How Well the Wood Accepts Pressure Treatment Chemicalĭifferent manufacturers use different pressure treatment chemicals but the common ones are alkaline copper quaternary (ACQ), copper azole (CA) or micronized copper azole (MCA). If you buy a wet post and then let it dry for a year, you will find that it will be many pounds lighter than when you bought it. A significant amount of the weight is actually water weight. The pattern is for pressure treated lumber to be sold very wet, almost to the point of dripping. But kiln dried wood has a moisture content between 5-10% and it doesn’t affect the weight much from piece to piece. What about weight? Won’t the heaviest post be the strongest because it will be denser wood? This is true for kiln dried dimensional lumber – a heavier piece typically means a tighter grain structure and stronger wood. You want to select posts with end grain that are tightly spaced from the middle part of the trunk because it is the heartwood that provides strength. Posts made from pith are more likely to warp, twist, bow, and splinter. The pith is weak wood because its job is to grow vertically very quickly and be flexible. You want to avoid the center of the tree – it contains the pith, which is the youngest wood in the tree. Most pressure treated lumber is either pine or fir so when you walk up to the stack of lumber, which post do you pick out? Of course, an oak post will last longer than a pine post, but we are not talking about the species of wood here. The denser the rings, the stronger the post and the longer it will last. You want to select rings that are close together. Most importantly check the end grain of the post. The knots lower the lateral strength of the wood and the included bark diminish the integrity by allowing easy access for moisture and insects to the interior of the post. The Strength of the Woodįirst, eliminate any posts that have excessive knots – especially if they have included bark. There are three main factors that will determine the life of your post: the strength of the wood, how well it accepts the pressure treatment chemical, and how well the post is protected from moisture.
Not all wood is the same even 4x4s that come from the same mill and have the same treatment can have dramatically different life spans when put into a demanding application such as holding up your privacy fence. How Long Will a Treated 4×4 Post Last in the Ground? Most of the trees brought into the mill are from 10-year re-growth and the posts will contain very wide rings from the sapwood layer that simply will not last. What about cedar posts? The quality of wood coming out of the lumber mills has declined significantly over the past 20 years – it used to be that you could get old growth heart wood (from the center of the tree) that would last for decades as a post, but today the quality is simply to variable to recommend using cedar or other naturally rot-resistant wood. Wood fence posts should be pressure treated and rated for ground contact. The post bears all of the stress from wind gusts and is exposed to moisture and insects underground that can cause the post to deteriorate and weaken. When we consider the privacy fence for most homeowners, the fence post is the most critical element. There are different grades of pressure treatment and proper selection depends on the severity of the location and how the wood will be exposed to the elements.
The three common types of pressure treatment are ACQ (alkaline copper quaternary), CA (copper azole), and the newest type is MCQ (micronized copper quaternary). Pressure treated lumber is chemically treated to inhibit fungal growth and insect activity. You can work through the calculations of ASCE/SEI 7-10 yourself, or just know that it is important to select the strongest building materials available.
When a storm blows through the wind loads on your fence can be incredibly high and all of that load is transferred onto the post. Nails, screws and other fasteners used with pressure treated wood should be double hot dipped galvanized or stainless steel ASTM A 153 Class D.2×4 rails in 8-foot lengths are commonly used. Pressure treated fence rails should be AWPA grade UC3B if left unpainted, grade UC3A if painted or finished with an opaque stain.Avoid peeler cores when selecting 4” x 4” x 8’ fence posts. Pressure treated posts should be rated for ground contact – AWPA grade UC4A for general purpose or UC4B for heavy duty.There are many different types of wood lumber and grades available at your local home improvement store so it is important to know the important items when purchasing the materials for your repair.