Many lumberyards stock treated wood in the broad category of #2 or better. If this is the case you may want to sort through the lumber piece by piece to find clear, straight boards. Because treated wood is often still wet when it is delivered to the site it will shrink slightly in width and thickness as it dries out. This can cause significant twisting, cupping, bowing, and warping especially in lower grade boards where knots and uneven grains are already present. It is possible that some boards will become so deformed that they will be unusable. You may want to store lumber by fastening boards together using a method called stickering to allow boards to dry evenly. You may also purchase kiln dried after treatment (KDAT) which is much more stable but also more expensive.
Premium - Highest grade for 5/4 radius edge decking.
Select - The highest grade available, contains very few detects. It must meet a minimum 1/12 grain slope and have all knots sound encased. It has a high consistency and the appearance is very good.
#1 – Will contain no splits larger than the width of the board. Knots may not be larger than 2 ¾". One hole is permitted every 3 ft.
#Standard - Midrange grade for 5/4 decking.
#2 – Grain slope must meet minimum 1/8 grain slope. Boards will contain no splits larger than 1.5 times width of board. Knots may not be larger than 3 1/2". One hole is permitted every 2 ft. Wane corners.
#3 – The lowest quality grade. Not suitable for deck construction.