The old saying goes, “they just don’t make it like they use to” holds very true for fence posts. It is indeed true that they do not make them like they used to.
Very old fences were made from redwood heart wood. It is very resilient to moisture damage and pests. Some of these can last 50 plus years with no issues. Many barbwire cattle fences are still standing that used redwood as posts. However, redwood increased in price over time and pressure treated posts became the new norm for fence construction, at least here in California. You will find some fences built with cedar posts but these are not as common.
Purpose of Treated Wood
Today, most fences in California use pressure treated posts for fence building. Fence posts are treated for two main reasons. First, the chemicals delay deterioration of the wood by fungus. Second, the chemicals make the lumber less appealing to wood destroying insects, like termites. Pressure treated lumber DOES NOT mean that the lumber will be more resistant to the intrusion of water. Pressure treated lumber will hold water just as much as non treated lumber. Most pressure treated materials are a type of pine. Water causing dry rot is the main reason why fence posts fail.
Treatment Changes for Posts
Initially most pressure treated posts were a green color. I recall these growing up. These were treated with Chromate Copper Arsenate (CCA) which was introduced in the 1940’s. These were good quality posts and some have lasted 35 plus years here in California. Unfortunately due to health concerns they stopped using CCA for residential use in 2003. However, it is still used for phone poles and docks where they need long life.
The industry switched to Copper Azole (CA) as the new treatment for posts. This is what we find at our local hardware stores here in California. These are the brown or red looking posts. Due to increase in copper the posts are more corrosive to fasteners than the CCA posts were. Unfortunately a side effect of changing to the CA treatment also created a shorter lifespan for fences. Some will argue that the posts last just as long, but in my personal experience I have seen this not to be true. There is a reason why commercial applications still use CCA treated lumber.
Use the Correct Fasteners for PT Wood
Fence posts are the foundation for any fence. Once posts are set, the fence rails are then attached to the posts. Fence rails should also use pressure treated lumber for longevity. It is important to check that the fasteners that you are using to build your fence, either nails or screws, are rated to be used with pressure treated lumber. You can determine if the fasteners you are using are correct by reading the manufacture label on the box.
So How Long Will They Last?
Here in the central valley, new fence posts will last 15-20 years with regular exposure to moisture. By regular exposure, I mean our winter seasons and regular irrigation of landscaping. Some fence posts will last 30 years because the only water that affects them is winter rains, no landscape irrigation. I have seen other fences where the post has to be replaced in under 10 years, along with an overall fence repair. This is usually caused by the ground continuously being wet due to over irrigation, or the entire back yard is concrete and water slopes to the fence line. My strong recommendation is to rebuild the fence with steel posts when it’s time to be replaced if the fence is located in a high moisture area.
How Do I Make Fence Posts Last?
Although this may be an unpopular opinion I do not install fence posts in concrete. The only posts I place in concrete are gate and latch posts. When building a fence I install posts 2.5-3’ in the ground and pack the soil tight. I have never had a fence blow over using this method. No matter if posts are in concrete or just soil they all fail at the same spot, the interaction between the top of the ground and the post. Hardly ever do they fail a foot under the surface. The number one thing to make posts live is to make sure water does not stagnate around the post. One major problem I see in yards is that the sprinklers are pointed towards the fence; this should be avoided if at all possible! If you want to give your post an extra layer of protection there are companies that sell a wrap that goes around the post at ground level to protect it from moisture.