Simplified system now in place for structural timber framing

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A simpler system for structural timber framing which protects timber against decay and is time and cost effective has come into effect this month.
This replaces the previous system, which can still be used until June 30, 2011, allowing a transition period for the industry.

The key change is that there will be a single class of timber treatment, H1.2, for enclosed radiata pine and Douglas fir framing. Currently there are as many as four different classes of timber, including untreated timber, which can be used to frame a building.

The changes are contained in Acceptable Solution B2/AS1, which sets out one way of complying with the durability clause of the Building Code.
They follow years of work with the sector and a consultation process which resulted in almost 240 submissions, with strong support for the proposed changes.

Acceptable Solution B2/AS1 was introduced in 2004, but building practices and timber treatment have changed and improved since then.
The Department of Building and Housing (DBH) worked with the timber industry for several years to develop a system that was simpler and clearer, and that maintained the current level of protection against fungal decay and insect attack.

“The use of four different classes of timber treatment for framing inside the building envelope was complex, and could lead to mistakes on site,” DBH chief engineer Mike Stannard says.
“The new system offers potential cost savings by simplifying inventories and reducing the likelihood of mistakes that can lead to rework. It is expected to make the consent and inspection process more straightforward.”

The exception to the new single treatment class is the use of H3.2 for cantilevered deck joists and framing, the same level of treatment as timber that is exposed to weather.
“Cantilevered decks depend on a single point of support, and there is less chance that leaks will be discovered and repaired. This indicates a need for a higher level of treatment,” Mr Stannard says.

The new system also allows untreated Douglas fir to be used for framing in simple houses of low-risk design. The DBH has defined the low-risk conditions in which Douglas fir can be used.

“Timber must be adequately protected against damage from moisture and insect attack so that buildings are durable and comply with the Building Code,” Mr Stannard says.
“At the same time, there are consumers who want the option of chemical-free framing in their home. Research shows that untreated Douglas fir is more resistant to decay than untreated radiata pine, and science and expert opinion support the use of Douglas fir in low-risk buildings.”

Other species such as Macrocarpa, Eucalyptus, Larch and heart Rimu, Matai and Beech can also be used untreated in certain situations, as described in New Zealand Standard 3602.
Mr Stannard says about 75% of submissions supported the proposals for a single hazard class for enclosed timber framing, and more than 90% agreed that H1.2 provided adequate protection from decay in enclosed situations.

“Some submissions raised the question of whether designers and builders should be allowed to use higher hazard classes for enclosed timber framing. We encourage the use of H1.2, but acknowledge that designers may have particular reasons to use higher hazard classes in some situations — and we have retained that option.

“However, there are general and specific costs in using higher levels of treatment than those warranted, and it is not expected that this provision will be widely used.
“Another question raised in submissions was whether roof trusses need to be treated, or whether untreated Douglas fir could be used.
“We decided that in the interest of clarity, simplicity and inventory rationalisation that roof trusses should be treated to the same level as other enclosed framing.”

Mr Stannard says about 75% of submissions supported the use of untreated Douglas fir for houses of low-risk design.
Some suggested allowing the use of untreated Douglas fir created unnecessary complexity, while others supported wider use of Douglas fir.
“We considered both matters carefully. Given the industry support for simplification, the risk to internal framing from internal wet areas and the risk of transferred moisture from external walls, we would not support the use of untreated Douglas fir in anything other than houses of low-risk design.

“However, some consumers want a chemical-free option, and we think allowing the use of untreated Douglas fir in houses of low-risk design does not compromise the new system nor make it unnecessarily complex.”

The transition period for industry where the previous Acceptable Solution can still be used will allow designers time to finish designs already underway when the new Acceptable Solution is published.
From the July 1, 2011, only the new Acceptable Solution will apply for consenting purposes.

The new Acceptable Solution is now available in the B2 Durability Compliance Document on the DBH web site at www.dbh.govt.nz.