How to Install Inside Window Casing
- 1). Mark the window casing setback, called the "reveal," from the inside edge of the top window jamb. Measure upward from the edge of the window jamb's inner face and mark a point 1/4 inch from the edge on the outer surface.
- 2). Repeat Step 1 at incremental points around the remaining three sides of the window jamb to use in positioning the window casing during installation. The most helpful points for guide marks is at each corner of the window about 1 inch from the ends and equally spaced about 12 inches apart over the length. This will help properly position the casing pieces as you nail them in place.
- 3). Cut the top, or "header," section of window casing. Measure the horizontal dimension between inside faces of the window jamb sides at the top of the window. Mark a length of casing material to this dimension plus 1/2 inch on the edge that will be to the inside of the casing frame toward the glass. Use a miter saw to cut a 45-degree angle on both ends of the casing with the dimension marks as the short point of both angles.
- 4). Mark the window's side casing pieces. Measure the window's vertical height dimension, from the top surface of the window's sill to the bottom surface of the window's header or top jamb. Mark this dimension plus 1/2 inch on the inside edge of two lengths of casing material if your window type has an internal sill style that does not extend beyond the wall plane or side jambs. Mark this dimension plus 1/4 inch on the inside edge of two lengths of casing material if your window type has a traditional sill style that does extend beyond the wall plane and side jambs.
- 5). Cut the side casing. For both types of windowsill, the top cut will be a 45-degree angle with the dimension mark being the short point of the angle. For the bottom cut on an internal type sill, the bottom cuts will be 45-degree angles with the dimension mark being the short point of the angles. For a traditional type sill, the bottom cut will be a square cut to sit flat on the sill.
- 6). Install the left-side vertical casing piece. Align the inner edge with the reveal marked in Step 1 and the short point of the top angle 1/4 inch beyond the top jamb edge. For installation with a traditional sill, the bottom edge will sit flat against the sill. For internal sills, the short points of the mitered ends will extend equally at each end. Secure the casing in place using finish nails through the casing edge into the window jamb spaced about 8 inches apart or as needed.
- 7). Install the top casing piece cut in Step 3. Align the left-end miter angle with the side casing piece installed in the previous step and the bottom edge with the reveal marks across the jamb. Secure the top casing in place using finish nails through the casing edge into the window jamb spaced about 8 inches apart or as needed.
- 8). Install the right-side vertical casing piece. As with the left side, align the inner edge with the reveal marked in Step 1 and the short point of the top angle tightly fitted against the top casing angle. Secure the upper two-thirds of the casing piece in place using finish nails through the casing edge into the window jamb spaced about 8 inches apart or as needed and leaving the bottom corner loose.
- 9). Measure and cut the bottom casing or apron molding section. Measure the horizontal inside dimension between the inside faces of the window jamb at the sill. For windows with an internal sill, the bottom casing piece matches the upper and side casing and each end is cut at 45 degrees to match the side casing pieces. For a traditional sill, the bottom casing is apron molding installed against the bottom surface of the sill, cut to the dimension measured from outside to outside of the side casing with a square cut on both ends.
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Install the bottom casing section or apron molding, whichever is appropriate. Secure the casing section or apron molding in place using finish nails through the edge into the window jamb spaced about 8 inches apart or as needed. The corner angles for bottom casing should be held tight to the side casing angles. Apron molding is positioned tightly against the bottom of the sill with the ends aligned with the side casing's outer edges. - 11
Complete the final nailing of the casing around the outer perimeter. Secure the casing's outer edges with finish nails through the casing into the wall framing, spacing the nails about 3/8 inch from the edge and 8 to 10 inches apart. Lightly sand the corner joints, as necessary, to flush the surfaces. All of the nails can be set using a nailset and the holes filled with painter's putty for painting.