APA - The Engineered Wood Association
Web stiffener blocks may be comprised of lumber, rim board, or wood structural panels. The minimum grade of wood structural panels is Rated Sheathing; minimum lumber grade is Utility grade SPF (south) or better. Any rim board product would also work satisfactorily. Ideally, the depth of the web stiffener should equal the distance between the flanges of the joist minus 1/8 inch - 1/4 inch. For bearing stiffeners, this gap is placed between the stiffener and the bottom of the top flange. For load stiffeners, the gap is located at the bottom of the stiffener.