It was warm and humid on the morning of January 27, 2023, in Auckland, the kind of weather that makes everything feel sticky. Then the rain began. By 5pm, cars were floating down the road in Henderson, entire suburbs were underwater, thousands of people were evacuating their homes, and every fire appliance in the city had been deployed. It would become Auckland’s wettest day in recorded history.
Aftewards, almost 1200 homes across the city were condemned as posing an “intolerable risk to life”. Some of those houses had been completely destroyed by floodwaters or landslides, some were damaged, and others were completely unscathed but stood at the foot of unstable hillsides considered to be high risk.
A house comprises 100 tonnes of waste on average – which meant the 1200 condemned homes were equivalent to a month’s worth of rubbish from the entire city, according to Architecture Now’s Andrew Barrie, a professor of architecture at the University of Auckland.
“Construction wood waste is a challenging issue in New Zealand,” says Gary Raftery, a senior lecturer in engineering at the university.
Most wood waste goes to landfill. At best, it’s chipped and used as fuel for cement manufacturing, or as compost or playground mulch.
Now, University of Auckland researchers are figuring out how to salvage, assess, recycle and reclassify wood waste as viable building products – using the 1200 homes condemned by the 2023 floods as a test case.
The goal: to make it possible for housing materials to go back into housing. “It’s about activating circular economy processes within our built environment,” says Alessandro Premier, one of a team of researchers at the School of Architecture, Planning and Design working on the project.
How to recycle a house
First, researchers must identify and quantify the house’s materials within its walls, floorboards, and structural framing. Premier and colleagues used LiDAR and hand scanners to create virtual models of the condemned houses – each representing a particular housing style. “So, we have a repository of digital models that we can use to understand how much material we can effectively reuse, and how we can maximise that”, says Premier.
Timber-frame detached houses in New Zealand are quite similar, says Premier. Though they come in a variety of typologies, each has common features. “We have the villas, the bungalows, the state houses – you’re probably living in one of them.”
The types of timber might be different – native timber was widely used up until the 1950s, then radiata pine became the dominant building material from the 1960s onwards. “But the construction typology in terms of the structural system is relatively similar,” says Premier.
The idea is that anyone deconstructing a house will eventually be able to look up the housing type and identify where there might be reusable, saleable, or recyclable materials.
Can old materials be trusted?
Salvaged timber can’t be immediately reused in the structural framing of a house, says Raftery, because it’s not certified to a certain strength, as new timber is.
“There’s a lot of technical challenges with using waste timber,” says Raftery. “The big issue is quality control. So if there’s no specific data to prove that the material that we’re extracting out can be repurposed in structural applications, then no engineer will really be confident or want to put their own reputation at risk.”
Raftery and his colleagues are working on changing that. Contractors disassembling Auckland’s condemned houses are providing Raftery with plenty of samples: salvaged timber to test. He plans to identify its mechanical properties: strength, bendiness, toughness, and so on. “There’s very little knowledge in terms of radiata pine as a species,” says Raftery. “We don’t really know what mechanical properties we can achieve, because over time timber loses strength – some species more than others.”
If there’s a demand for salvaged timber, and if its strength can be guaranteed, it becomes feasible to do the work of extracting it from homes, removing metal or other contaminants, testing and recertifying it. “Then the motivation is there,” says Raftery.
The challenge of creating a new market
Making second-hand materials available for purchase is one thing: making people want to buy them is another.
While structural components are tricky to reuse because they must be recertified for safety, other components are tricky to reuse because they’re not perfect. Market demand is for brand-new products, says Premier. “There’s a psychological approach that is related to embracing the ageing of materials,” he says.
It’s possible for tastes to shift, he says; they already have, at least once. “In the past, especially when natural materials dominated, architects and clients were pretty much aware that materials were ageing, and they knew that, and they were accepting that,” he says. “Now it’s a little bit more difficult for us to embrace this imperfection because we live in an industrialised society where everything needs to be perfect and new.”
Some elements of the condemned houses can be used straight away. “There are some materials, like timber slats, that can be reused for flooring easily—but it will have an old look, and clients must like it,” he says.
What’s missing is awareness about the importance of reusing materials, and the environmental difference they can make, says Premier. “It’s very important that the architects, designers, the other stakeholders involved, understand the value,” he says. “It’s creating a second-hand material market that can be complementary to the standard products commercially available.”
The opportunity of using second-hand materials
Reusing and recycling building materials is environmentally meaningful because it locks carbon away for decades, instead of releasing it back into the atmosphere – which happens when wood is burned, for instance.
“In terms of sustainability, every building is designed for 50 years minimum,” says Raftery. “And that means that if you’re able to [use] waste timber for another 50 years, it locks that carbon in for five decades again, rather than it being used in mulch in playgrounds or composting or something of that nature – because that’s not going to lock the carbon in for that same level.”
It isn’t just flood-damaged homes that could be part of a recycling system. A wide swathe of New Zealand housing is either coming to the end of its life or being updated to accommodate city growth. “There are a lot of different regions around Auckland that are undergoing sort of a generation change – Glen Innes, East Tamaki – where there’s multi-residential housing being placed instead of older houses that are getting demolished,” says Raftery.
As our cities grow, it’s possible to use elements of what came before as a trace of the past, says Premier. Housing deemed for demolition could act as a kind of “urban mine” that locks in valuable reusable materials – preserving heritage all while contributing to sustainability.
