Sawn or Milled Timber Print E-mail
Sawn timber is produced by sawing a log longitudinally to create pieces of sawn timber each with a square or rectangular cross section. The cross section is usually one of the "industry standard sizes" although on occasions, the timber may be sawn to a special non-standard cross section. The sizes vary from those used in Domestic Construction right up to those used in Heavy Engineering Construction which includes the large sizes used in structures such as bridges, wharves, warehouses, factories and railway lines.

Similarly, the length of each piece will be one of the "industry standard lengths" which are multiples of 300 mm.

Although sawn timber can be purchased as "un-graded" or "run of mill" or "non-structural", it is generally sold as a graded product. There are several methods grading which predict the strength of the piece of timber and each of those methods is set out in an Australian Standard.

Visual stress grading Each piece of timber or wood product is inspected on its final size and assigned to a structural grade according to a set of limits on visual characteristics. The type and extent of visual characteristics are used as indicators of material properties and the limits are based on their effect on strength, appearance, durability or utility.

The stress grade is then given on the basis of the structural grade and the species, group of species or particular population of a species.

Visual stress grading is generally employed for grading round, hewn or sawn products.

Machine proof stress grading As each piece is passed through a machine it is subjected to a load similar to that which it will be required to meet in service. The piece either passes or fails. Generally a "fail" means that the piece has broken and so some preliminary sorting before proof grading is essential to avoid a large amount of broken timber that may well have been of value had it not been broken.

Machine proof stress grading of timber and wood products for the market place is not common. It is sometimes used for grading sawn products of domestic construction sizes.

Mechanical stress grading Each piece is passed through a machine to indicate its modulus of elasticity. This is then used as an indicator of the material properties. The piece must also meet specified limits on visual characteristics before it can be assigned to a stress grade.

Mechanical stress grading is generally employed for grading sawn softwood timber and wood products of domestic construction sizes. Some sawn hardwood timber and wood products of domestic construction sizes are mechanically stress graded.

The grading of sawn timber and wood products for structural purposes provides for a range of grades. Designers, specifiers and builders are then able to choose an appropriate grade for the job. Different grading methods often provide different sets of grades. For example, some are expressed as grade 1 or grade 2; some as structural grade 1, 2, 3, 4 or 5; and some as F4, F5, F7, F8, F11, F14, F17, F22, F27 or F34.

In some applications such as a verandah post or an exposed beam, the appearance of the structural piece of timber may be important. Consequently, some of the visual grading rules for timber of domestic construction sizes provide for an appearance grade.

Timber for domestic construction may have a sawn or a dressed surface. Timbers for bridge or wharf decking are sometimes specified with one face dressed.

Specialty Timber Products (sometimes referred to as milled products) include products that may have either a sawn or dressed surface and are graded primarily for appearance although some do have a strength requirement. They include flooring, cladding, lining boards, joinery products and domestic decking.

Grades for specialty timber products

Both the grading rules for softwood (AS 4785 Series) and the grading rules for hardwood (AS 2796 Series) specialty timber products are based on the premise that each of the grades described meets the minimum strength and serviceability requirement of the product being graded. The grades differ from each other in the amount or size of feature permitted.

Preservative Treated Sawn or Milled Timber. Some sawn or milled timber and wood products are preservative treated against a number of hazards including borers, termites and decay. See: Treated Timber .

When structural timber is supplied as dressed, the nominal size may be reduced due to the process of dressing to a "finished dimension" or "finished size". Design calculations should be based on the finished dimension.

Sizes of Sawn or Milled Timber and Wood Products

Sawn timber Sizes for sawn timber (with a sawn or a smooth surface) are usually a nominal size with a positive and a negative tolerance. The tolerances are tighter for timber specified as sized or gauged timber or as dressed timber. However the sizes for dressed timber (and to a lesser extent for sized or gauged timber) are smaller than the sizes for sawn timber of the same nominal size. A piece of sawn timber will lose some of its dimension as part as the dressing process leaving the dressed piece with a finished size less than the nominal size. For example, when a piece of sawn timber with a nominal size of 100 x 100 mm is dressed all round (DAR) it will have a finished size of say 93 x 93 mm. The dressed piece of timber has historically been described as either 100 x 100 mm dressed or dressed to 93 x 93 mm finished size. It is far better to specify dressed timber by its finished size.

Specialty timber products
Sizes for specialty timber products including flooring, cladding, lining boards, joinery products and domestic decking, usually relate to the cover width of the product. That is to say, the width of that part of the product that will be exposed to view after the product has been installed. For example, a flooring board with a cover width of say 85mm may have an overall width (including the tongue) of say 93mm and was produced from a seasoned sawn board with an actual width of say 100mm. Such a flooring board has historically been described as either 100mm flooring or 85mm flooring. It is far better to specify the product by its cover width.


 
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