The rules adopted by the National Hardwood Lumber Association are considered standard in grading hardwood lumber intended for cutting into smaller pieces to make furniture or other fabricated products. Some grades commonly used for this material are Selects, No. 1 Common, No. 2A Common, etc. Stress grade lumber Pieces of lumber graded for structural uses are put into classes with similar mechanical properties called stress grades. The allowable properties depend upon the particular sorting criteria and on additional factors that are independent of the sorting criteria.
Allowable properties are usually much lower than the properties of clear, straight-grained wood.
The allowable properties are inferred through visual grading criteria or determined nondestructively by a machine grading criteria. Visually Graded Visual grading is the oldest stress-grading method. It is based on the premise that mechanical properties of lumber differ from mechanical properties of clear wood. Growth characteristics, that affect properties and can be seen and judged by eye, are used to sort the lumber into stress grades. Typical visual sorting criteria include density, decay, heartwood and sapwood, slope of grain, knots, shake, checks and splits, wane and pitch pockets.