How to Build a Cornice Box and Crown Molding

For this episode of HouseCalls, Ron drove down the eastern seaboard to Plymouth, Massachusetts, the home of the Plymouth Rock. Here, Ron met Richard and Susan Bodurtha, who live in the old caretaker's house situated across from the Monument to our Forefathers built in the 1800's. This Monument is inscribed with the name of each individual who came over on the Mayflower. While the monument has weathered the centuries beautifully, this 150 year-old home was in need of a little attention.

The Bodurtha's wanted to put up crown molding in the living room and build a cornice box above the window and around an old radiator pipe. Ron was more than happy to lend a hand

Part 1: Cornice Box
1. Measure the window:

• For this particular project, Ron needed to make sure that the inside of the cornice box safely cleared the pipe by ½ inch on that side of the window, so that measurement determined how far out the cornice box would need to go on the other side as well. Since it is not likely that you will have a pipe running along your window, you only need to make sure there is room for the curtain rod and curtains to be hung inside the cornice box.

• Use a level to draw a plumb (or vertical) line on either side of the window down from the ceiling where the inside of the cornice will be attached to the wall. This box was made of ¾ inch plywood. Be sure to allow for the thickness of the wood when making your measurements.
• Measure how far the cornice will need to come out from the wall again allowing space for the curtain rod.
• Measure how far down you will want the cornice to come. It should at least cover part of the upper sill.

2. Cut wood:

• Rough cut wood to a bit larger than the measurements for the box.
• Use a power miter box set at a 45-degree angle to make bevel cuts on both ends of the cornice front and the joint end of each side.

• Also cut the piece of wood that will serve as the top of the cornice. Although this piece will not show, it contributes to the structural integrity of the box and makes it easier to mount to the wall with brackets. Also, in cases where the cornice does not extend all the way to the ceiling, the top piece will keep dust off the top of the drapes.
• Bevel cut ¾ inch trim to add to the lower edge of the cornice.

3. Glue and nail the box together:

• For a really strong hold, apply wood glue with a brush to the beveled edges. Then use corner clamps to hold the piece steady while you secure it with a nail gun and 6-penny finish nails. Using a combination of glue and nails allows you to continue the project without waiting for the glue to set up completely.
• Again using the combination of glue and nails, attach the top of the cornice box. In this particular case, Ron used a jigsaw to cut out the opening, again allowing ½ inch clearance around the pipe.
• Glue and nail the trim onto the lower edge of the cornice box.

• Use a stud finder to locate the wall studs. Use a pencil and a level to draw a straight and level line. Attach two two-inch right angle brackets to the wall studs along this line to mount the cornice to the wall. Line up the ends of the box with the vertical lines drawn on either side of the window and attach the cornice on the brackets. Use 3/4-inch screws to secure the top of the cornice to the brackets from inside the box.

Part 2: Crown Molding
1. Check plaster and patch if necessary

• Because the Bodurtha home was very old, Ron came across a place where the ceiling plaster was sagging considerably. If you find this in your situation, you will need to scrape away the old plaster and replace it with quick drying patching plaster.

2. Attach corner blocks:

• It is typical of a lot of old houses that the walls and ceilings are not perfectly straight. To allow the crown molding to fit properly, cut and attach wooden corner blocks every few inches along the ceiling wherever you can find a stud or some bracing. To attach them, first put some construction adhesive on the back and then nail them to the wall studs.

3. Cut crown molding:

• Cutting crown molding can be confusing, even for Ron. The key is to go slowly and to keep double-checking your work. When you are cutting crown, you are going to be cutting it in the same diagonal position in which it will be installed, except with the bottom side up. Use crown molding stops on your miter saw to hold the molding in this diagonal position and make the cutting easier
• Measure each piece of molding along the backside (the side that touches the wall) and begin making your beveled cuts.

4. Hang molding:

• Mark the position of the wooden corner blocks on the wall, so you know exactly where to nail. Attach the molding to these blocks with finish nails.
• Ron continued the crown molding around the top of the cornice for an elegant touch.

5. Finishing touches:

• Once all the crown molding is in place, fill the nail holes using your finger and a dab of interior spackle.
• Allow the spackle to dry completely, which usually takes about 30 minutes, and then sand it lightly with sandpaper.

• Conceal any gaps between the top of the molding and the ceiling by filling them with a bead of painters caulk. Smooth out the caulk, once again using your finger.
• Use masking tape along the edges where the molding meets the ceiling and the wall and paint your new cornice and molding.

There is nothing like a little molding and a cornice box to change the look of an entire room, as Susan and Richard Bodurtha discovered. With Ron's expert help and guidance, they were able to transform this ordinary old room into something quite elegant in just one day!