Specialty Timbers New Zealand

Durability

Timber must be durable, for a service life in applications where the wood is subject to decay. All sapwood is inherently perishable, whereas (depending on timber species) heartwood can be anything between perishable and very durable.

Natural durability or preservative treatment?

Naturally durable timber does not require preservative treatment for many above-ground building applications and NZS 3602 provides "Acceptable Solutions" for naturally durable timber species according to the application. Importantly, some applications require timber that is durable for at least 50 years in service (for building elements that are hard to replace, e.g structural timber). Other applications require at least 15 years durability (for elements that are easy to replace, e.g. weatherboards or decking).

Some perishable timbers (e.g. radiata pine) can also be treated with preservatives to provide durability against biological threats such as decay and insect attack. Level of preservative treatment is prescribed according to the environmental conditions the timber building element is subject to. These are known as "hazard classes" (see below).

Both hazard classes and naturally durable timbers are specified in NZS 3602 for a range of building applications.

Natural durability

The Australasian durability classification system provides four classes of natural durability:

Expected life (years) of naturally durable timbers based on 50 mm or 20 mm square testing stakes*
Durability Class	Ground contact stakes		Exposed above ground
Durability Class	50 mm	20 mm	50 mm	20 mm
Class 1	> 25	> 10	> 40	> 25
Class 2	15 – 25	6 – 10	20 – 40	15 – 25
Class 3	5 – 15	2 – 6	10 – 20	7 – 15
Class 4	0 – 5	0 – 2	0 – 10	0 – 7

Results from testing in New Zealand have classified the following timber species:

In-ground natural durability classification for the heartwood of NZ-grown species (50 mm square stakes)*
Class 1 (very durable) > 25 years	Class 2 (durable) 15 – 25 years	Class 3 (moderately durable) 5 – 15 years	Class 3 (moderately durable) 5 – 15 years	Class 4 (non-durable) < 5 years
Hardwoods	Hardwoods	Hardwoods	Softwoods	Hardwoods
Eucalyptus cladocalyx	E. amygdalina	Black beech²	Japanese cedar²	Paulownia elongata
Robinia	E. botryoides	Blackwood²	Kaikawaka²	Paulownia tomentosa
	E. cornuta	E. globulus²	Kauri²	Poplar
	E. globoidea	E. sieberi²	Larch²	Tawa
	E. muelleriana	Gleditsia²	Lawson cypress²	Silver wattle
	E. pilularis	Southern rata²	Lusitanica²
Softwoods	E. radiata (p)	E. fastigata¹	Macrocarpa²	Softwoods
Silver pine	E. saligna	E. delegatensis¹	Matai²	P. nigra
Totara	European oak	E. fraxinoides¹	Redwood²
	Hard beech	E. obliqua¹	Rimu²
	Mountain beech	E. pyrocarpa¹	Tanekaha²
	Red beech	E. viminalis¹	Western red cedar²
	Sweet chestnut	E. regnans	Douglas fir¹
		Hinau¹	Leyland cypress¹ (p)
		Mangeao¹	Miro¹
		Pukatea¹	Pinus contorta¹
		Silver beech¹	P. muricata¹
			P. radiata¹
			P. strobus¹
			P. ponderosa¹
1 Species with durability towards the lower end of the range. 2 Species with durability towards the upper end of the range. (p) Provisional classification, species still in test. *From Durability of New Zealand grown timbers, David Page and Tripti Singh, NZ Journal of Forestry, February 2014, Vol. 58, No. 4

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Reconciling durability classes with Hazard classes

Hazard classes are used to prescribe type and level of preservative treatment required for the timber components of a building:

Hazard classes^
H1.1	Timber used in situations protected from the weather, dry in service and where resistance to borer only is required.
H1.2	Timber used in situations protected from the weather but where there is a risk of moisture exposure conducive to decay.
H3.1	Timber used outdoors above ground, exposed to the weather – generally in non-structural applications; i.e. fascia boards, weatherboards.
H3.2	Timber used outdoors above ground, exposed to weather or protected from the weather but with a risk of water entrapment; i.e. structural applications, decking, fencing and pergolas.

Radiata pine, being a perishable timber, requires preservative treatment to be used for any of the hazard classes above. Preservative treatment levels are provided in NZS 3602 for timber components used in buildings.

Naturally durable timber species are also listed in NZS 3602 for timber components that require no preservative treatment for use in buildings.

^ Higher hazard classes are available for in-ground preservative treatment for radiata pine. Applications requiring a minimum of 50 years service in-ground are not suitable for most specialty timbers grown in NZ.

Alternative preservative treatment options

A range of preservation options are available to improve durability of timber, such as boron treatment (for interior applications) and thermal modification (for decking and cladding applications).

Boron treatment of timber

Boron-based preservative compounds (borates) offer significant environmental and health advantages over other preservatives, with low mammalian toxicity and low environmental impact. In contrast, conventional wood preservatives comprising an active ingredient and a solvent (e.g. LOSP), give more cause for concern because of the organic solvents which carry the chemicals, and the chemicals themselves, which tend to be eco-toxic.

Borates are inexpensive, highly effective timber preservatives against both fungal and insect attack where they are not subject to leaching⁺. They also have flame retardant properties.

Borates are salts of boric acid. Borax is the sodium salt of boric acid and is mined as nearly pure sodium borate. Borax is easily converted to boric acid and other borates. Borax itself has many commercial uses including water softening, and has flame retardant properties. Disodium octaborate tetrahydrate (DOT) is often used as a wood preservative.

Borates are water soluble salts and traditionally water has been used for diffusion of boron into "green" (i.e. freshly sawn) timber before it is dried. However, this method is not appropriate for seasoned timber where it is not desirable to increase moisture content, such as where dimensional stability is critical, or where grain-raising is to be avoided, such as joinery products.

All the reviewed scientific data indicates that borates have been proven effective against all known wood destroying organisms were they have been applied, such as in North America, Europe, Japan, Australia and New Zealand.

Borates are odorless and colourless. Proper application lasts for the lifetime of the wood⁺. Repeat treatment is not necessary. However, because boron-based treatment is water-soluble it can be leached out of wood if exposed to excessive moisture. Borates do not leach out of structural wood used for interior applications under typical conditions⁺ and should remain available to repel wood-destroying organisms for the life of the building. Leaching in not of concern for borate-treated structural wood, provided long as the wood is sheltered from rain or sealed against moisture.

H 1.2 treatment under NZS 3640 requires complete sapwood penetration of boron and an average cross section retention of 0.4% BAE (Boric Active Element) of the oven-dried wood. NZS 3640 Chemical Preservation of Round and Sawn Timber allows H 1.2 boron treatment of all species for which H 1.2 is specified in NZS 3602. This includes cypress species (macrocarpa, lusitanica, Lawsons cypress) for most structural applications.

Traditionally boron is applied to green sawn timber under dip-diffusion, pressure-diffusion or pressure treatment processes. Allowing the treatment to diffuse for sufficient time until full penetration of sapwood has occurred (usually between two and eight weeks) before seasoning the timber will meet H 1.2 requirements.

Timber is also required under NZS 3640 to be verified as meeting the minimum preservative levels by an independent quality assurance programme.

+ Borates are not used for preservative treatment of wood exposed to repeated wetting, such as in outdoor settings and other weather-exposed situations.

Glycol borates

Glycol borates are formulations of inorganic borate salts in a carrier of glycol. The glycol base acts as a carrier ensuring deep penetration of boron but without wetting the timber. Glycol borates are highly effective timber preservatives. Because penetration (diffusion) of boron continues over time these treatments are even suitable for application to completed joinery items.

Glycols are organic compounds in the alcohol family and are used as penetrants for boron into dry timber. Ethylene glycol is moderately toxic but propylene glycol is a non-toxic alternative also used in skin creams, food additives and toothpaste. Both forms are effective at carrying borate into wood fibres.

Glycols break down quickly in the environment and present no environmental hazards.

See Home-Brew Solution of Borates

Glycol borates applied to timber may initially penetrate only 3mm into the timber creating a protective envelope around the outside of a piece of timber. When the timber is moistened or wetted, the preservative is diffused into the timber. This is different from treatment systems that conform to NZS 3640, where complete sapwood penetration of boron takes place (H1.2).

Applied industrially with penetrants, glycol borates can meet the requirements of H1.2. Such products currently marketed include Osmose's Framepro, Arch Wood Protection NZ Ltd's FRAMEBor, Copcote's Tru-core and TimTech's BG-Dry.

The performance of any treatment hinges on the level of retention of the active ingredient which must be higher than the lowest expected threshold values. To be effective in controlling decay, timber treatment levels would need to remain above the minimum levels suggested by NZ research (0.15% BAE).

Acetylation

Acetylation of wood has recently been commercialised in Europe with the worlds first cost-effective large-scale production facility producing acetylated wood.

By reacting the wood with acetic anhydride, which comes from acetic acid (i.e. vinegar) the wood (known as "Accoya") becomes more dimensionally stable and also very durable. However radiata pine treated with this process is currently expensive in New Zealand.

Thermal modification for durability

Wood can be heat treated at between 190° and 230°C to become more dimensionally stable, stiffer and durable. However the wood also becomes weaker (bending strength). The wood also becomes a rich dark brown colour and thermal modification can also be used for producing wood with a darker appearance.

Creosote

Creosote is produced by the high-temperature distillation of bituminous coal. The distillate consists of solid aromatic hydrocarbons, and tar acids and bases. Creosote is toxic to fungi and insects, is relatively insoluble in water, and is a low cost wood preservative. It is a traditional penetrating surface treatment which imparts durability to wood. The pungent odour, oily finish and its propensity to leak and stain surrounding material makes it less desirable in some situations. Creosote was traditionally used where timbers are in contact with the ground such as railway sleepers, fence palings and battens. It is also effective in marine structures. Creosote is no longer available in New Zealand because of it has been deemed carcinogenic.