Investigations prompted by the fatal Grenfell Tower fire in London last year have culminated in new guidance about what cladding to use on New Zealand buildings.
This follows the mass cancellation of Codemark certifications for aluminium composite panels because their fire resistance claims were not backed up.
The Ministry of Business, Innovation and Employment's new guidance lays out what fire testing on cladding is acceptable.
The 18 pages of guidance also focus on how the spread of fire up the outside of a high-rise must be addressed.
It says that to meet the Building Code's safety demands, the combustibility of the entire wall - including framing, insulation and interior linings - must be considered.
"In practical terms it will make it very difficult to have a wooden framed building, since wood is 'combustible' even if protected by a fire cement or plasterboard sheet," one fire engineer said.
The Grenfell fire prompted building authorities in many countries to question the combustibility of the cladding allowed on multistorey buildings.
The Auckland, Wellington and Christchurch city councils identified hundreds of buildings with either the very combustible type of panel used on Grenfell, or a semi-combustible type; and in many cases the councils had no record of what type was on them.
Confusion over the cladding has caused costly delays in issuing building consents, or designs being redone.
This sparked questions to the government over what the acceptable fire testing protocols were, the ministry said.
"The guidance does not intend to provide a fire-engineered design solution for individual construction details but covers broad principles requiring consideration in their development," its guidance said.
"Some of the principles are based on a simplistic risk assessment approach."
It lays out three different pathways for meeting the Building Code.
A fatal flaw in the Codemark certificates was they treated the aluminium panel in isolation instead of considering how the entire wall assembly would react to fire.
The guidance now shows that the whole wall must be considered including:
- combustibility of insulation
- combustibility of framing (eg timber frame)
- composition of rigid air barrier
- building underlay
- uninterrupted vertical cavity
- continuity of products
"In order for an external wall cladding system to be certified for fire safety performance, it needs to be constructed to replicate the details of the test," the guidance said.
"This includes, for example, framings, substrate, flashing details, gaskets, sealants and fixing mechanisms."
The cladding controversy revealed that MBIE had been told its flagship Codemark certification system was badly flawed, and it is now overhauling it.