How to Build a Window Mantle

Measure the distance between the outside edges of the window’s vertical trim at the top of the window.

Use a power table saw or a hand saw to cut a length of 1-by-4-inch frieze board to the width of the window frame, using 90-degree angle cuts on both vertical sides.

Cut a 1 1/2-inch wide piece of bullnose stop molding to a length that is 1 inch longer than the width of the window frame. Cut the molding to length with 90-degree angle cuts on both sides, creating a crosshead strip with a 1/2-inch reveal.

Measure out a length of 1-by-3-inch top cap molding that’s 7 inches wider than the width of the window frame. Use a power saw or a handsaw to make 90-degree angle cuts on either side of the cap.

Cut a piece of 2 1/2-inch crown molding to 6 inches wider than the window frame. Cut it to size with 90-degree angle cuts.

Butt the bottom edge of the crown molding against the top edge of the frieze board so that 3 inches of molding extend beyond the board’s sides. Mark the bottom of the crown molding where it meets the board.

Use a power table saw or a miter box and saw to make 45-degree angle back-cut miters that extend outward from the marked points at the bottom of the molding.

Line the top edge of the frieze board with a thin layer of wood glue. Lay the board on its backside. Center and butt the back-most part of the top cap’s bottom side against the glue-covered surface, so that each of its ends extend 3 1/2 inches beyond the edges of the board. Nail the cap into place with 6d finishing nails.

Set the unit on its top, so that the frieze board is vertical. Line the bottom edge of the board with wood glue. Center and press the top side of the bullnose stop molding against the glued surface, so that it extends a 1/2 inch beyond the edges of the board. Use 6d finishing nails to strengthen the joint.

Line the bottom and top edges of the crown molding with wood glue. Angle the centered molding between the frieze board and the top cap so that its narrower bottom edge is flat against the board, and its wider top edge is flush against the bottom of the cap. Use 6d finishing nails to secure the molding along both edges.

Front-cut a piece of scrap crown molding with a 45-degree angle miter, so that the cut angle runs opposite to one of the mitered ends of the crown molding. Repeat the process for the other side.

Fit each mitered return into place and mark them where they meet the back of the mantle. Use a handsaw to trim the pieces with 90-degree angle cuts that will sit flush against the wall.

Glue the pieces into place, creating returns that finish the molding by wrapping it to the wall. Until the glue dries completely, use masking tape to hold the returns in place.

Remove the masking tape and lightly sand all of the mantle’s edges and joints. Before installation, finish the mantle to match the rest of the window trim.