More in Remodeling
The hollow mantel is actually a plywood box with the back side missing; the back will fit over a 2x4 attached to the stonework over the fireplace. Determine the desired size for the finished box and the dimensions for each of the sides.
Use a table saw to cut 3/4" birch plywood for the sides of the box. Miter all of the cuts 45 degrees for nearly invisible joints between the boards.
Use wood glue and a nail gun to join the plywood pieces and form the mantel box. Apply glue to all of the angled edges and make sure they're lined up properly before nailing. Prime and paint as desired.
Cut a 2x4 to the correct length to support the mantel. Drill two holes through the ledger large enough to accommodate lag screws.
Center the ledger over the fireplace, make sure it's level and mark the holes in the ledger on the stonework. Set the ledger aside and use a hammer drill with a masonry bit to drill two holes in the stonework large enough to accommodate lead anchors.
Insert lead anchors in the holes in the stonework. Position the ledger and use a socket wrench to drive lag screws through the ledger holes into the lead anchors.
Fit the mantel over the ledger, center it over the fireplace and check it for level. Drive screws through the mantel into the ledger to secure it.