How to Trim Window Casings

Window trim softens unframed windows and adds elegance to stark rooms.

Using casing to trim a window gives the window, and room, a simple but elegant finish. Most renovations and new construction use picture framing casing to finish trim windows. There are a variety of casings available and these can be painted or stained in an even larger number of colors. Some careful measuring and basic home improvement tools and skills are all you will need to complete the job.

Things You'll Need

• Trim casing
• Measuring tape
• Pencil
• Miter saw
• Nail gun or hammer
• Finishing nails
• Caulk
• Putty

Instructions

1. Use the tape measure to measure the perimeter of the window. Measure the sides, top and bottom. Subtract 1/2 inch from each of your measurement to allow for a reveal around the window. This ensures that any gap between the window and buck frame will be hidden. Write down your measurements.

2. Mark the measurements on a piece of casing. The marks should be on the side of the casing that will be toward the window. Be sure there is enough extra casing beyond the marks to account for the 45-degree cut that you will be making. Your measurements are the shorter points on the casing.

3. Cut the casing at the pencil marks using a miter saw set at 45 degrees. For the opposing cut, you will need to move the miter saw so that it is making the opposite cut.

   Repeat Steps 2 and 3 to cut the side and bottom pieces.

4. Make a pencil mark, lightly, on the window at each corner, 1/4 inch in from the corners. This will help you line up the casing. Hold the top piece of casing in place and use finishing nails to secure it. You want to nail into the buck frame of the window.

5. Repeat the process to mount the side and bottom pieces. The 45-degree angles may be slightly off if the window is not square, but this can be easily remedied by using putty to fill the small gaps. Use putty to also fill the nail holes.

Tips & Warnings

• If the window you are casing has a sill, the bottom cut of your side pieces will be straight and no amount should be subtracted from that measurement. Your bottom piece of casing should then be measured to suit the window sill rather than the window.

• Always follow manufacturer's safety instructions and wear proper safety equipment when working with power tools.

Resources

• Ask the Builder: Door and Window Trim Tricks