Installing mitered window casing without a stool or an apron is relatively simple, as long as everything is square. If not, you'll need to make some adjustments.
This type of molding has no stool or apron—just four casing pieces that frame the window as if it were a work of art. Picture-frame casing is often used for casement windows, where a stool would not be very usable anyway, but it’s not uncommon for sash windows in a modern house.
In theory, this type of casing is simplicity itself: Just cut four pieces to the correct lengths, all with 45-degree miters, and nail them up. And if your window jamb is perfectly square and your saw cuts perfect miters, installation might go just that smoothly. However, things are rarely perfect in this world, and if an angle is off only slightly, with picture-frame molding the imperfections are multiplied by four. So with the following steps, proceed carefully to ensure you keep all the joints nice and tight.
1. CHECK FOR SQUARE. As with any casing installation, check the window jamb for square. For picture frame, inspect all four corners especially carefully, noting even tiny imperfections. As much as possible, adjust shims to make the corners square; time spent doing this will save installation time and help produce a neater job.
2. MARK THE REVEAL. Use a plywood gauge guide or a sliding square to mark the desired reveal on all sides.
3. MEASURE FOR ALL FOUR PIECES. If the measurements for the two sides or the top and bottom differ, adjust the jamb to make them equal.
4. TEST THE ANGLE. Cut two scrap pieces of casing at 45 degrees, and hold them at all four corners to be sure they follow the angle of the jamb. If not, you may need to adjust your miter saw.
5. FIRST PIECE. Cut one end of a piece for the top or bottom. Measure for cutting the other end by burning an inch, as shown. Test the fit on both the top and the bottom.
6. SECOND PIECE. If you are satisfied, use the first piece as a template to mark the tip (only the tip) of the second piece.
7. TACK TOP AND BOTTOM. Use a hammer to “tack” the top and bottom pieces in places so they are aligned with the reveal lines. Partially drive two or more nails so you can remove them if you need to adjust the casing.
TIP: Use “hard trim” nails to tack the casing; they make smaller holes than regular finish nails do. Or just use a small gauge pneumatic nailer for tacking trim temporarily.
8. MEASURE FOR THE SIDE PIECES. This measurement should be the same on each side.
9. ADJUSTING THE FIT. Cut a side piece 1⁄8 in. or 1⁄16 in. longer than your measurement. Hold the board in place and test the fit. You may find that it only needs to have one end cut slightly to length. Or you may see that an angle or two needs to be changed very slightly. Make the adjusting cut with the miter
TIP: If you have a handsaw rather than a power miter saw, making micro-adjustments will be nearly impossible. In that case, it’s best to try to make the first cut accurate. Then make small adjustments with a plane or sander.
10. INSTALL THE SIDE PIECES. You may need to remove one or more nails to adjust the position of the top or bottom piece in order to make a tight joint. Once you are happy with the joints, drive finish nails to attach the casing all around. Drive a pair of nails, one into the jamb and one into the wall framing, every 16 in. or so. If you are hand nailing, use 3d hard-trim finish nails for attaching to the jamb and 6d nails for attaching to the wall.
Excerpted from Trim (The Taunton Press, 2015) by Steve Cory.
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