Does pressure-treated lumber need waterproofing?
Yes. Pressure-treated wood is pretreated with chemicals and is effective against wood destroying organisms, but it has little effect on damage caused by moisture. Wood is a naturally porous material. As wood dries, it shrinks. When wood absorbs water, it swells. These cycles cause wood to crack, warp and split—this is typically what is meant by “water damage.” Water repellents or sealers slow down the rate at which moisture is absorbed and released, reducing stresses within the wood.
