How to Build a Simple Fireplace Mantel

Measure out 6 inches from each side of the fireplace opening and draw a small pencil mark. You can erase the mark later, but need the mark to guide in drawing the outline for the sides of the fireplace mantel.

Stretch a chalk line over the pencil mark. Use a level to ensure the chalk line is perfectly horizontal on the wall. Pull the chalk line taut and then pluck the line from the middle to produce a line of chalk on the wall to outline the outer edges of the mantel you will install.

Measure 2 inches up from each top corner of the fireplace opening and draw a small pencil mark at each location. Stretch a chalk line 6 inches past each of the marks and line the chalk line string up with those lines. Level the chalk line horizontally and pluck the taut line to produce a line to mark the top of the mantel's shelf.

Measure the top of the fireplace opening and cut a section of wood that measures 1 foot longer than the width of the fireplace opening. Use a carpenter's square to cut the ends of the piece into 90-degree angles.

Set the cut board into place aligning the top of the board with the chalk line across the top of the fireplace opening. Have an assistant help you hold those pieces into place while you locate the wall studs underneath the outer 6 inches of the board, on both sides, using a stud finder device.

Trace the bottom of the board to mark where you should align the tops of the L-shape brackets and move the board out of the way. Set one L-shape bracket into place on each side of the fireplace opening. Align the bracket over the wall studs and against the pencil line marking the bottom of the mantel's shelf.

Drive 2-inch wood screws through the mounting holes in the bracket and into the wall studs. Set the cut board into place on top of the brackets and have an assistant hold the board as you drive 1 1/2 inch screws through the mounting holes at the top of the bracket, up into the mantel's shelf board.

Measure the distance between the floor and the bottom of the mantel's shelf. Cut two pieces from the remaining two boards that measure the exact distance from the mantel's shelf to the floor. Cut 90-degree angles into both ends of both pieces.

Align the two boards into place vertically between the floor and the shelf. Align the sides of the board with the edge of the fireplace opening and the vertical chalk lines.

Drive 4-inch wood screws through the boards and into the wall studs behind the boards. Sink the screws one-fourth inch below the surface of the boards.

Fill the holes in the board in with wood putty or wood filler. Smooth the wood into the holes and wipe away any excess putty around the holes.