Like any wood that comes into contact with the soil, fence posts are prone to decay. If you can’t get decay resistant wood, such as locust, cedar, redwood, Osage orange, red mulberry, or tamarack, use pressure-treated lumber or-as a last resort-apply a nontoxic preservative to the posts yourself.
Before starting work, drive a stake at each end of the fence line and stretch a string to serve as a guide in aligning the posts. Level the string to determine how high each post needs to be, and position the posts by dropping a plumb line from the string.
At least one-third of a post’s total length should be underground (2 feet in any case). With a post-hole digger, excavate a hole twice the diameter of the post, its sides angled outward at the bottom. Make it deep enough to put a large flat stone beneath the post.
Set the post on the stone and pack 4 to 6 inches of gravel around it. Then align the post with the guide string, plumb it, and brace.
If frost heaving is a problem in your area and flooding is not, fill the hole with heavy subsoil to within 6 inches of the top, using a 2 x 4 to tamp it down firmly as you go. Then force several large stones around the post to keep it in position. Fill the rest of the hole with topsoil, mounding it ,up around the post. (In warm areas, in sandy soil, or in places where the water level is high, use concrete instead of soil and rocks.)
To protect the tops of posts from moisture, chamfer the edges or attach drip caps of wood or metal. Attaching rails
The simplest kind of rails are nailed onto the posts. Use the guide string to keep them level; secure them with galvanized nails. The next simplest are split rails that slide into holes drilled through the posts. Rails with tenons that fit into mortises must be assembled as the posts are being set.
Repairs and replacements
A wobbly post can sometimes be firmed by driving wooden wedges around its base. Usually, however, wobbly or skewed posts are rotten beneath the surface. To avoid replacing such a post, sink a new, shorter post next to it and bolt the two together; or, if they do not touch, insert a spacer block between them and wrap both with perforated metal strapping. Saw the old post off at the soil line.
To replace a mortised rail, splice a new rail onto the mortised end. Glue and screw the joint in place.