Fireplaces usually stick out from the surrounding wall at least a little. The bump out might be the full thickness of a brick or two, or perhaps little more than the thickness of the existing tile surround. Either way, it is easiest to build the mantle by attaching it one piece at a time to the surround, rather than to construct the whole mantle first and then install it.

Each column consists of two boards, one down the face and another that forms the corner. If the bump out is fairly small, this corner board can extend to the wall. Cut the face boards to size, allowing for each one to extend ¾ inch beyond the edge of the fireplace to cover the side board. Then measure and cut the side boards. Attach the boards with nails, screws or, if you have a biscuit joiner, biscuits. Choose your fastener based on the type of finish you plan for the mantle. If you are painting, then it is easy to fill large screw holes with putty. If you are using nice wood that you intend to cover with a clear finish, then use biscuits, which are hidden, or finish nails, which produce small holes. Attach the columns to the fireplace with construction adhesive and suitable fasteners (masonry screws or anchors will be necessary for brick or tile surrounds).

Attach the Frieze

Cut the frieze to fit between the columns. The frieze does not have to be a rectangle. Especially if you are using MDF or plywood, it is easy to cut an arched frieze, which looks particularly nice over an arched firebox opening. Measure and cut carefully so that the fit is tight, then attach the frieze to the fireplace just as you did the columns, making sure that the top is perfectly level.