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Interior Jamb Extensions for Window Trimout

Extension jambs bring a window or door frame flush with the interior drywall
May 08, 2023
Read time: 3 mins

In this article, Richard McMurray of DFW Crown Molding in Dallas/Fort Worth, Texas, discusses the process of extending a window jamb to flush it with the drywall. This is important to ensure that the window casing sits flush with the window unit and drywall.

Richard notes that the drywall protrudes past the jam creating a voided space. Extending the jam to bring it out flush with the drywall is necessary. This is done by filling in the gap with a jam extension.

Measure the gap and rip the stock to fit.

Richard got lucky because the gap is 3/4-inch, so he can use a piece of nominal trim stock to flush with their drywall, so they do not have to rip or plane anything down. This is essentially their jam extension. They are extending the jamb to bring it out flush with the drywall so that the casing can sit flush with the window unit and the drywall.

There are a couple of ways to do this with the jam extension. Richard notes that they can install it flush or give it a little bit of an offset, like an eighth-inch reveal or quarter-inch reveal. Either way looks fine, but Richard prefers the cleaner look of a flush installation. He notes that it does require a little bit more prep work, but as long as the parts remain flush, it's not much extra work—just passing over it with an orbital sander and then respraying the window.

Richard notes that the width of the jam is an inch and a half, and they will rip their extensions down to an inch and a quarter. This is because they do not want to fight with all the foam, and they want to keep it smooth right where it is.

This creates less work for the crew. He rips their extensions from offcuts of Windsor One trim stock and keeps the factory edge on them. This makes it easier when the crew goes to prep and paint because they already have a smooth, clean edge versus using the rip side with saw marks that would have to be joined or sanded to look finished.

Extend the stool to match the jambs.

Richard notes that they do not need an extension for the bottom because their stool and apron will go there. Richard installs the head and side jambs with a 16 gauge finish nailer to install the head and side jambs. He recommends starting on one side and ensuring the extensions are flush before tacking them. He works from one side to the other and puts nails every six to eight inches.

He notes that he did not glue the surfaces because it is not needed. 

"If you have ever shot something with 16 gauge nails like this and then tried to pry it off, it is extremely hard," he notes.