Different grades of sawn timber
There are many different timber grades to meet various requirements. These are grouped into two main categories – appearance grades and structural grades.
Appearance grades of timber are used for finishing and furniture. This is mainly clear grain but can also have minor blemishes and small, tight knots.
Clears grade is free from defects, such as knots, burrs, and other characteristics of a tree. This timber grade is also sometimes specified as Dressing A [DA], Select or Prime grade. Clears grade is the best possible finished appearance.
No.2 clears grade
No.2 clears grade is graded to the best face and 2 edges. The reverse grade of the board may contain knots and defects as described in Dressing Grade best face. This can also be known as premium grade. Outdoor Premium Grade timber can meet this specification as it is clean and presentable for exposed rafters and high-quality decking projects.
Dressing and furniture grade
A good appearance grade with a few sound defects allowed. The reverse face may contain all defects described in Merchantable Grade best face, yet the pith can only be up to 6mm wide. The pith is at the very centre of the tree and is the oldest part of the tree. The pith can only be up to that size due to its greater tendency to crack as opposed to the other wood, and the impact it may have on appearance.
Figure 1: An example of pith – The brown streak along the bottom of the board.
Pith does not tend to impact durability, but it does affect the appearance of the wood.
Figure 2. “Wane”. The diagram shows the end section and where wane is found and the picture depicts what it actually looks like on timber .
This is a standard knotty appearance grade. The reverse face may have stick marks, edge defects, pith up to 10mm and spike knots less than 20mm wide. Spike knots are limbs that have been cut across or cut lengthwise showing the endwise or lengthwise section of the limb or the knot. These knots generally have splits and grain deviations near them. Standard grade can be used in applications such as “budget” decking jobs.
Figure 3: An example of a spike knot penetrating the board.
Structural grades are used primarily for construction, where strength and durability are important. The main factor influencing a structural grade is size of knots. Structural grades are typically used for the framing component of a house build.
Industrial grades are used in packaging for various products such as pallets, cable drums, and concrete boxing work. This grade can contain larger knots and defects. Industrial grade is typically the lowest grade of timber you’ll find at a retailer in New Zealand.
OPG (outdoor premium grade)
OPG Timber has a great appearance with occasional features that are excellent for high-end visual applications. This type of timber is rougher headed, fewer cracks and more uniform this also means that it accepts paint and stain better. OPG timber is machined with very fine ribs. It is usually kiln-dried and exterior treated. You will find OPG grade used in high-end decking and pergola jobs, where a nicer finish is required.
Figure 4: (above) Outdoor premium grade timber.