Building a Fireplace Mantel
Install the frieze board over the furring strips at the desired elevation. Check the frieze board for level and nail it into the furring strips.
Lay out the bottom edge of the crown molding on the frieze board. Transfer these layout marks from the frieze board to the front of the mantel legs. Mark the bottom edge of the crown or bed molding all the way around the mantel legs.
Begin the crown molding build-up with a piece of bed mold or brick mold cut at inside miters to the length of the frieze board. Follow the mantel legs with the first layer of molding all the way around the mantel legs. Measure the pieces at the point where they touch the mantel surfaces.
Mark the overlap for the next level of crown molding. Install the second level of crown in the same order as the first, beginning with the center piece and working out to the edges. Apply carpenter’s glue at all miter joints. A good technique at this stage is to cut a piece of crown just a little over size. After testing one miter for fit, hold the piece in place and mark the short point to indicate the precise location of the second cut.
A small overlap requires precise nailing. You may want to add a strip of wood to the top of the first layer of molding so you have more wood in which to nail. Measure the first round of moldings at the points where the crown will overlap.
Measure and cut the mantel shelf. Note: The difference between the width of the shelf and widest point of the crown (from the wall to the front of the crown at the legs) is the “reveal.” Add twice the reveal to the longest point on the crown to determine the full length of the mantel shelf.
Use a router to shape the edges of the mantel shelf. Test the mantel shelf in place and scribe it to the wall if necessary. Fireplace framing and firebox flanges often cause bumps along the wall, which the mantel shelf must fit. Use a power planer to remove the wood to the scribe marks.