Floating houses gaining ground
You wouldn’t know it from the street but a house in Kaiapoi, north of Christchurch, is designed to float.
Built on a flood and quakeprone 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 floodprone areas and locations 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.
Ashby helped create a sitespecific design for the Kaiapoi house, which looks like an ordinary home with one big difference – it is flood-proof.
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 pontoon is 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 floodwaters recedes.
Ashby said the biggest barrier to building floating houses was the cost. The pontoon and the subfloor structure of the Kaiapoi house cost about $100,000.
The price has put at least one Christchurch homeowner’s plans on hold. He was told the foundations and flotation for a 90sqm home he planned for a Kerrs Reach property would cost about $160,000. The cost needed to drop to about $60,000 before it could be considered viable.
Ashby said there were also problems with the fact many planners were not familiar with floating houses so 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.