How to Case a Window With a Metal Frame

Measure the depth of the window from its front left drywall corner to the metal face of the window. Cut 1-inch boards to this depth for all of the windows you will be casing. Use a table saw to cut along the length evenly.

Measure the width of the window. Cut your trimmed wood to this width. Position the board against the underside of the drywall with the flat wide side touching the drywall. Nail the board into the header using finish nails. Use two finish nails evenly spaced apart in pairs for every 8 to 12 inches of board length.

Measure the height of the window from the windowsill to the underside of your first board. Cut two boards to this height. Position one board on the left side of the window tight to the drywall. Finish-nail the board to the studs. Repeat for the right side.

Measure 1/8 inch from the bottom of your first width board that you nailed to the underside of the header. Draw a horizontal line across the board 1/8 inch from the edge. Measure and draw a vertical line 1/8 inch from the inside left corner face of the left-side board from the windowsill to the top board. Measure and draw a vertical line 1/8 inch from the inside right corner face of the right-side board from the windowsill to the top board. This is called a reveal line. Your reveal lines should cross each other at the top right and left corners.

Measure from where the left vertical reveal line crosses the top horizontal reveal line to where the right reveal line crosses the top horizontal reveal line. Set your miter saw to 45 degrees with the blade toward you and to the right side. Place a piece of window trim molding faceup in the miter saw 2 inches from the left side. Press the molding against the fence and cut the molding. Measure from the bottom corner using your crossing line measurement. Make a mark on the molding.

Swing the miter saw to your left at 45 degrees. Slide your faceup molding to the left until the blade lines up with your cut mark. Cut the molding. Hold the molding up on your horizontal top reveal line. The top points should each face outward and the bottom corners should line up with the left and right crossing lines. Nail the molding to the wall using finish nails, one every 8 inches.

Measure from the left corner down to the windowsill. Position your molding faceup in the miter saw. Move the saw to 90 degrees. Slide the molding to the left just far enough to the left to cut off the angle. Cut the board. Transfer your measurement along the board and mark. Pivot the saw to the left at 45 degrees. Slide the molding to the left and line up the blade with your mark. Cut the wood. Line up the board along your left reveal line with the corners aligned. Nail the board in place. Repeat for the right side.