Install Perfect Window Trim

Long-time carpenter Travis Larson explains how he installs window trim

Man installing trim around a window | Construction Pro Tips
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You don’t even need a tape measure!

Like most veteran carpenters, I’ve trimmed hundreds—maybe even thousands—of windows. So trimming out a window is second nature to me. But when I see windows that were trimmed by a rookie or watched rookies trim a window, I realize it’s not quite as easy for others.

The truth is, I rarely even use a tape measure. Nope, it’s all done by eye, using a sharp pencil, a miter saw and an 18-gauge nailer. I’ll show you how to pull off a window trim job that’ll look every bit as good as a vets—without hours of frustration.  We’re working with standard trim, between 3/8 and1/2 in. thick—the types you’ll find at any home center.

-Travis Larson

Marking the length of a piece of trim | Construction Pro Tips
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Step One: Mark the length

Cut a 45-degree angle on one end of the trim and hold it so the short end of the angle overhangs halfway, or 3/8 in., onto the jamb. Then mark the other end flush with the inside of the jamb. That’ll give you a 3/16-in. reveal.

Stapling trim to the window for good spacing | Construction Pro Tips
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Step Two: Get the spacing right

Hold the trim 3/16 in. away from the jamb at both ends and along the base of the trim. Nail the trim to the jamb with 1-in. brads spaced about every 6 in. Nail the thick part of the trim to the framing with 2-in. brads.

Checking fit and length at the miter | Construction Pro Tips
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Step Three: Check the fit, then cut to length

Cut a 45-degree miter on one end of the trim board. Adjust the miter as needed for a perfect fit. Then scribe the cut length 3/16 in. past the bottom of the jamb. Nail the trim onto the jamb first and then to the framing, as you did with the top piece.

Gluing and pinning for a solid miter | Construction Pro Tips
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Step Four: Glue and pin for a solid miter

Glue and pin together the miter from both directions with 1-in. brads. Wipe the glue squeeze-out with a damp rag right away.

Trimming the side of a window opposite the first side | Construction Pro Tips
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Step Five: Trim the other side

Repeat all the same steps on the other side of the window, fitting first the top miter, and then marking and cutting the bottom one. Nail the trim into place.

Fitting the bottom miter to length | Construction Pro Tips
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Step Six: Fit the first bottom miter

Cut an overly long piece of trim and cut a miter on one end. Overlap the far end to check the fit. Mark and recut the miter as needed for a perfect fit.

Checking the bottom miters for fit | Construction Pro Tips
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Step Seven: Fit the opposite miter

Cut a test miter on the other end and check the fit. Adjust the miter as necessary until you’re satisfied with the joint.

Scribing the miter for a perfect fit | Construction Pro Tips
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Step Eight: Scribe for length

With the saw still set for the previous miter, flip the trim over and scribe the length for the end that has that miter. Transfer the mark to the front side and make the cut.

Knocking back projecting drywall with a hammer | Construction Pro Tips
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Dealing with problematic drywall

If you have drywall that’s “proud” (sticking out past the jamb) or recessed behind the jamb, you have to deal with it before trimming or the trim won’t lie flat. Here’s what to do:

If the drywall projects more than 1/8 in. crush in the drywall with a hammer. Just be sure the crushed area will be covered by trim. In this situation, your miters won’t be 45 degrees. You may need to go as low as 44 degrees to get a tight miter.

Cutting away drywall for better trim fits | Construction Pro Tips
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If the drywall projects past the jamb 1/8 in. or less...

and is close to the window jamb, just chamfer the edge with a utility knife. Check to see if you’ve pared off enough drywall by holding a chunk of trim against the drywall and jamb. If it rocks and won’t sit flush against both surfaces, carve out some more.

Shimming trim in places where the drywall has recessed | Construction Pro Tips
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If the drywall is recessed behind the jamb...

...don’t nail the trim to the framing at first. Only nail it to the jamb and pin the mitered corners together. After the window is trimmed, slide shims behind each nail location to hold out the trim while nailing, then cut off the shims. Caulk the perimeter of the trim to eliminate gaps before painting.