How to Build an Arched Cornice

Take measurements using a yard stick. Measure the width of the window, and add 2 inches to the measurement to extend beyond the window frame. Measure the depth, or how far you want the cornice to sit out from the wall. Allow room to cover the window frame and any drapery hardware already in place. Double this measurement and add it to the total width of the plywood you are purchasing. Determine the height you want for the cornice. Take the measurement from above the window frame to the bottom of the cornice box. An arched cornice will have the arch opening in the center that is shorter than the sides of the box.

Purchase plywood that is one-half inch thick and the size needed according to your measurements.

Cut four pieces of plywood to match your measurements. Cut two pieces of plywood according the measurement for your depth. Cut a piece of plywood to the width and height to create the front piece of the box. Cut another piece to this same width by the depth of the cornice box sides.

Determine the size and shape of the arch desired. Draw the shape on the lower edge of the front piece using a pencil. Cut the arch out using a jigsaw.

Attach the top and sides of the cornice box together to the large front piece with wood glue. Line up edges so they are flush. Secure the frame with hammer and nails.