How to Frame an Arch Above a Gate

Measure the width of your gate from inside one support post to the inside of the other support post. Your new arch can attach to the top of taller gate posts, or you can add taller posts directly behind your existing post for a completely new arch.

Place a sheet of plywood horizontally on the worktable. Measure 2 inches from the bottom edge, and draw a line parallel with the edge. Starting 8 inches in from the left side, mark the line. Transfer your gate width measurement and make a second mark. Most posts are 4 inches square. Measure to the left of your first mark 4 inches and mark. Measure to the right of your second mark 4 inches and mark.

Measure between your inside marks and divide by two. Nail a short nail into the plywood at this point. Tie a string to the nail. Stretch the string to the left to the inside left mark. Tie a pencil to the string to line up with this mark. Hold the pencil upright and straight and draw a half circle arch. Stretch the string to the left to the outside left mark. Tie the pencil again so that the lead crosses over the mark and draw a second arch four inches away from the first arch. Cut out the inside arch, leaving the 2-inch flat strip along the bottom edge. Cut out the outside arch leaving the strip and cutting past the horizontal line to the bottom edge of the board. You will have a D shape. This is your arch template.

Place craft paper on the table. Position the template over the paper so that the bottom flat edge aligns with the bottom flat edge of the paper. Measure the circumference of the half circle and divide by four. Mark off the four sections on the template. Draw a straight line from the nail through each mark on the template and on the paper. Measure 1/4 inch from the inside curve of the template on the line and make a mark, "A." Measure 6 inches from the mark to the outside of the outside curve and make a second mark, "B." Repeat this for each radiating line. Remove the template.

Draw lines from one "A" mark to the next. Draw lines from one "B" mark to the next. Connect up missing line areas from "A" to "B" mark. You will create a template that is half an octagon shape for boards 6 inches wide. Use your template to cut ten pieces of board. They will all be the same size. Use a miter saw for the cuts.

Position five of the octagon piece boards on their 6-inch flat sides with their ends butted. This will form more than your half circle. Place your template over the boards so that it is two inches from the left end of the octagon boards, and screw the template temporarily to the boards. Draw the outline of your arch onto the boards. Unscrew the template, and number your first set. Lay out your remaining five octagon pieces the same way. Place your template six inches from the left end and screw the template to the boards. Draw the outline of your arch. Remove and number your second set. Cut out your curves using a jigsaw.

Assemble your first set of curved pieces on the table. Position the template over the pieces and screw it in place. Turn the template over so that it is on the table. Spread exterior glue over the layer of curved pieces. Assemble your second set over the first set so that the joints do not align. Clamp the boards together. Drill pilot holes and screw through the side of the top arch into the bottom arch. Use eight exterior grade screws per curved board evenly spaced. Allow the glue to dry for 24 hours.

Bring the arch to the posts. Fit and clamp the arch to the back side of the post. Draw a line along the top back edge of the post and across the front of the arch. Cut the arch along the cut lines. Drill pilot holes through the arch and into the post at a 45-degree angle. Screw the arch to the post top.

Measure, cut, and nail trim to cover the screw holes. Use a miter saw and exterior grade trim.