Extra – Extra is the highest grade. Only very small bright knots are allowed on the surface of timber, which do not deteriorate the overall structure of the material. It is an expensive grade and material of the highest quality: no blue stains, rot, worm-holes, eased arris, cracks, mechanical damage are allowed.
А – A is high grade. On the surface of the wood, only small knots that have an effect on the structure of the material in general are allowed; they should be of light color. No blue stains, rot, worm-holes, and eased arris are allowed. Part-through cracks no wider than 2 mm and no longer than 200 mm are allowed.
A/В – A/B is the middle grade. Knots may be of various sizes, but no significant cracks or chips (20 mm) are allowed.
С – C is the lowest and the cheapest grade. Knots, pitching, resin streaks, and other defects are allowed on the surface. Such timber is often used under a layer of higher quality cover, which allows saving on the aesthetic aspect of the coating, but not on its strength.