beams on your house.

Corrosion

Your building site is classified as being in one of the four zones above or in a sea spray zone. This zoning affects the materials you will use to build you home, especially claddings and metal fastenings.

Sea spray zones and geothermal areas cause materials to deteriorate much more quickly. For example, steel fastenings will corrode or rust more quickly than stainless steel fastenings when exposed to sea spray. Fastenings should be carefully selected depending upon the site requirements.

The zone your home is in may also affect cladding and flashing type and will affect the guarantees manufacturers offer on their products.