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1. The natural resistance of timber to bio-deterioration due to fungi, insects and mechanical breakdown caused by weathering, checking and splitting.

2. In building, the efficacy of details in preserving or protecting the fabric of the building from decay or deterioration.

Durability class: Durability is expressed as one of four classes. The value for each species is based on trials of the resistance to both decay and termites of untreated heartwood in contact with the ground. The classes are:

  • Class 1: Timber of the highest natural durability, expected to have a life of at least 25 years and up to 50 years in the ground.
  • Class 2: Timber of high natural durability, expected to have a life of about 15 to 25 years in the ground.
  • Class 3: Timber of moderate natural durability, expected to have a life of about 8 to 15 years in the ground.
  • Class 4: Timber of low durability, expected to have a life of 1 to 8 years in the ground. The sapwood of all species is regarded to be Class 4


Treatment Types

CCA: Copper Chrome Arsenate, a wood preservative.

LOSP: Light Organic Solvent Preservative, a wood preservative.

ACQ: Alkaline Copper Quaternary, a wood preservative.

TanE: Copper Azole based timber treatment incorporating copper and an organic azole co-biocide

Treated Timber Hazard Level Guide: Treated timber should be used to the correct hazard level for its intended application. Note that the hazard level of treatment is not related to the stress grade or other engineering properties of the timber. The hazard levels are:

Treatment Chemicals & Hazard Levels

Type Hazard Level
H1 H2 H3 H4 H5 H6
Water Boron          
Solvent LOSP      
Double CCA +

Hazard Levels and Applications

Hazard Class Exposure Specific Service Conditions Biological Hazard Typical Uses
H1 Inside, above ground Completely protected from the weather and well ventilated, and protected from termites Lyctid borers Susceptible framing, flooring, furniture, interior joinery
H2 Inside, above ground Protected from wetting. Nil leaching Borers and termites Framing, flooring and similar, used in dry situations
H3 Outside, above ground Subject to periodic moderate wetting and leaching Moderate decay, borers and termites Weatherboard, fascia, pergolas (above ground), window joinery, framing and decking
H4 Outside, in-ground Subject to severe wetting and leaching Severe decay, borers and termites Fence posts, garden wall less than 1m high, greenhouses, pergolas (inground) and landscaping timbers
H5 Outside, in-ground contact with or in fresh water Subject to extreme wetting and leaching and/or where the critical use requires a higher degree of protection Very severe decay, borers and termites Retaining walls, piling house stumps, building poles, cooling tower fill
H6 Marine waters Subject to prolonged immersion in sea water Marine wood borers and decay Boat hulls, marine piles, jetty, cross-bracing, landing steps, and similar
1. Examples shown in this table are not exhaustive.
2. Not all preservatives are suitable for all hazard levels

Industry Specified Grades

Structural timber is graded into categories (stress grades) which can be used to determine member sizes. The stress grade will depend on the natural properties and characteristics of the species (e.g density, knots, sloping grain).

Stress grades for hardwoods and some softwoods are designated by an 'F' number (F5, F14, F17, F27 etc.) Softwoods are also stress graded with an 'MGP' number or 'SP' number.

'F' grades can be allocated by visual grading, machine stress grading or proof grading.

'MGP' and 'SP' grades can only be allocated by machine stress grading.

Specific span tables apply to each individual grade. Please phone us for further information.

Stress Grades for Hardwood

  Seasoned Hardwoods Unseasoned Hardwoods
Common Stress Grades F17 and F27 F14 and F17
Occasional achievable Stress Grades F34 F14 and F22
Common Grading Method Visual Visual and Proof

Moisture Content

In all common applications, timber contains moisture. Even timber that has been in service for 100 years will contain similar amounts of moisture to seasoned timber that has just been put into service. The reason for this is that the moisture in the air (humidity) maintains a certain level of moisture in the wood. The moisture present in freshly sawn (i.e green) timber, straight from the log, is much higher and as a consequence of this, the air absorbs moisture from green timber until a balance is achieved.

Moisture content is defined as the mass of water present in the timber divided by the mass of the timber with all water removed, expressed as a percentage.

GOS (Green Off Saw)

Sawn timber direct from the mill.


When we refer to seasoned timber, we are usually referring to timber that has moisture contents in the range from 15% - 19%. The sawn timber is stacked with strips between each piece to allow natural wind and climate to dry the timber to the surrounding ambient temperature.

KD (Kiln Dried)

Kiln dried in a kiln where the temperature and air flow is controlled.

Moisture Content of 9% - 14%


RS (Rough Sawn)

The sawn profile from the sawmill.

DAR (Dressed All Round)

The finished machined profile with sharp, square corners.

DPR (Dressed Pencil Round)

The finished machine profile with 4mm radius

T & G (Tongue & Groove Flooring)

Standard flooring profile with secret nail and end matching. Note the size is the top cover.

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"I've always been very happy with the timber supplied by Marshall Timber Traders. Their hardwood quality and service is excellent"

Luke Fitcher - Award Winning Builder
Hammerhead Building Projects (Kingscliff)

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Marshall Timber Traders
28 Ozone St. Chinderah NSW 2487
Tel: (02) 6674 8828