There are two primary methods of installing window trim. The first is called a wrap-around style, which consists of placing an identical piece of trim on all four sides of the window frame. This style is most often used in small rooms. It doesn't leave bulky pieces of trim protruding into the room. Since you are emphasizing the glass in the window instead of the trim, it can make the room appear a bigger. Here's how it's done.
Step 1 — Measure
Measure the top of the window frame from inside edge to inside edge and add 1/4" to that measurement. This will allow you to create a reveal of 1/8" on each side when you install the piece. Your local hardware store will be able to help you select the appropriate trim based on your measurements and a picture of the window.
Step 2 — Angle the Trim
Using your miter saw, cut one end of a length of trim to 45 degrees. Measuring from the bottom (or the shortest) edge of the angle you just created, make a mark using your measurements from earlier. Make sure to check your measurements twice, you won't be able to adjust the wood after you've cut it. Adjust your miter saw so that the next cut will be a 45 degree cut that results in your mark being the short end of the angle. Hold the piece in place around the window to ensure that you figured the reveal correctly, and repeat the process.
Step 3 — Nail it in Place
Beginning with the top, level and secure each piece with a few finishing nails. Don't drive a bunch of them yet, so you can remove it to make slight adjustments if it isn't completely square. Once every piece is in place and you are happy with each miter, drive a nail on the inside and outside edge of the face every 8" to 12".
The second way to install window trim is by utilizing an apron and a stool. The process is similar to installing a wrap-around style casing except for the bottom, where you will use the stool. You can buy stool material already milled from your local lumber yard or home improvement retailer. The stool has a groove milled into the bottom so that it will rest on the inside edge of the sill.
The stool and apron installation is found in a home where the woodwork plays a predominant role in the decorative feel of the house. Stools vary in size and can be be used as small shelves The presence of a stool and apron helps give a room depth and texture. Here is a step-by-step break down of how to install the stool and apron.
Step 1 — Measure
Measure the distance between the insides of the window frame. Next you will need to account for the reveal, typically 1/8". Double that amount, since there is a reveal on each side of the window, and add a total of 1/4" to your stool width. The next measurement to add is the width of the casing that will be on the side of your windows.
You are going to want the stool to extend at least to the edge of the casing, so add the width of the casing twice, as there will be casing on each side.
The last measurement to add is the horn. This is a distance that you determine based on how you want the window to function and to look. Typically the horn is 1/2" to 3/4". Remember to add it twice.
Step 2 — Cut the Stool
Once you have added all of those measurements together, cut the stool stock to length. After the overall length is correct, notch the stool so that it will slide around the casings and the window frame. Take the measurement between the window frames and mark that distance on the stool. Then measure to see how deep the notch needs to be. Lay this out on the stool and cut the notches with a jigsaw.
Step 3 — Install Stool and Casings
Once the stool is cut to fit, secure it to the sill with finish nails. The side and head casings will be installed just like they were in the wrap-around style.
Step 4 — Create the Apron
Once the side casings are in place, you can create the apron. Measure across the window from the outside of the casings. Using this measurement, cut an additional piece of casing and install it underneath the stool. This decorative piece will serve as the apron, and will also lend some additional support to the stool. After the apron is nailed to the wall, drive several finish nails through the sill and into the apron to secure it.
You can create a clean and neat looking corner by "butting" the ends of the casings into each other. One key thing to keep in mind, though, is that profiled casings will not work for butt joints. The casing must be flat and of a uniform height.
Another option is to use plinth blocks, sometimes called rosettes, at the corners. These are small squares or rectangles of wood, often decorated with a floral pattern or a bull's-eye that allow you to butt the casing right up against it on both sides. This plinth block eliminates the need for any difficult joinery. Simply set the blocks at each corner so that they leave the appropriate reveal and then measure the distance between them. Cut the casing to that length and install.
Go over each piece of trim in detail to make sure every nail head is set. Once the heads are all set, you can then go over them with wood putty and fill the holes. You can use a lightweight spackling compound, which is easy to sand and dries quickly, if you are painting. If you are going to stain and varnish your trim, then you will need to find a wood putty that matches the color of the stain you are going to use.
After the wood putty or spackling compound has dried, use a 150-grit sandpaper to buff it smooth, and you are ready to paint, stain, or seal.
The biggest challenges that come with choosing to install your own window trim is deciphering some of the jargon. Once you get them there will be nothing standing in your way!
Brian Simkins is a freelance writer living in Chicago. He enjoys using his 14 years of home improvement experience to educate and equip new home owners.