How to Install Molding on Arched Windows

Measure the distance from the top of the windowsill to the point the arch meets one straight vertical window leg. Place a pencil mark on a length of wood trim at the measured length plus 4 inches. Cut the wood trim to length with a handsaw. Cut a second piece of wood trim to length the same length.

Hold one piece of trim against the wall, with the inside edge of the trim sitting 3/16 to 1/4 inch away from the edge of the window frame. Secure the trim by hammering in one 1-inch brad nail every 14 to 16 inches with a claw hammer. Leave the head of each nail protruding 1/16 inch away from the front face of the trim. Install the second piece of trim. Leave the same 3/16 to 1/4 inch away from the edge of the window frame.

Align a straightedge with the bottom left corner of the right-side trim and the top left-hand corner of the window frame. Ensure the straightedge extends past the top left side of the right trim. Drag a pencil along the straightedge. Move the straightedge to the bottom right corner of the left-side trim. Draw a pencil line on across the face of the second trim piece. Do not allow the straightedge to move as you mark as this will cause the cut angle to change. Cut along each pencil line with the handsaw.

Set the center of a full length of flexible trim with the center of the window frame arch. Keep the bottom of the trim 3/16 to 1/4 inch away from the edge of the window frame. Secure the trim by hammering one brad nail at its center with the hammer. Pull one side of the trim along the arch. Place one nail every 6 inches as you move to hold the trim in place. Set the last nail 8 inches from the end of the trim. Work toward the other piece of trim.

Align the handsaw blade with points the flexible trim intersects an installed piece of wood trim. Cut the flexible trim miter slowly with the saw to avoid damaging its surface. Repeat the procedure to cut the miter on the second piece of trim.

Slide a wood shim under each mitered side of the flexible trim if it sits lower than the wood trim installed on the sides of the window frame. Secure the ends of the trim with a brad nail placed 1/2 inch in from each end. Set each brad nail below the surface of the trim with a nail set. Cut the ends off the wood shims -- if installed -- with a sharp-bladed utility knife before caulking the trim to the wall.