Timber, the original low-carbon building material (and keeping old framing sound)

Long before anyone talked about embodied carbon, New Zealand houses were built from wood. Kauri, rimu and mataī framed the villas and bungalows of the nineteenth and early twentieth centuries; plantation radiata pine took over in the decades that followed. Timber is still the default framing material for New Zealand homes, and it is increasingly being used for bigger buildings too.

Why timber counts as low-carbon

Every building material carries a carbon cost from making, moving and installing it. Steel, cement and aluminium need a lot of high-temperature energy to produce. Timber needs comparatively little: trees grow using sunlight, and turning logs into framing mainly involves sawing and drying.

Timber also holds carbon. Roughly half the dry weight of wood is carbon taken from the atmosphere as the tree grew, and that carbon stays locked up for as long as the timber stays in the building. A house frame that lasts a century is a small, long-lived carbon store.

That is why sustainable-building guidance, such as BRANZ's Level site, puts so much emphasis on material choice. Level notes that the government intends to cap the whole-of-life embodied carbon of new buildings, which will make low-carbon materials like timber more attractive still.

New Zealand is well placed to use it. The country's planted production forests cover around 1.8 million hectares, about nine-tenths of it radiata pine, according to the Ministry for Primary Industries. Engineered products such as glulam beams and cross-laminated timber panels now let architects use wood for apartment blocks, schools and offices where concrete and steel used to be the only choices.

Durability is the catch

The carbon case for timber only works if the timber lasts. A frame that rots or is eaten from within has to be replaced, and replacing it costs both money and carbon.

New Zealand's Building Code deals with this in clause B2 Durability. It requires building elements to last for set minimum periods with normal maintenance: 50 years (or the life of the building) for structural elements and anything hard to get at or replace, 15 years for moderately accessible elements such as claddings, and 5 years for easy-to-replace items. Framing is in the 50-year category.

Timber can meet that requirement because of three things working together:

1. Keeping it dry. Wood that stays below about 20% moisture content resists decay. Most timber problems start with a leak, a lack of ventilation or ground moisture.
2. Treatment where needed. Preservative treatment protects timber in situations where it could get wet or be attacked by insects. New Zealand uses a system of hazard classes that match the treatment level to the risk.
3. Good design. Eaves, flashings, cavities and ventilated subfloors keep water away from the frame.

The leaky-building crisis of the 1990s and 2000s showed what happens when those safeguards fail together. Changes during that period allowed untreated, kiln-dried radiata pine framing in many situations; when monolithic claddings let water in and it could not drain or dry, that framing rotted quickly. Treatment requirements for framing were tightened in the mid-2000s.

Older houses: what to keep an eye on

Many New Zealand homes are older than the current rules, and their framing has been doing its job for decades. Keeping it sound mostly comes down to moisture and insects.

Moisture

• Fix leaks promptly. Roof leaks, failed flashings and leaking showers are the most common causes of hidden decay.
• Keep the subfloor ventilated. Don't block vents, and consider a ground moisture barrier if the subfloor is damp.
• Watch for signs such as musty smells, soft or discoloured skirting boards, or doors that start sticking.

Borer

The common house borer is a small beetle whose larvae tunnel through timber for two or three years before emerging as adults, usually over summer. The signs are small round exit holes, typically around 2 mm across, often with little piles of fine powdery dust beneath them.

Borer prefers untreated softwood and the sapwood of some native timbers, so older houses with untreated framing and floors are most at risk. Places to check include:

• the subfloor framing, piles and the underside of floorboards;
• the ceiling space, especially rafters and ceiling joists;
• older furniture and stored timber, which can bring borer with them.

Old exit holes don't necessarily mean an active problem; borer damage can be decades old. Fresh dust, newly appearing holes or timber that feels soft or crumbly under a screwdriver are signs to have the timber properly inspected by someone who knows what they are looking at (this short explainer video shows how to tell whether borer is still active and how it is treated). Badly damaged framing may need replacing; light damage can often be treated.

It is worth having the framing checked before major work such as re-roofing, insulating or installing solar panels, which all depend on sound timber to fix to.

Building new with timber

If you are building or renovating, a few principles keep timber in the low-carbon column:

• Use the right treatment for the location, as set out in the Building Code's acceptable solutions, and keep treated and untreated offcuts separate.
• Design out moisture: eaves, drained cavities and good flashings.
• Ask about certification for sustainably managed forests.
• Plan for maintenance: paint and stain protect exterior timber, and B2 assumes normal maintenance is done.

The long view

Timber was New Zealand's first building material and it is likely to be one of its most important future ones. Its low embodied carbon and carbon storage make it attractive, but only if the frame lasts as long as the building. Keep it dry, treat it where it needs treating, and keep an eye on the old frames that are already doing the job.