Trim Carpentry 101: Installing Window Casing

By Bruce W. Maki, Editor

When a conventional sill is not used, window casing is essentially a big picture frame. Four pieces of wood cut with a miter at each end. Pretty simple.

The first step was to mark the reveal. In this case, I used 3/16". I use an adjustable try-square for this procedure. The inside corner of the casing will be placed on the pencil marks.

The bottom jamb was an exception. I marked the reveal 3/16" from the bottom, because... ... the huge gap at the bottom is tricky to cover, unless the casing is positioned very carefully.

Then I filled the gaps around the jambs with fiberglass insulation. It pays to keep small scraps of insulation. Here I avoided using expanding foam insulation. It can make a mess because the amount of expansion is hard to predict. And it ruins carpet.

When I install window casing I make a little sketch on a piece of paper (3x5 file cards are convenient) and write down the inside dimensions of the casing rectangle. Then I add twice the trim width to arrive at the outside dimensions, because it is far easier to measure the long dimensions of a mitered piece. (It is darned difficult to hook the tape measure on the short side of a mitered cut.)

I mark the length and then make a 45 degree line with a speed square. This line makes it easy to get a precise cut on the miter saw. Note the fine-toothed blade. It produces a smoother cut.

The four pieces were laid out on the garage floor. I placed a scrap of plywood (in this case, a spare cabinet shelf) to raise the trim up so the brad nailer can be aimed properly.

I did several casings without using the plywood spacer trick, but often the brad nails would bend and exit the back of the trim, even though they were (sort of) aimed sideways. I observed that I was not able to aim the nailer perfectly sideways when the trim was on the floor, so the hard wood grain would force the thin nail to bend.

I shot 1" brads into the corners. Note the two tiny holes. Nobody ever notices these when the job is done.

The casing with all corners nailed.

This window casing was so big that I absolutely needed a helper to carry it into the room. Without a helper, I would have to assemble the casing in the room. This window casing was almost 7' long.

Installation begins with a 1" brad to hold a top corner in place. With both top corners positioned and nailed, I continued down the side, driving a nail every 8 to 12 inches to hold the casing into the jamb extensions.

This is not just a simple matter of blasting in a few dozen air nails. The casing needs to be carefully positioned so the reveal is balanced all around. Sometimes a corner joint will open up. The trim is flexible enough that it can be bent to force a gap to shut, and then fastened to hold the corner in place. Then, a foot away from the corner, the trim can be bent back the other way. 

The top piece was arched slightly, and a nail was visible underneath. I was able to pull the mid-section down (the corners were already fastened) and nail it in place.

There are a lot of compromises necessary in this work. In the above example, the reveal ended up being slightly different along the top piece. But that is less objectionable than having a big nail show through.  In hindsight, I suppose I could have put some wood filler in the nail hole.

I would rather have an uneven reveal than have corner joints open up. Few people notice a reveal that varies from 1/8" to 3/16", especially when the trim is stained wood, where the grain pattern distracts the eye.

For painted trim, I would keep the reveal consistent and uniform, and then caulk the gaps.

After the inboard edge of the casing was nailed all around with 1" brads, I drove in some 2" finish nails to the outboard edge. I only use 3 or 4 nails per piece of trim, just enough to keep the trim from rocking.

The finished window. The homeowner will be staining the jambs and sashes.

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Copyright © 1999, 2005  HammerZone.com

Written October 31, 1999 
Revised January 10, 2005