Recognised grade marking on a piece of timber is its “Passport” for use in structural applications and assures acceptability by authorities such as Building Regulation, Building Control, NHBC etc. These standards require that graded material is supplied and marked under the control of a recognised scheme operated by a Certification/Notified Body such as Exova BM TRADA. Visual grading is carried out by operatives that have successfully passed a process of training and examination supported by annual assessment of continued competency.
Exova BM TRADA has been providing Q-Mark strength grading certification to the timber and construction industry since the early 1970s and more recently including CE marking.
Visual strength grading is the process of assessing timbers by reference to a Grading Standard which defines limiting characteristics that can be determined by visual inspection. These include knot size and position, fissures, distortion, wane, slope of grain, rot/insect damage plus a range of other abnormalities that can affect strength.
There are a number of National Standards for this purpose, such as, BS 4978, BS 5756, INSTA 142, DIN 4074, IS 127, NLGA/NGRDL. The Standard for controlling Visual Strength Grading operations within UK/ Europe, which is also now being adopted in other countries, is EN 14081-1. This Standard is supported by various national grading rules: EN 1912 which links the available grades and species to Strength Classes and EN 338 which provides the Characteristic Values (used by structural engineers) of Visually Strength Graded timber to the Classes C14-C35 for softwoods and D18-D70 for hardwoods.