Although machine stress graded timber is given a grade allocation by the machine itself, a person still needs to visually check each piece before it’s grade stamped, to make sure the machine hasn’t missed any important defects.
Traditional grading machines that measure the deflection in a board are not always accurate in assessing defects that affect shear strength, like sloping grain, resin pockets, and fractures.
Even scanners can miss certain problems, such as bow, spring and twist.
It’s a good idea for everyone who handles machine stress graded timber to have a general understanding of the visual over-rides that apply, because there may be times when a piece will get through the quality control process that isn’t up to grade.
There are also various problems that can appear in a piece or get worse after it has been graded and left the sawmill – especially those defects that are affected by changes in moisture content, such as, bow, spring, twist, checks and splits.
If you come across pieces that clearly don’t make the standard required, you shouldn’t use them, even if they’re stamped with that grade. This particularly applies to timber going into wall frames, roof trusses and other building structures.
If you find that a lot of pieces in a pack are not making the grade they’re stamped with, you should contact the mill that supplied the timber and ask them to send out a rep to discuss the problem. Bear in mind, though, that the mill is only responsible for ensuring that the timber was in compliance with the grade requirements at the time it was graded, and not for any changes that have occurred since it left the mill.
The link below will take you to a summary of visual over-rides that apply to machine stress graded softwoods that are kiln dried and planer gauged (or dressed). These are taken from the Australian / New Zealand Standard: AS/NZS 1748.1:2011. The species you’re most likely to use this with are radiata and slash pine. For definitions of the characteristics listed in this table and methods of measurement, see the next lesson: Visual stress grading techniques.