Preventing fence-post rot
I’m about to install a fence and have purchased cedar 4x4s for the posts. But I recently read a tip in your magazine about replacing cedar posts that had rotted underground. I thought cedar wouldn’t rot. What’s the best way to do the job so that I don’t have to replace the posts 10 or 15 years down the road?
Pat Lemley, Salisbury, MD
Eric Nelson, a landscaper in Bethlehem, Connecticut, replies: Cedar is an excellent material for fence posts, but it’s rot resistant, not rotproof. Keeping the post dry is key to its longevity.
Surrounding the post with gravel may be beneficial if the existing soils allow water to drain away from the hole. Using gravel around a fence post can be counterproductive, though. If the soils are heavy and the percolation rate slow, this technique will direct water into the post hole, where it will remain for some time, hastening rot. If a filter fabric isn’t used between the existing soil and the gravel, the gravel ultimately will become contaminated with soil, diminishing its effectiveness.
Concrete poured around the post will stabilize the fence. Where the concrete stops and the soil contacts the wood will be where the cedar will start to degrade. The concrete would need to be poured even with the finish grade to minimize the chance of rot. And if the soil drains poorly, the concrete will absorb moisture, expediting decomposition.
The ideal solution is to eliminate contact between soil and wood. Using a metal post anchor set in concrete is probably the most effective method for achieving maximum longevity. To help keep the post from wicking water, choose an anchor that spaces the post bottom away from the concrete. Anchors are readily available and are typically used for deck-support applications. An example is the Simpson Strong-Tie HDC connector. It affixes to a concrete footing with a 1/2-in. foundation anchor bolt and runs up two sides of the post, where screws complete the connection. The concrete base should be about 8 in. in dia. and extend below the frost line, or at least 30 in., to provide enough resistance to keep the fence upright in a wind.