Properly treated, wood can last as long as stone. The secret is to seal the wood from the inside out, not just to put a coating on the surface.
Treated Yellow Pine:
Wolmanized lumber is actually chemically treated yellow pine. It has been pumped with copper sulfides and salts. This leaves the wood resistant to rotting and is structurally strong.
The disadvantages of yellow pine is that it is a water based wood. As the water molecules evaporate, the wood shrinks. When it rains, the wood expands. This expansion and contraction causes the wood to go into stress. This stress causes the wood to warp, crack and split. The avarage yellow pine board shrinks by 40% in weight, becoming spongy. Saturating te board with an oil-based penetrating sealer will fill some of the collapsed cell structure with oil preservatives. This process stops the warping, cracking and splitting.
Cedar and Redwood:
Cedar and redwood are oil based woods. These lightweight oils evaporate slowly. This evaporation begins on the outside of the wood and over the years, drying toward the inside. The fact that the board dries slowly and has a core oil still inside means that cedar and redwood are resistant to warping, cracking and splitting and will stay straight over time.
However, cedar and redwood are subject to rotting. As lightweight oils in the woods evaporate, the cells collapse. When the deck gets wet, these cells soak up water, causing the deck to decay. These woods always rot in areas where there are water traps like between boards or capped boards. These decks will rot from the bottom up.
To seal cedar and redwood decks, it is necessary to use an oil-based penetrating sealer. It is necessary for the sealer to look for dry cell structure. The sealer needs to follow the nails throught the board, penetrating through all the knot holes and wrap around the ends of the board.