How wind design works, and why nobody can promise cyclone-proof

People searching for cyclone proof homes usually want one thing: a house that is still standing after the storm. That is a fair thing to want. The honest route to it is not a label. It is a wind design for your site, a frame built to that design, and a builder who installs every part of it correctly.

Why nobody can honestly promise a cyclone-proof home

A cyclone-proof promise would mean a building that survives any storm, on any site, however it was built. Nobody can know that. Storms vary, sites vary, and a house is only as good as its weakest connection.

What can be done is narrower and more useful. An engineer works out the wind loads your building has to resist on its site. The building is then designed, detailed and built to resist them.

So when a seller describes a cyclone rated home or a cyclone resistant house, ask what the rating refers to. Which site was it designed for? Who set the wind loads, and which parts of the building does it cover?

The load path, in plain language

Wind pushes on walls and tries to lift the roof. That force has to travel from the roof sheets down to the ground through an unbroken chain of parts and connections. Engineers call this the load path. Every link in it has to be strong enough, and fixed to the next link.

• Roof cladding and its fixings. The sheets catch the wind first, and the fixings hold them to the roof framing.
• Roof framing. Trusses or rafters collect that load and carry it to the walls.
• Roof-to-wall connections. Where the roof framing meets the top plate of the wall, the uplift has to be held.
• Walls and bracing. Wall frames carry the load down, and braced bays stop the walls racking sideways.
• Hold-downs. Brackets and anchors tie the frame to the foundation so it cannot lift or slide.
• Foundations. The slab or piers hold the building down and pass the load into the ground.

Render · design intent

A chain fails at its weakest link. A well-designed frame does not help if a roof sheet is under-fixed or a hold-down anchor was never set in the slab. The small fixings at the top matter as much as the steel in the middle.

What the engineer needs to know about your site

Wind design is specific to a site and a building. Two identical houses on the same island can need different designs. The engineer looks at:

• Site location, which sets the wind conditions the design starts from.
• Exposure, meaning how open the site is to the wind from each direction.
• Terrain, meaning what surrounds the site: open water, open ground, trees or other buildings.
• Topography, because hills, ridges and slopes can speed up the wind.
• Building shape, including height, roof form and pitch, overhangs and verandas.
• Importance, meaning what the building is used for and how much is at stake if it fails.

From this, the project engineer determines the design wind loads, working to the loading standard and requirements the local building authority accepts. We do not publish wind speeds or wind regions for particular islands, because they belong to the engineer and the authority, not to a website.

Engineering may be arranged through trusted independent engineers where required. It is quoted separately and remains subject to availability and project location. Those engineers are independent businesses, not our employees.

What a steel frame contributes, and what it depends on

The frame is the middle of the load path. We roll the wall frames and roof framing in New Zealand to the approved drawings. The bracing positions, the screw pattern at each connection and any hold-down brackets are set by the design and shown on the erection drawings. They are not left to judgement on the day.

Installation is where designs succeed or fail. The local builder sets the anchors accurately in the slab, installs the bracing in its designed positions, and puts every screw in the right place to the right depth. Foundations, cladding and roofing are designed and supplied by others, so they need the same wind design behind them.

Questions to ask before you order a cyclone rated home

• Who is doing the wind design for my site, and what site information do they need from me?
• Is the design based on my actual site, or on a general assumption?
• Which parts of the load path are in the framing package, and which are supplied by others?
• Who designs the foundations and the hold-down anchors?
• Are the roof cladding and its fixings specified for the same wind design as the frame?
• Who checks the work on site against the drawings?
• What does the local building authority need to see before and during construction?

You can start before any of these are answered. Tell us the island, the nearest port and anything you know about the site’s exposure and access. We will tell you what the quotation and the engineering still need.