If your cedar fence posts are rotting at the bottom, you need to replace them. The rot probably developed because the posts were installed improperly. So if you install your new posts the same way the old posts were installed, you'll just have to do the whole thing over again a few years down the line.
Cedar has a reputation for durability, but unless a few guidelines are followed, cedar posts can fail in as few as five years. Three factors contribute to this early failure: poor drainage, low-quality wood and poor protection against insect damage. To get the most out of your new posts, here are five things you can do:
1. Soak the bottom of the posts in a wood preservative containing copper napthanate, such as Cuprinol. Available at some paint stores and home centers, this wood treatment is specifically designed for in-ground applications.
2. Place about 6 in. of aggregate in the bottom of the posthole to allow for drainage. The bottom of the post should extend a few inches into the aggregate as shown.
3. Pour the concrete so that it's above the soil level. Trowel the top smooth and slope it so that water runs away from the post.
4. Apply high-quality exterior acrylic latex caulk, or silicone specifically designed to adhere to concrete, at the base of the post. This will seal the gap between the concrete and post that's caused by freeze/thaw cycles.
5. Don't use posts that contain sapwood. Sapwood is lighter in color (usually yellow) than heartwood, which is dark. Instead, use heartwood, because it's denser and more insect-resistant.