The jury is out on whether SkyCity's fire-damaged International Convention Centre will have the same roof design.
The roof on one of New Zealand's largest buildings was made of bitumen, rubber, plywood and a highly compressed straw-like substance.
Those are the materials which burned so fiercely when the roof caught alight last October and subsequently burned for 10 days.
The complexity, size, design and composition are factors partly blamed by experts for the fire's ferocity and duration.
That raises questions about whether the same type of design and construction is planned in the $336 million-plus repairs.
Ross Taylor, chief executive of Fletcher Building which owns NZICC head contractor Fletcher Construction, said no definite decisions had been made yet about the replacement.
"The roof design and specifications were compliant and it is absolutely appropriate to put it back there. Once we look at it, we will consider options and what allows us to build more effectively and quickly. We're thinking it through now as we speak," Taylor said from Sydney.
Roof materials similar to the NZICC's were common in Australia and New Zealand "and it's not an unusual product", Taylor stressed.
A Fletcher Construction spokesperson said changes to the design were being considered, however no final decisions had been made.
"If the roof does require changes from its original design, these will need to be consented by council. No timeline has yet been set for roof completion."
The Australasian Fire and Emergency Service Authorities Council review of the NZICC fire cited the roof design and materials as factors which contributed to the fire's complexity, duration and Kiwi firefighters' inability to create a fire break to stop it burning.
A "compressed straw-like product" inside that roof and its composition had an effect on attempts to extinguish the fire, that report said. Plus that straw material became extremely heavy when water-laden, creating other issues.
"The skeleton of the roof had thousands of metres of high-grade timber to support its top layer. The roof was 'non-walk on', engineered to carry less than 75kg/sq m."
A weather seal was formed by a two-layer rubber membrane on plywood. That plywood had stainless steel mesh stapled to it. Under that was 100mm of thermal insulation supported by wire mesh. This lay on hundreds of large joists "packed with a compressed straw-like product to add further insulation. Below that were acoustic panels on top of an absorption blanket," the report said.
"The complex roof construction made it impossible for the Fire and Emergency NZ tools available to effectively cut through the roof to try to create a fire break," the Australian report said.
"While it may have been possible to breach the roof using the tools and technical expertise available to Fletcher staff, not only was that deemed impractical for safety reasons but it is doubtful that it would have been possible to apply extinguishing media directly on to the fire, given the complexity of the roof construction," the report noted.
Furthermore, the roof became so weak firefighters could have fallen through it, while the water-logged roof straw could have meant the entire roof falling into the vast building.
"The crews on the roof were unaware of the roof's load limits and that the fire could seriously compromise its integrity".
It appeared strong enough and they didn't see any signs they were in danger, the report said.
Convention centre owner SkyCity Entertainment Group declined to answer questions about the roof design.
Warren and Mahoney, Woods Bagot and Moller Architects designed the centre and Beca were the engineers.
John Tookey, AUT school of engineering professor, said the straw, bitumen and plywood combination was very much a standard solution in a lot of jurisdictions around the world.
"Forty years ago, my father installed a roof like this on his garage. These types of roofs have been around for decades. It works really well as a rule in terms of ease of application and thermal performance as well as cost. You can cover a large amount of space with this type of roofing solution relatively quickly and cost-effectively. It is also very robust which is a good thing in seismically active zones," Tookey said.
Once it has been correctly installed, it is not particularly more of a fire hazard than most other construction materials, Tookey stressed.
"Remember that most construction projects have large amounts of timber incorporated as a default material for internal structures in particular," he said.
"That said it does have that sort of 'medieval feel' as a solution – maybe we are biased as a result of the Three Little Pigs story? Anyway so long as there is appropriate fire suppression and fire fighting facilities/forces available – no major problem.
"In the case of this fire it was as a screw-up during installation with a wind blowing to literally fan the flames into life," Tookey noted.
Another construction expert, who didn't want to be named but heads high-rise projects worth tens of millions of dollars, took the opposite view.
"My understanding is that the architects chose the compressed straw product because it had very good sound-absorbing properties: convention or function centres require high-quality acoustic environments," he said.
Many alternatives exist but the ply/bitumen/rubber/wood/steel/straw combo had a low carbon footprint and is eco-friendly "to appease the greenies as opposed to other more traditional non-green products".
The roof had good fire-rating properties from the inside where most people would think a fire would start.
"No one considered a fire starting atop the roof," the construction boss said.
Warm roofs were becoming the in-thing as the New Zealand building code and standards improve post-leaky buildings.
"We've seen the invention of façade engineers, a profession which didn't exist five years ago, now considered one of the biggest hand-brakes on project feasibility, along with nervous banks and risk-averse and inept councils," the building chief said.
"I'd be surprised if the same roof is specified again. However this would then require an amendment to the building consent, and potentially additional costs, something that the Insurance company wouldn't be up for, unless a code-acceptable/acoustic-compliant alternative was found that was more cost-efficient," he said.
The roof structure and materials used could have helped the centre get environmental ratings or points, making it a "greener" building, he said.
"Is SkyCity likely to re-specify the same design? Given that it has already been fully designed and signed off then it would be the most cost-effective thing to do. It is very much quicker than arguing for an exemption for a particular new solution," the boss said.
But there was a mentality for many clients and builders about lightning striking twice in the same place.
"I can fully see how funders and insurers may wish to back away from this solution for the look of the thing as much as anything else. However, there is no real reason why this should be the case. Alternatives with synthetic non-flammable insulations are out there but – no surprise – are usually more expensive," the building boss said.
Another industry expert said he doubted insurers would allow the same type of roof to be built. James MacQueen this month released BDO's annual construction sector report, surveying nearly 100 directors and senior managers of major building firms.
"This is speculation on my behalf but I'd expect the insurers would seek to claim back money," MacQueen said.
The roof design would change and be stringently peer-reviewed because of the fire, MacQueen expects.