How to Build a Window Cornice
Your window cornice project will begin with the cutting and assembly of the three sided lidded box. After the basic box has been assembled, the chosen trim is added and the cornice is finished. The window cornice is then ready to be mounted to the wall by screwing it to a 1x2, which is screwed to the wall above the window opening.
1. First measure the curtain and curtain rod to get the length, width and depth you will need. The area should be measured with the curtains open in order to account for their thickness when bunched together. Add an inch or so to the depth of the area and to the width of the curtain rod in order to be sure that the finished cornice will cover everything nicely.
2. Next, miter the ends and front pieces of the box. The easiest approach to building the box is to square up the board, then tilt the miter saw in order to cut the 45 degree bevel needed for the first end piece. It can then be squared up to cut the second end.
3. Glue the mitered corners and then nail them together using one inch brads. Measure the assembly carefully in order to determine the proper length for the top.
4. When mitering the trim, be sure to start by cutting and mounting an end piece first, followed by the long front trim and finishing with the other end. This will help to ensure that you can check the fits and get good miters for each corner. Each miter should be fit, then scribed the length of each piece.
5. Many homeowners find that cutting crown molding can be a bit of a tricky proposition. The trick to cutting the crown molding perfectly is to rest the molding upside down while cutting the 45-degree angles.
6. When finishing the box after the wood has been mitered, it is a good idea to fill any nail holes and prime the area before starting to paint. If the cornices have any elaborate details, such as blocking, it is best to use several light coats of spray paint, rather than painting with a brush. A highly detailed stained or natural wood finish can be finished with a shellac, polyurethane stain or spray lacquer. The cornice ledger can be pre-painted to match the wall before being put in place.
When it comes to designing the cornice, it is important to consider the type of décor in the home, and to choose the type of wood, type of molding and type of finish accordingly. For some designs, the top will overlap the sides, often with a routed edge, while for others it will be nailed to the top or inside of the box so that it will not show from underneath. In these cases the tops are best made using 1x4s, 1x6s or 1x8s. When choosing the best combination, there will be no need to cut the tops to width.