It's taken 25 years of research but engineers say they now have the answers to fix buildings from the 1980s and 90s that are prone to earthquake damage.
Researchers from Canterbury and Auckland universities have been investigating ways to strengthen precast concrete floors, which often pancake during earthquakes.
In a lab at the University of Canterbury, a typical precast concrete floor was put to the test, being pushed and pulled as if in an earthquake.
The risk with pre-cast floors is they can become detached from their supports, the team said.
But in the lab, these floors hold up, thanks to beams and cables reinforcing it underneath.
Engineers say it's the best evidence yet that putting in extra support works.
"We now believe we are in a position where we have reliable guidance that engineers should have confidence in to set out how to go about designing those retrofits and ensuring that they're installed properly," Nicholas Brooke said, vice president of the Structural Engineering Society.
It's huge progress from 20 years ago when the floors, also known as hollow core, were first tested using something called the Matthews test, he said.
Those floors quickly collapsed in a simulated earthquake.
"Hollow core flooring is a reasonably brittle form of construction. So compared to most reinforced concrete structures it has more propensity to fail suddenly," Brooke explained.
The Northridge earthquake in California 25 years ago first alerted engineers here to the dangers posed by precast floors.
They've been studying the issue ever since and it's hoped the latest research gives building owners greater confidence to retrofit.
"Amongst all the uncertainty and concern we have about our buildings, which is very justified, this provides a bit of light at the end of the tunnel, this is possibly the positive way forward, how we deal with our existing building stock," Jo Horrocks said, EQC Chief Resilience and Research Officer.
But earthquake strengthening is expensive. The work on Wellington's town hall has just blown out to $180 million and fixing the central library, which has precast floors, will cost about the same price.
It's those sorts of costs that pose a tough call for owners – to demolish or save?
"My hope with this research is that we won't see buildings abandoned or demolished because it tells us exactly what we need to do. The research team have focused on cost-effective retrofits to the greatest degree," Horrocks said.