Four state houses that started to float in the Auckland Anniversary weekend flooding two years ago are still propped up with poles in Māngere, awaiting their fate.
Questions are being raised about the polystyrene blocks in theirfoundations and engineers contacted by RNZ said these acted like floaties when water reached them.
But Kāinga Ora said the houses were built to building code and it was the extreme rainfall that was to blame for their damage.
State houses in Māngere that started to float in the Auckland Anniversary weekend flooding in 2023 are still awaiting their fate. Photo / RNZ / Marika Khabazi
State housing tenant Kelly Dey had a harrowing escape from her two-storey house when it lifted and tilted, with water coming into the house and waist deep at street level.
Hers was one of four homes in a social housing development near a stream in Māngere that started to float.
“It wasn’t heaps of water, but you could see it coming in the front door and then I went back upstairs and then when I came back downstairs to have a look, that’s when I noticed that the house was tilting.”
With just the clothes on her back, she got out with her four children and headed for higher ground.
Kelly Dey lived on Ventura St with her four children until the night of the storm. Photo / RNZ / Marika Khabazi
Dey said in the following days, a friend who works in construction raised concerns about the polystyrene in the foundations which had become visible.
RNZ obtained Auckland Council’s property file for the development, which has close to 200 reports, and shared it with three independent engineers.
Whangārei-based geotechnical engineer David Buxton, of Northland Geotech, ran buoyancy check calculations for the house Dey lived in.
He found that using the council-required benchmark of a one-in-100-year flood, which Kāinga Ora would have used, the house would not likely float.
Questions are being raised about the polystyrene blocks in houses' foundations but Kāinga Ora said they were built to building code and extreme rainfall was to blame for the damage. Photo / RNZ / Marika Khabazi
Knowing the house had flooded, he also calculated what would happen if water reached floor level, and found the house would easily float.
“If you imagine yourself standing in the sea at low tide with a block of polystyrene, and then as the tide comes in trying to hold it there, at some point it’s going to lift and float you up.
“That’s what happened with this house, [the water] has surrounded a big block of polystyrene and as the water’s risen, it’s just floated up.”
Auckland-based structural engineer and managing director at Compusoft Engineering, Nic Brooke, said Buxton’s calculations were reasonable.
“Overall they suggest that the houses would be stable at the one-in-100-year level flood but then become buoyant and at risk of floating when the flood level is somewhere between that level and at floor level,” Brooke said.
“It means that the risk is acceptable in terms of what’s required by councils and building codes. Unfortunately for any aspect of engineering we design for a certain level of risk and if events beyond that level of risk occur behaviour becomes less certain.”
Pito Place is within the flood plain of Te Ararata Creek in Māngere and almost the entire street has been condemned, forcing out some families who have lived there for generations. Photo / RNZ / Marika Khabazi
Buxton said the polystyrene blocks, in this case GeoFoam, were a common building material for home foundations but in this case, there was simply not enough weight to hold the house down when it flooded.
“They caused the floating but they were there for a reason.”
He said the polystyrene blocks were used as a lighter foundation material to lift the floor level without sinking in the soft soil.
“The GeoFoam was there so the house didn’t apply too much weight to the ground and cause the house to settle and sink,” Buxton said.
“It’s a valid reason for being there and it’s a perfectly acceptable and common product that was used, it’s just that it was done in a way that allowed it to float.”
Brooke said floods could cause buildings to act like boats if there was not enough weight or another mitigation such as piles to hold them down.
“The foundation material here, the lightweight foam, will increase the buoyancy but that’s not necessarily problematic provided there’s enough weight above to hold it down.”
He believed looking outside the box was important for considering natural hazards such as flood zones and earthquakes.
“It’s useful to make it a learning example for the profession and just to serve as a reminder through guidance notes or practice advisories to remind engineers and councils and other professionals involved that they should be checking the buoyancy of houses and other structures to make sure ... the house won’t start floating.”
Buxton said it would be good to have guidelines for using polystyrene blocks in flood zones, akin to what existed for liquefaction.
“There’s guidance to say ‘don’t just look at particular scenarios, step back and look at the wider situation’. Is there a place in the scenario that suddenly liquefaction is going to occur and there’s going to be a massive change in performance and is that going to be acceptable or not?
“The parallel would work here, do a buoyancy check for a one-in-100-year flood, but we should also step back and say ‘at what point would this building float and is that acceptable for this scenario?’
“At a strict, ‘this is the criteria, this is the check that we’ve done’, it passes that check. What it doesn’t stand up to is a wider look back, a sanity check to say ‘hey is this a good idea?’”
An engineering geologist who viewed the property file but did not want to be named questioned the use of polystyrene blocks for houses in a flood-prone area that needed ground stability measures.
The general manager of research for the Building and Research Association (Branz), Chris Litten, believed anyone building in flood zones should take a wider perspective than what was required to sign off a dwelling with the local council.
“Ideally you shouldn’t build in a flood zone but we recognise that actually people do need to do that and live in those sort of places. Anyone who’s designing and building needs to take a step back and think about all the possibilities to mitigate the flooding.”
He said Branz had no concerns about particular building products but urged experts to take a holistic view when building on flood-prone land.
“There’s a lot of options so it’s just not being rigid and following the building code but taking a step back and thinking what’s going to happen if flooding happens and try to mitigate it as best you can.”
Despite her harrowing escape the night of the floods, Kelly Dey still calls Māngere home.
She said planning for one-in-100-year floods was not enough.
“They should be doing it not for just 100-year flood. They should be doing it for at least a 200- to 300-year flood so that it is safe for the residents.”
Kāinga Ora said no development projects under way in Māngere had polystyrene blocks in their foundations.
Engineering NZ said polystyrene was used in various CodeMark foundation systems recognised by the Ministry of Business, Innovation and Employment as meeting the requirements of the Building Code.
Kāinga Ora’s position
Kāinga Ora rehoused 246 households forced out of their state homes after the January floods in Auckland 2023. Two years on, some of those homes have been demolished or are earmarked unliveable, including at least 45 in Māngere.
The agency’s general manager of construction and innovation, Patrick Dougherty, said Māngere was well-known as an area of Auckland where ground stabilisation was needed before building.
Kāinga Ora construction projects were all built to legislative and council requirements, he said.
“Resource consents for Kāinga Ora developments include investigations and recommendations by engineers for ground stabilisation and flood path mitigations as part of the consenting process.”
Five homes in part of its social housing development in the suburb were red-stickered after the flooding. Four of those homes had shifted from their foundations.
They all had varying levels of polystyrene blocks in their foundations, depending on ground levels.
“All five properties were in Stage 1 of the social housing development and were built to the required building codes and standards of a one-in-100-year storm event,” Dougherty said.
“What occurred in January 2023 was considered a one-in-200-year storm event. Those four properties were the only Kāinga Ora homes to have their foundations impacted by the force of the unprecedented floodwater.”
In another statement, the agency said it commissioned an independent geotechnical review of the flood damage in February 2023, which noted rainfall was extreme and exceeded Auckland Council’s code of practice.
“There is no evidence to suggest the material used in the construction of the foundations caused the damage; it was the force of the water that caused the damage.”
It said the review found flotation calculations were noted as reasonable, with no errors detected.
The agency refused to provide RNZ a copy of the report, citing legal privilege.
Kāinga Ora also said there was no legal dispute over the houses.
