How to Make a Pediment Over a Window

Measure the width of the window opening. Add to this measurement the width of the two pilasters that will flank the window. Add another inch to the total to account for a half inch overhang of the pediment on each side of the window. For example, if your window opening measures 30 inches and your two pilasters each measure 5 inches, your subtotal is 40 inches. Add another inch to allow the base of the pediment to extend a half inch beyond the pilasters on each side. This overhang produces a balanced, classical appearance. In the example, the total measurement of the pediment is 41 inches for a window opening of 30 inches, two 5-inch wide pilasters and a 1/2-inch pediment overhang.

Cut 1-by-6-inch lumber according to your total measurement. Lightly sand the edges with sandpaper.

Measure the center top of the board and mark this measurement with a pencil. For example, if your board is 41 inches long, measure and mark the board at 20.5 inches at the top of the board. This mark constitutes the zenith of the triangle you will create.

On the left side of the board, measure 2 inches down from the top corner. Make a mark with the pencil. Repeat this for the right side of the board.

Lay a straightedge on the board to connect the mark on the left side of the board and the mark on the top center of the board. Draw a line with a pencil. This constitutes the top left angle of the triangle.

Lay the straightedge on the board to connect the mark on the right side of the board and the mark on the top center of the board. Draw a line with your pencil. This constitutes the top right angle of the triangle.

Cut the board at your lines to create a triangular-shaped pediment. Lightly sand the edges with sandpaper.

Place a sliding bevel on the left side of the pediment so the metal blade rests on the top of the pediment where you just cut the line and the bevel handle presses against the straight left side of the board. Tighten the bevel so the blade does not shift. The sliding bevel gives you a template of the angle measurement. You will transfer this angle measurement to the miter saw to cut the beadboard cap for a tight fit.

Place the sliding bevel on the miter saw with the blade pointing toward you and the bevel handle up against the miter saw shelf guide. Adjust the miter saw angle to correlate with the bevel angle measurement.

Select a section of beaded corner molding slightly longer than half the width of the pediment. You will cut the right side of the molding at an angle and temporarily allow the left side of the molding to overhang the edge of the pediment. Place the molding onto the miter saw. Cut the beaded corner molding at the angle measurement set on the saw.

Squeeze a thin line of wood glue into the inside corner of the beaded corner molding. Place the beaded corner molding onto the left top side of the pediment, taking care to lay the freshly cut, angled edge of the molding onto the center top mark of the pediment board. Press into place.

Drill a hole every 6 inches through the beaded corner onto the top of the pediment. Hammer the finishing nails into the holes.

Repeat the steps for cutting and securing the beaded corner molding for the right side of the pediment. Glue, drill and nail the molding to the right side of the pediment.

Adjust the miter saw to a 90 degree angle. Chop off the extra length of the beaded corner molding that is overhanging the left and right sides of the pediment. Lightly sand with sandpaper.

Add architectural millwork, if desired. Prime and paint or stain the pediment, if desired.