How to Cut Window Casing

Window casing is the final trim detail for a new or replacement window installation, framing the window opening to conceal the space between the jamb and the adjacent wall framing. Whether you've chosen a trim style from the wide assortment of shapes and widths available at home improvement centers, or the trim has been supplied by the window manufacturer, installing the casing trim completes your new window.

Things You'll Need

• Tape measure
• Pencil
• Hand square
• Window casing molding
• Window apron molding
• Miter saw
• Hammer
• 6d finish nails
• 8d finish nails
• Nail set

Instructions

1. Use a tape measure to mark a line on the face of the window frame ¼ inch from the inside frame edge. Mark the top and both sides at 8- to 10-inch spacing and at each corner. This will be the setback line, or the "reveal," of the window casing.

2. Measure the inside vertical dimension between the top of the window sill and the bottom edge of the top frame on the right-hand side of the window.

3. Start from a square cut end and mark the inside edge of a piece of the window casing trim with the dimension from Step 2 plus ¼ inch. This is the short point of the upper right corner angle, so make certain to leave enough space beyond the mark for a 45-degree angle cut.

4. Cut the casing piece marked in Step 3 on a miter saw at a 45-degree angle at the the mark made in Step 2, making the mark the short side of the angle.

5. Position the casing trim piece cut in Step 4 against the face of the right side window jamb. Align the inside edge of the trim piece with the ¼-inch reveal marks placed in Step 1, with the bottom end resting on the window sill. Attach the trim piece to the window jamb with 6d finish nails approximately ¼ inch back from the inside edge, and spaced approximately 8 inches apart and 1 inch from each end.

6. Measure the inside horizontal dimension at the top of the window frame, and between the inside surfaces of both sides of the window.

7. Mark the inside edge of a piece of the window casing trim with the dimensions determined in Step 6 plus ½ inch. This is the top casing piece, so make certain to leave enough space beyond the marks at both sides for opposing 45-degree angle cuts.

8. Use the miter saw to cut the casing piece measured in Step 7 at a 45-degree angle at both marks, making the mark the short point of both angles.

9. Hold the casing trim piece cut in Step 8 in place against the face of the top of the window jamb. Align the inside edge of the trim piece with the ¼-inch reveal marks placed in Step 1, and the right end miter cut tight to the right side casing piece installed in Step 5. Attach the top trim piece to the jamb with 6d finish nails approximately ¼ inch back from the lower edge and spaced approximately 8 inches apart and 1 inch from each end.

10. Repeat Steps 3 through 5 for the left side casing piece. Make certain the upper left corner joint is tight as you nail the casing trim in place.

11. Complete the final casing nailing with 8d finish nails, inserted ¼-inch from the outer edge and spaced approximately 8- to 10-inches apart.

12. Determine the length of the sill apron to be installed below the window sill. Measure the horizontal dimension, outside to outside and just above the sill, of the two vertical casing trim pieces installed in Step 5 and 10.

13. Start from a square end and mark a length of sill apron trim to the dimension determined in Step 12. Cut a square cut at the marked length.

14. Place the apron trim piece cut in Step 13 beneath the bottom of the window sill, making certain the thickest side of the molding is up against the bottom surface of the sill. Attach with 8d finish nails spaced 8 inches apart at both horizontal edges, and approximately 1 inch from each end.

Tips & Warnings

• Adding a light layer of wood glue to the faces of the mitered corners ensures that the corner joints will remain tight.

References

• This Old House: How To Trim Out A Window