Pressure treated southern yellow pine is one of the most popular woods used in fence building. It is readily available, inexpensive, and has a long life span if installed properly. Most southern yellow pine lumber is treated with Alkaline Copper Quaternary. The process of “pressure treating” involves immersing the lumber in the chemical and placing it in a pressurized chamber. The pressure causes the chemical to penetrate the wood. The wood is then given a retention level. A retention level simply means how much of the chemical is retained by the wood.
The higher the retention level, the better resistance to the elements. Different outdoor projects call for different retention levels. Marine grade lumber, used for piers or boat marinas in salt or brackish water, has a retention level of 2.5. Ground contact lumber, or lumber to be put in the ground, has a retention level of .40. Outdoor use lumber, such as 2×4’s and pickets, has a retention level of .25. It is important you select the lumber with the proper rating for your project. When installing a wooden fence, it is very important that all posts are rated for ground contact, or .40 retention.
It has happened more than once that someone, a home owner or even a professional, has gone to the lumber yard for fence posts. He or she sees the 4×4 posts for about $7.00 each, and right next to them are landscape timbers for about $3.00 each. The lumber looks about the same and it seems like a good way to cut down on material costs. This is a big mistake! If you look at the retention levels, you will see the 4×4 lumber will be rated for ground contact at .40. You will also see the landscape timbers are rated for outdoor use at .25 retention level. The landscape timbers are not treated to be put in the ground around constant moisture. This will cause the timbers to rot and become brittle, and eventually break at the ground’s service. Pickets and 2×4’s need to be rated for outdoor use, or .25 retention level.
Since they will not be below grade, this rating is suitable for their use. You may want to raise the pickets an inch off the ground when installing them so they are not in direct contact with the ground. This will prevent the picket from soaking up moisture from the ground which results swelling and rotting. Although you need the proper retention level, there is no need to over do it. .25 retention for the central area of NC is fine for pickets and 2×4’s. There is no need to spend time hunting down lumber with a higher retention level and paying more for it. In some cases, over doing it can cause violations. The EPA mandates all marine grade lumber, or lumber with a retention level of 2.5, must be placed in salt water or brackish water. If it isn’t, they can give you a fine. The chemical level is too high for normal use and can pollute the soil and nearby water sources.
When building a wooden fence, make sure you get lumber with the proper retention levels. The key word is “proper”, not too low and not too high. The retention levels in this article are suitable to the Raleigh area of North Carolina. If you live in a different part of the country, you may want to check the local building codes for retention levels in your area.