What does ‘checking’ mean?

During the drying process, the ends and surfaces dry first and this means that they shrink. But, the core remains swollen. The different rates of drying cause cracking at the ends and surface of the lumber. This reaction is called “checking”. Why does this matter? A relatively small amount of checking is deemed acceptable under the grading rules for lumber and the amount allowed, varies by grade. The amounts allowed are based upon the effect that these defects have on various strength properties of wood.