Pressure Treated lumber (PT lumber) is a very convenient and affordable choice for your exterior construction applications. It offers durability, rot resistance and termite protection. The chemical process which infuses these characteristics, however, can have some undesirable consequences. One such consequence is twisting over time.
How Pressure Treated Lumber is Created
Originally, manufacturers preserved Pressure Treated lumber with a chemical known as Chromated Copper Arsenic. The EPA eventually decided that the toxicity of this chemical was quite dangerous and recommended that such wood be treated with a penetrating oil finish bi-annually. But over time, lumber manufacturers stopped much production of the arsenic-based preservative in favor of less toxic chemicals, such as Alkaline Copper Quaternary, Copper Azole, and Micronized Copper Azole. But no matter which chemical the lumber supplier uses, the application process is the same. Prior to pressure treating, the manufacturer places the wood into a tank which can depressurize. This tank removes any air in the wood and then floods those air voids with one of the above chemicals. After treatment, the wood has a high moisture content for a period of time. This moisture can become the bane of builders.
Moisture Problems and More
After chemical pressure treatment, PT lumber must dry quite a bit before you use it in construction. Some builders use the wood before it dries and hope that the anchoring of nails and fasteners will prevent its twisting as it dries. However, this procedure is not the optimum for best results. If you use PT lumber before it dries, the customer may experience very twisted results down the road. The warped pieces are not structurally compromised, but they are an eyesore. Another issue Pressure Treated lumber can experience is called checking, which means that the wood splits and splinters. But both twisting and checking can be minimized, if you follow some simple steps.
As you select the Pressure Treated wood for your project, plan ahead. Not only does the grain and pith affect twisting and splitting, but the grade and type of lumber plays a part as well. When choosing the grade and type, look for Pressure Treated lumber Grade 1 or higher, and also look for the Kiln Dried After Treatment (KDAT) marking. Both of these factors will ensure you have a high quality wood and lower moisture content than is standard. In addition, examine each individual piece carefully as you select it. Check that the grain is straight and runs parallel to the long face of the board. Also, look at the pith, if it is part of a 4X4 or 6X6 piece. The pith is the center of the growth rings, and the inner rings have a tendency to not be as stable. Another thing to be aware of is that a piece which includes the pith will be more likely to check. If you must buy lumber which has the pith, be sure it is centered and matches on both ends of the piece.
Final Prep for Use
After heeding all of the above and you purchase your Pressure Treated lumber, you can do a few more things to ensure that your wood stays on the straight and narrow. First, if you did not buy the kiln dried version, you will likely need to allow the PT lumber to dry out a bit before you use it. Step one is to paint the ends to prevent moisture from escaping unevenly. Next, stack the wood on stickers and add weights to help stabilize it as it dries out. The drying process can take several weeks, so plan ahead. Also be sure to buy a little more than you will need, as some pieces may twist anyway and will not be usable. Finally, if this all seems like too much work, consider using cedar or redwood instead, as they are more stable.
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