Glass Trim Window

Installing casing trim around door openings and window frames can be a bit more difficult if you are looking for a professional looking finish. Certain techniques need to be employed to be sure the trim is cut to the exact form fitting precision, and using the right tools are necessary to ensure the job can be done quickly and smoothly.

Casing trim must be installed around all door openings first, before any baseboard trim can be laid down. The baseboard will then butt up against the casing for a nice looking finish. All casing should be ideally painted or stained finished before the install, as painting it afterwards can be more tedious.
Having the proper tools are an important part of a clean and efficient install. The typical measuring tape and pencil are your staple tools, along with a hammer and nail set. Have the appropriate length finishing nails for the thickness of your trim. For a standard sized casing trim, have 1 1/2" nails and 1" finishing nails. Having an electric chop saw, or miter saw is important, along with a small step ladder. It will usually take many cuts to achieve the perfect fit and a hand saw and mitering block will be more difficult to use efficiently. Finally, if your casing will have a painted finish, you have the luxury of using a paintable caulking to hide all seams and cracks for the ultimate clean install. Have a caulking gun and the necessary amount of caulking in a color that best matches your wall or casing color.

For a door trim casing install, start with the two vertical trim pieces first. Obviously there is a gap between the edge of the drywall and the finished door frame. Ideally we would like to cover this leaving the trim about 3/16" back from the inside edge of the finished door frame.

Now, assuming your door frame angles are approximately 90 degrees, cut the top of the casing piece at a 45 degree angle. Keep the shorter side on the inside and the longer side of the cut on the outside, of course. Don't forget to allow for the 3/16" spacing for the top board. The best way to do this is to stand the trim piece up on end, in place, and mark with a sharp pencil, on the inside side of the trim, a little tick at the appropriate height. Make your miter cut at a 45, using this pencil tick as a guide; it's important to be precise here. There's nothing wrong with cutting it a bit "long" to start, test fit it, and then working your way down bit by bit until the proper length is achieved.

Nail the board on using both sizes of finishing nails. Use the 1 ½" nails on the outer portion of the trim, nailed into the jack stud behind, and use the 1" finishing nails on the inner portion of the trim, nailing through to the finished door frame. Use nails every 1' to 2' apart, or as necessary, and counter sink with the nail set. Do the same with the trim piece on the other side of the doorway.

This leaves just the top casing piece to fit in between the two side pieces, and 45 degree angle joins. I like to start by cutting one end of the top trim piece at a 45 and test fit that end. It's helpful to have a helper hold the other end of the piece up as you check the fit. If you don't have another ladder for your helper, they can stand on a solid chair, or even a sturdy coffee table. Make sure the trim piece is horizontally in line with the top of the door frame, about 3/16" above the lip. The angled end join should fit leaving only a smooth flat seam line. If not, then you will have to make a cut at a slightly different angle to compensate.

Switch sides with your helper and mark the angled cut for the other side. The top casing piece will overlap the side piece, so you will be able to line up and mark the top and bottom for the exact angled cut. Set your miter saw up to the appropriate angle as measured, don't assume 45 degrees, use the angle measured. Make the cut about ¼ too long and then check for angle fitment, using a helper to hold the other end up again. If the angle was correct, whittle cut bit by bit, until the trim piece fits snugly and perfectly between the two side casings and nail it home. The same technique can be used for window trim, except the bottom piece will be like the top piece.

If there were any imperfections or crack lines exposed, use the caulking to runs beads around all sides of the casing, and smooth out with a finger. Only do this for painted trim, the wood finish trim may look awful if you try to caulk the cracks. You can also fill the angle join cracks with the caulking, as well as the nail holes, or just use wood filler. You can then repaint or touch up paint the casing, or if the casing color was a close match, just leave it. Good luck!!