4. Cut and fasten 1 x 2s to the 1 x 4s, positioning the 1x2s 3/4 inch beneath the top of the post, leaving a 3/4-inch reveal at the bottom of the cornice. Then add 1x8 flush with the top edge of the 1 x 2 and add 1 x 4 trim 1/2 inch below it, using 6d galvanized casing nails.
5. Add 1x4 and 1x2 trim to the cornice with 6d galvanized casing nails. Construct the cap by using construction adhesive to edge-join two 14-1/2-inch pieces of 2 x 8. Trim the cap with 2 x 2 fastened with construction adhesive and 10d galvanized casing nails. Caulk the cap thoroughly. Cut two 10-inch pieces of 2 x 8 for each post, notching them with a jig saw to accept the base of the arch. Attach the cap with 3-1/2-inch galvanized deck screws, drilling pilot holes to avoid splits. Attach the arch base pieces in the same way.
6. Measure the distance between the inside of both arch base pieces. Divide the distance in half to determine the inside arch radius. Lay out the sections of the arch, making two sets of templates to cut the arch sections from 1 x 12s. For best results, trim the end of one section and place it over the untrimmed end of the next section before marking the joint.
7. Join the sections together to complete the arch, using construction adhesive and 1-1/4-inch galvanized deck screws. Predrill and countersink the screw holes. Use epoxy wood filler to fill any gaps and cover screw heads. With a belt sander and a wood rasp, smooth the edges of the arch. Position it on top of the posts in the notched base pieces. Anchor each end of the arch by drilling horizontal pilot holes through the notched bases and inserting four 3-1/2-inch galvanized deck screws, two from each side. Hinge the gate to the posts and fasten 1 x 2 stop with 1-1/4-inch screws.