How to Build a Mantel Shelf With Crown Molding

Measure the width of the top of the fireplace. Add 6 to 8 inches to either side of the top of the fireplace. For example, if the top of your fireplace is 30 inches and you add 6 inches to either side, you'll have a total measurement of 42 inches.

Purchase a piece of 5/8-inch plywood. Cut the plywood to the measurement taken in Step 1 and 1-foot to 2-feet wide.

Select a style of crown molding that suits your taste. Cut two pieces of the crown molding to equal the depth of the plywood. Cut the front edges of these pieces with a miter saw at a 45-degree inward angle.

Cut a piece of crown molding to equal the length of the front of the piece of plywood. Cut each of the edges of this piece of plywood with a miter saw at a 45-degree outward angle.

Apply wood glue to the front and side edges of the piece of plywood. Secure the pieces of crown molding to the edges they have been cut for; the side pieces on the sides of the plywood and the front piece on the front of the plywood. Position the crown molding so that the bottom edge runs along the bottom edge of the plywood. Fit the miter edges of the side and front pieces of molding together.

Hammer finishing nails through the crown molding and into the plywood at each of the ends of the shelf. Hammer additional nails every 4 to 6 inches along the front and sides of the molding.

Hold a nail set tool over the finishing nails and gently tap a hammer into the end of it to countersink the finishing nails into the crown molding.