How to Trim a Window
If you are planning on installing a new or replacement window you will need to know how to complete the job by installing wood trim around the window. Window trim adds architectural elements, and sometimes function, to a window with the casing and sill. This step by step guide to installing window trim should help you complete a first-class project.
Types of Window Trim
Typically there are two ways to install window trim. The first is called a wrap or surround where identical pieces of trim are installed on all four sides of the window like framing a picture. This style is most often used in small rooms or contemporary designs. Since the trim is flat against the wall there is no trim protruding into the room and makes the window appear larger and it can make the room appear larger as well.
Design Options for Window Stool and Apron
Window Trim Components
Wrap Around Casing
1. Measure the top of the window frame from inside edge to inside edge.
2. 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. You can also mark off the reveal on the edge of the window jamb at each corner and then measure to each mark for the trim length. You can also adjust the reveal to your taste or to compensate for an inside corner or some other building component that you need to work around.
3. Using your miter saw, cut one end of a length of trim to 45 degrees.
4. Measuring from the bottom, or the shortest, edge of the angle you just created, measure the distance that you determined in step #2 and make a mark.
5. 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. The cuts on casing edge closest to the window should angle in on both ends.
6. Hold the piece in place to make sure that you figured the reveal correctly. Level the piece and then tack it in place with a few 4d finish nails. This will allow you to make any adjustments if the window isn't square.
7. Cut the miter on the next piece of trim and set this cut against the first piece of casing you installed. Mark the miter cut on the casing from the reveal line on the window jamb.
8. Drive a nail on the inside and outside edge of the casing every 8" to 12". Use 4d finish nails when going into the window jamb and 6d or 8d nails when nailing through the drywall into the framing.
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 a different piece of wood called a stool which is installed along the bottom edge of the sash to form a sill. You can buy stool material already milled from your local lumber yard or home center. Some window manufacturers have a groove in the bottom of the window for the stool to fit into so you can purchase the correct type of material for the sill. In most cases the stool has a groove on bottom edge that goes against the window and fit over the exposed framing.
Window Stool stock
Stool and apron window casing is a more traditional approach and stools can be used as small shelves depending on the width of material used. The stool and apron helps give a room depth and texture.
Stool and Apron Window Casing
1. Place a side casing against the wall, aligned with the reveal line on a side jamb. Mark the wall at the casing's outside edge. Repeat for the opposite jamb. Measure between the marks and add 2 inches for a 1-inch horn and 1-1/2 inches for a ¾ inch horn.
2. Cut the stool to this length with your miter saw set to 0-degrees (perpendicular) or with a hand saw.
3. After the overall length is correct, hold the stool flush against the wall to mark the notch out. Transfer this distance between the window frames to the stool.
4. Measure the depth of the notch or angle the stool so you can place one end up against the window jamb and mark the depth. Transfer this mark to the other end and cut stool and cut the notches with a jigsaw.
5. Once the stool is cut to fit, secure it to the sill with finish nails.
6. Measure and cut the head casing and install it over the window.
7. The side casings will have a square end on the bottom and rest on top of the stool. Cut a miter at the top of the side casing and hold it against the head casing. Mark the location of the top edge of the stool onto the side casing. Cut the casing to length and repeat the same process for the other side.
8. To complete the window you will need to install an apron under the stool. Measure the distance to the outer edge of the two side casings and cut a piece of casing to this length 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. Sometimes the apron ends are cut on an angle for aesthetics, a good angle is better 15 and 22 ½ degrees.
If you want to use decorative casing, but you aren’t very good at mitering the corners you can use plinth blocks, sometimes called rosettes, at the corners. These are small blocks of wood, often decorated with a floral pattern or a bull's-eye, which allow you to butt the casing right up against it on both sides. This plinth block eliminates the need for miter joints and also adds an architectural style to the window. 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.
To complete the installing of the window trim use a nail set and hammer to set the nail heads below the face of the trim.
Once the heads are all set fill the holes with wood putty or glazing compound. If you are planning to stain the trim you will need to use stainable wood putty or fill in the nail holes with a wood filler stick after you are done staining the trim.
If you use wood putty sand off the excess with 150 grit sandpaper.
Hopefully you will be able to tackle this rewarding home improvement project. See my other related articles regarding finish carpentry and windows here: