Natural or Treated Timber - What choices do we have?

We recently received a call on the BBE helpline from a family in the process of designing a healthy house. They were concerned about the information they had received from their local Council stating that, with the latest changes to the Building Code, untreated timber was no longer acceptable. We were able to reassure them that this information was incorrect and that there are still options available for the use of untreated timber in the building or renovation of our homes.

Generally it can be said that timber does not like getting wet, so good detailing is the key to timber survival, rather than relying on treatment. That is how old buildings have survived. But the ‘leaky homes’ crisis of recent years has brought the problem of rotting houses, built with untreated pine, to the public’s attention. Although poor workmanship and inappropriate architectural design and detailing have been identified as primary reasons for leaking houses, the first legal steps to deal with this problem focused on reintroducing treated timber (pinus radiata) in certain applications. If some councils now try to deter people from using untreated timbers altogether this might just be an attempt to play it safe in the face of legal claims from affected home owners.

Treatment methods

Timber is very versatile and one of the few truly renewable building materials. But it is susceptible to decay. That’s why certain species need treatment in order to increase the durability and pest resistance. There are different treatments and hazard levels of risk depending on the application. Some timber is in continuous ground contact, some is exposed to the weather but off the ground and in other situations it is used in interior applications well away from moisture sources. The different treatment levels take these specific moisture conditions into account.

The standard treatment for pinus radiata used to be Copper Chrome Arsenate (CCA), which contains copper, chromium and arsenic as the main ingredients. Chromium and arsenic are both highly toxic. The treatment also can migrate into the soil it is in contact with. It is of particular risk to builders during construction when the timber is still wet.

In recent years new treatment options have been developed that are less toxic:

The active ingredients of Alkaline copper quaternary (ACQ) are copper and a quaternary ammonium compound. Quats are biodegradable in soil. They are also lower in toxicity than the co-biocides used in CCA and have a lower environmental impact.

Light organic solvent preservative (LOSP) contains insecticide, a fungicide and sometimes a mouldicide, which are applied to timber through a controlled vacuum-pressure process that does not release waste into the environment.

Copper azole contains recycled copper and tebuconazole. Their application does not release waste into the environment either.

Although the new treatment methods are less harmful to the environment we still have to keep in mind that physical contact, leachage and outgassing can all have a detrimental affect on our health.

Untreated, natural timber

So what are our options if we want to use untreated timber to create healthier homes?

There are several species of timber available in NZ that comply with building durability standards and do not need extra treatment. That is, they have a high resin content or are dry or dense enough to resist insect and fungal decay:

1. Internal applications

Douglas fir, macrocarpa (sapwood), Lawson’s cypress (sapwood) and kiln dried pinus radiata can be used for internal framing, ceiling joists and battens and internal beams. The one condition is that they have to be assembled under full cover and never exposed to construction moisture or weather.

These same timbers can also be used for roof trusses, exposed rafters and other roof framing with minimum exposure to weather during construction.

2. Timbers exposed to the weather (equivalent to H3)

Macrocarpa (heartwood), Lawson’s cypress (heartwood) and eucalypt species are all suitable for exterior applications like weatherboard, board and batten cladding options, pergolas or deck railings. Western red cedar can also be used for weatherboard cladding but it is mostly imported from Canada and not all of it is sustainably grown. If it is a certificate should be available. Eucalypt timber makes excellent decking. Being a sustainably grown hardwood there is no need to use tropical hardwoods.

3. Timbers with ground contact (equivalent to H5)

Timbers that can survive in the ground are totara and jarrah. Neither is acceptable from an ecological point of view, as they are not sustainably grown. There are sources of second hand timber – old fence posts of totara and old lampposts of jarrah. But as they are already old their continued durability would have to be checked and approved by an engineer.

Timber tongue and groove floorboards are good looking, clean and warm when properly insulated. Harder woods such as eucalypt species are better as they are more resistant to denting and make for hardwearing, naturally resilient, beautiful floors. You can also use demolition native timber, macrocarpa and Lawson’s cypress. Pinus radiata can be injected with cellulose to create a hardwood-like weight and texture, also excellent for flooring.

High quality timber is needed for doors and windows, preferably heartwood. Macrocarpa, Lawson’s cypress and eucalypt saligna are acceptable, but cedar is often needed for larger sashes as it is lighter and more stable. Again, ask for a certificate to make sure it is sustainably grown. Finger jointed pinus radiata can be used if you are happy with a paint finish.

Demolition joinery comes loaded with style, charm and layers of paint (beware of lead in old paints!), but often needs quite a lot of work to get them cleaned down and operating smoothly.

Most joiners can make solid wood kitchens, but experience with alternative timbers is preferable.

An Eco-Building Products and Services Directory with a list of saw millers and other sustainable timber sources is available for $20 from the Building Biology and Ecology Institute (BBE) and bbe architects.