Floating houses ‘solution’ for NZ
You wouldn’t know it from the street but there is a house just north of Christchurch that’s designed to float.
Built on a flood and quake-prone site, the house sits on solid ground but its design allows it to float, if necessary.
Ashby Consulting Engineering principal engineer Colin Ashby believes floating houses are an ideal solution for flood-prone areas and those where the ground could be subject to liquefaction.
They are used in other parts of the world, particularly Holland and parts of the United States and Canada, but are rarely considered in New Zealand.
The single-storey rectangular home is about 140 square metres and has timber framing and weatherboard/lightweight cladding. The roof is lightweight corrugated steel and the chimney is also a lightweight design.
In place of underfloor framing, the house sits on a catamaran pontoon made of polystyrene, wrapped in a waterproof material and sheathed in plywood for protection from ultra-violet light and the weather.
The pontoons are attached to piles to stop the house from floating away, but it can rise in a flood, resist water flows of up to 3m per second, and settle back in the same place after the floodwater recedes.
Ashby admits the biggest barrier to building more floating houses is how much they cost.
The pontoons and the subfloor structure of the Kaiapoi house cost about $100,000 – too expensive for many others to consider.
The price tag has put at least one Christchurch homeowner’s plans on hold. He was told the foundations and flotation for a 90sqm basic boxlike home he planned for a Kerr’s Reach property would cost about $160,000. The cost needed to drop to about $60,000 before the project could be considered viable.
Ashby said there were also problems with the fact many planners were not familiar with floating houses and provisions for them were not included in district plans.
The Resource Management Act was originally drafted to make things easier, but people often had to ‘‘jump through hoops and pay a fortune in resource consent fees’’, he said.
Christchurch-based New Zealand Planning Institute board member Jonathan Clease said floating houses were untried in New Zealand, so he was not surprised councils were wary of approving consent.
Authorities had to be cautious about rebuilding in areas known to be at risk of hazards such as flooding, especially if they were considering building solutions that had never been tried in their districts before, he said.
Christchurch City Council head of resource consents John Higgins said there was nothing preventing floating foundations being used in the city from a resource consent perspective.
They still needed to comply with the same provisions as other houses, including zone requirements, minimum floor levels and recession plane angles, he said.
Offshore and Coastal Engineering Ltd director Gary Teear wants to put a floating show home on abandoned red-zoned land.
Teear’s company designed the tourist boat harbour at Milford Sound, which has floating wharves, and the floating berths for tourist vessels at Pearl Harbour near Manapouri.
He has offered to buy a piece of red zone land under the proviso he gets consent to build a demonstration floating house.
He believed there was great potential for floating houses near the Avon-Heathcote Estuary, where there was liquefaction during the earthquakes and ongoing concern about rising sea levels.
Floating foundations need not cost as much as the Kaiapoi house, and could cost less than those used in a conventional house, Teear said.