A wood fence with a flat cap presents a clean line. It is one of several design options for trimming a fence's top. Along with eye appeal and a modern look, this design is functional. It keeps the tops of the pickets from warping and twisting as they dry and weather. Although accurate measurements and cuts are required, the finished look sets your fence apart from others.
Mark your fence line by hammering a wood stake into the ground at each end. Be sure your fence line is 3 to 4 inches inside your property line. Tie one end of a mason line around one stake, stretch it tight to the other stake and secure it to the stake.
Measure 8 feet from one end of the fence line and spray a mark on the ground indicating the location for a post. The spot is on your side of the mason line. Continue measuring and marking the ground every 8 feet until you reach the other end.
Dig all the postholes with a posthole digger. The holes should measure 20 inches deep and 10 inches in diameter. Pour a small amount of gravel into each hole and place a 4-by-4-inch post into each hole on your side of the mason line.
Mix a bag of quick-setting concrete mix in a wheelbarrow, according to the directions on the bag. Each bag fills two holes. Have a helper hold the post and fill the first hole half way. Moving the post back and forth and poking the mix with a stake eliminates air pockets. Finish filling the hole until the mix is a couple inches from ground level.
Check that the post is centered and one-eighth inch away from the mason line. Place a carpenter's level on the post's sides checking for plumb. Move on to the second and all subsequent holes and repeat the process to set all the posts. Allow the posts to cure for a minimum of 24 hours.
Attach the Rails and Pickets
Measure down 3 1/2 inches from the top of the first post and place a pencil line on the post's inside. On the same post, measure down 63 inches from the top and place another line. These lines indicate where the upper and lower 2-by-4-inch brackets are attached. Mark the insides of all posts. Align and center the tops of the brackets with each pencil line and attach them to the posts with 2-inch galvanized nails.
Hold up the 2-by-4-inch flat-cap rail between the top of first two posts and make a mark indicating how much to trim off. Hold up a 2-by-4-inch rail between the upper and lower brackets of the first two posts and make a similar trim-off mark on each one. Make the cuts with a circular saw.
Attach the upper and lower rails with 2-inch galvanized nails. Lay the cap rail flat on top of the top rail and attach it to the rail with 3-inch galvanized nails. Repeat these steps until all the rails and flat cap rails are installed.
Attach the pickets to the rails with 1 1/2-inch deck screws. Position the first picket against the rails at the first post and butt it up under the top, flat cap. Hold a level against the picket to check for plumb. Use a drill with a screwdriver bit and screw two screws into the top and bottom rail.
Repeat the process of attaching the pickets for the length of the fence. Occasionally check the pickets for plumb. Shovel and tamp down some of the dirt around the base of each post. Attach a 4-by-4-inch fence post cap to each post to finish the project.