Firefighters were right to let the blaze atop SkyCity's Convention Centre burn itself out, says a civil engineering expert who had consulted on the $700m building's fire risk.

University of Auckland Associate Professor of Civil Engineering Charles Clifton was also puzzled at how the fire broke out in the first place yesterday – and expected the biggest clean-up expense would be cleaning up smoke and water damage in the under-construction centre.

Focus: Firefighters continue to battle the blaze at SkyCity Auckland. Video / Grant Murray / Supplied

Overnight, firefighters opted to "sacrifice" the centre's main roof level so its structural integrity would weaken enough so they could attack the fire more safely.

Fire and Emergency New Zealand assistant area manager Dave Woon said that once the building roof had burned to a point where there were no hazards - mainly falling objects and material - crews could get into the building and shift their focus.

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Clifton - an expert on steel design and construction who carried out fire consulting work for Fletcher Construction on the roof – backed this decision.

"By minimising the amount of water put on the fire, they will minimise the water damage through to the lower floors and reduce the cost and difficulty of cleaning up," he said.

It also saved them the very difficult and dangerous task of getting on to the roof itself to try and stop it spreading, which would have been near impossible, he said.

"Trying to do this from underneath would be completely impossible. Hopefully they can stop the fire spreading to the roofs on different levels."

Anatomy of the centre

Clifton said the roof system comprised panels with compressed straw insulation and acoustic insulation, which were embedded between cold formed steel purlins, like that seen in most large single storey buildings.

But in this case, the purlins were within the panels and surrounded by the straw insulation and acoustic insulation.

They had a fire resisting bottom layer to suppress fire getting into the roof from underneath.

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Over the top was a plywood layer, and all of this formed a sandwich panel that was put into place and then covered with the bitumen membrane outer layer.

Clifton said the sandwich panel construction was unusual and had been chosen for its thermal insulation and acoustic insulation.

While he wasn't sure why Fletcher Construction chose to use the combination of materials in these panels, he suspected the straw was chosen over other forms of insulation for environmental reasons and also because the material it was less toxic in fire.

When he was shown details during a tour of the roof, the company told him they were "very aware" of the potential combustibility of the straw and ply during construction if a fire broke out, and were taking precautions to prevent it.

The bitumen membrane, he added, was a common roof membrane on near flat roofs of multi-storey buildings.

The panels themselves were supported on structural steel rafters, which in turn were supported on the main structural steel trusses.

An engineering expert suspects the biggest cost of the fire will be smoke and water damage to the building. Photo / NZ Herald
An engineering expert suspects the biggest cost of the fire will be smoke and water damage to the building. Photo / NZ Herald

There were also bracing members in the roof that make the building rigid for earthquake and wind resistance.

The trusses were supported off steel columns back down to the lower levels and the ground.

After the insulating panels have burned out, Clifton expected the main columns and trusses would have no structural fire damage or distortion.

"Some of the intumescent paint covering them, which is designed to expand in severe fire and provide thermal insulation to the steelwork, may have started to expand - but I expect not very much of this; the temperatures at that level won't have been very high in fire terms."

He suspected many to most of the bracing members and rafters would also be similarly undamaged structurally, and be able to be treated the same as the main members.

"It will be easier to replace any members with visible distortion than to straighten them," he said.

"The video footage shows that the lightweight purlins, which are exposed once the bitumen, ply and straw has burned away, look to have minimal damage, however these have been part of the sandwich roof panel construction and I expect it will be easier to replace any that are still straight with new purlins in the new sandwich panels, than to try and rebuild them into the new roof."

Blowtorch mystery

Clifton figured the biggest expense would be cleaning up the smoke damage and water damage to the roof steelwork and to the lower levels of the structure.

In a much more severe fire in a multi-storey steel framed building in 1981 in the UK, only 7 per cent of the cost of rehabilitation happened to be in replacing structural members.

The rest went toward cleaning smoke and fire damage to the claddings and linings and other non-structural elements like walls and ceilings.

"I expect in this case the cost of replacing structural steel members due to fire damage will be close to zero per cent."

Opting to sacrifice the roof saved firefighters the difficult and dangerous task of getting onto it, an engineering expert says. Photo / NZ Herald
Opting to sacrifice the roof saved firefighters the difficult and dangerous task of getting onto it, an engineering expert says. Photo / NZ Herald

Clifton couldn't comment on the cause of the fire, but said that he'd seen roofers laying a similar bitumen membrane over an atrium roof below his office at the university.

"It is a time-consuming process which involves heating, with a blow torch, the joins between the bitumen strips to bond them together," he said.

"The blowtorches are very noisy when in operation doing this and it is very unlikely that someone left a torch on at full power while they went on a break."

He noticed when the roofers went away for short periods to get new materials they would sometimes leave the torches on a box with the pilot flame on and pointed away from anything combustible, but this flame was low intensity.

"I expect we will find out what has happened but it is unlikely, from my observational experience, that the fire would have started when new strips of bitumen layer were being bonded to already laid strips."

Fire cost: 'Sky's the limit'

Another engineering expert, Professor John Tookey of Auckland University of Technology, said commercial construction sites had a range of risk factors coming together to make fire a serious threat.

Those included subcontracted companies working for short periods in unfamiliar environments, and using potentially dangerous processes involving cutting and forming of materials at high temperatures.

Added to that was the fact fire suppression systems hadn't even been put in place yet – as was the case at SkyCity.

Around the world, there had been a litany of construction-related fires to hit major projects over recent years.

Renovators at Windsor Castle in the UK caused a huge fire that burned for 12 hours, and similarly, workers on the reconstruction of the Mackintosh School of Art in Glasgow started a fire that almost completely destroyed the architectural icon.

The fire broke out at the convention centre's seventh storey yesterday and spread across the roof. Photo / NZ Herald
The fire broke out at the convention centre's seventh storey yesterday and spread across the roof. Photo / NZ Herald

Most recently, the world looked on in horror as renovation work at Notre Dame Cathedral in Paris resulted in damage likely to take decades to make good.

While ramifications of the convention centre fire were yet to be established, Tookey expected the direct effects would be "huge".

"The consequential effects of smoke and water damage are likely to be every bit as substantial if not more," he said.

"When these losses and the business-related cashflow impact of cancelled conferences are calculated, the final bill could see the sky as the limit."

He saw the future of the SkyCity project as being in the hands of the underwriters and loss adjusters involved.

"The convention centre has been a ground zero for the financial exposure of Fletcher Construction over recent years," he said.

"There is no doubt that the company is very keen to see the back of this project. However it is not hard to imagine that this troubled project is likely to see a future sequence of litigation related to liability. Unfortunately this is one set to run and run."