Synopsis: An experienced builder and trim carpenter explains how to install window jamb extensions, stools, and casing.

We constantly look through windows. A well-trimmed window enhances the view the same way a picture frame enhances the art it surrounds. As with a picture frame, window trim should stand up to critical scrutiny but not draw your eye. Doing a good job trimming windows is a matter of following an efficient sequence, understanding what has to be done when and slowing down enough to do your best work.

I usually trim windows in one of two ways. They either have stools, the horizontal piece at the window’s bottom where the cat sits, or they don’t and are picture-framed with casing. In either case, since the energy crisis of the 1970s brought on the demand for thicker insulation, most houses I work on are framed with 2×6 studs. Windows, though, are generally still configured for 2×4 walls, so I have to extend the jamb to bring the window flush with the drywall.

I usually trim the widest, tallest windows on a job first. This way, if I cut something too short on the larger windows, I might be able to recut and use the trim on a smaller window. I also identify the most visible windows on the job, usually the kitchen, living room, entry and master bedroom, and try to use the best-looking stock in those high visibility areas.

First, mark all the details on a story pole

Before cutting stock, I trim any drywall that extends past the framing and remove any blobs of taping mud. Drywall dust is extremely abrasive and will quickly scratch trim, particularly prefinished material. Because of this risk, I take a minute to sweep the room clean after I prepare the openings.

Then, assuming that I’m trimming more than a handful of windows, I make up story poles. Story poles are lumber or plywood scraps with all the relevant dimensions for a job marked on them full scale.

I refer to story poles throughout the job for measurements. Story poles reduce the possibility of errors in measurement that can happen when reading a rule or tape. I use two story poles for each size of window: one each for height and width.

After labeling the marks and double checking all reveals and measurements, I cut the poles to length at their widest marks. A word of caution: Sometimes the reveal between the bottom jamb extension or stool and the window will differ from the other three reveals. Casement windows often have cranks at the bottom, and you need to be certain that you have left the space for them to turn.

To get trim straight, you might have to install crooked jamb extensions

I start by ripping jamb extensions, usually nominal 1-in. stock, slightly wide on a table saw. This width leaves some meat for trimming. I set my blade at 2° or 3° to bevel slightly the edge of the jamb extension that meets the window. The slight point of this bevel is on the visible side of the jamb extension and helps to ensure a nice fit.

For more photos and details on how to trim a window, click the View PDF button below. 

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