Wood Fence Posts – Why do they Rot and Break?
Wood Fence Post 101
We have received quite a few questions about the past about how exactly does a wood post rots, how do they fall apart? Why do they always fall apart in the footings and not above ground?
The arrows on this image represent rain, water from your sprinklers, water on top of the ground, or existing water in the ground that remains there for long periods of time.
This water or moisture will actually head towards the older concrete fence post base and it once it does the concrete actually starts to absorb it into the concrete just like a sponge.
You may ask -- why not just bury the post in the dirt? Realistically the concrete will provide you with a little resistance but at the same time it will retain moisture for long periods of time. The dirt might not, but it all depends on the type of soil you have (sandy, clay, etc.)
Your fence post will last when you protect yourself from the moisture. Understand that the concrete will actually suck the moisture like a sponge out of your soil. It also comes in from the wood itself, especially if your sprinkler is constantly reaching that post or you live in an area where you get a lot of rain. The water will absorb into the wood and work its way down towards the bottom of the fitting. We have seen some cases where the wood post swells up with so much water it will crack the concrete.
When you’re up against the elements, it can feel like a losing battle. However, if you understand what causes the wood post to rot, you can learn how to these problems and extend the life of your fence. Start with:
- Choosing the right type of wood
- Using water training techniques
Cedar vs. Pressure Treated Wood Posts. Which is best?
If you can, stick with pressure treated pine or fir type wood that is rated for ground contact. While cedar’s tight grain, resistance to rot, and lack of knots make for optimal fence material – it is less durable against soil than pressure treated pine (in posts). For this reason, it may be a good idea to use pressure treated pine for the posts and cedar for the rest of the fence.
Before you Buy a Post, Inspect it Carefully
This may seem like an obvious one, but you would be surprised how many fences warp simply because the wood was not inspected before installation.
Big box stores tend to buy the cheapest wood post fence product. Higher quality lumber companies stack the boards to be treated with deliberate small gaps, strap and treat it, then place it in a kiln for drying. When unstrapped fewer boards want to kink and twist, and that from natural grain twist.
The cheapest wood fence posts tend to be tightly bundled up without any space around the boards and commonly using a water-based treatment, so the penetration of the treatment is uneven and boards will range from 100% moisture saturation on one side to 25% or less on the other. When unbundled at the store it looks OK for a few days, but starts warping soon after. Because the treatment was uneven, weather and sprinkler activity makes it worse, causing even more warp and twisting.
If you are hiring a fence contractor to do the work, be sure to check the contract for guarantees against warping.
Avoid rot with a few draining tricks:
Make the diameter of the hole bigger on the bottom. You can do this easily using a post hole digger. First start with the vertical hole then start to fan at an angle to get to the cone shape.
When the post dries and shrinks in winter, water will enter the hole, and it won't drain out through the bottom. The gravel allows that water to move away from the post and into the ground.
