Answer: All treated wood comes from the stock of untreated wood. But, to inject the toxins into the wood members, indentations are made in a vertical pattern running the length of the board approx. 1/2″ long x 1/8″ wide x 3/32″ deep. These indentations are made in rows about 3/8″ apart and cover the exterior of the board. The indentation patterns slightly weaken the board. These indentations, along with the colors and smells are what have come to signify pressure treated wood. Since 2x wood products are most prone to splitting vertically through the boards, the indention pattern of the pressure treated wood tends to promote splintering especially when toe nailed or screwed.
As said at the top of this article, treated wood comes from the same sticks as untreated or ‘green’ lumber ( I know, pressure treated looks green but it’s just a coincidence). The absolute best sticks are deemed ‘no. 1’ or they are taken to be kiln dried, these are usually used for exposed interior wood or exposed overhang outriggers. The rest is deemed ‘no. 2 or better’. The best of the ‘no.2 or better’ is kept for framing materials. The boards that have the most knots, wanes, and twists are usually picked to be treated. There are several reasons for this, first being that the treatment will re-soak the boards while treating making the more twisted and curled boards more pliable, and able to reform when tightly banded.
Second reason for choosing the lowest quality lumber to treat is because the colors of the treatment hides imperfections that would otherwise be noted by building professionals causing them to pass by that piece. The last, and possibly most important reason, is because pressure treated lumber was originally referred to as ‘ground contact lumber’ and was not designed to carry loads.
Although pressure treated wood is often called out on plans to carry loads, the designers and engineer account for the lower quality lumber by upgrading the size of the timbers, and may call for ‘no.1’ boards to be treated. Most counties will allow treated wood to span the same load as untreated, however. The strongest type of treated wood on the market today is dug fir, but in some cases you may find hem-fir which is not intended for ground contact. Recently states have passed legislation to decrease the amount of toxins by approx. 50 percent to fight ground water poisoning.