How to Install Window Trim Properly

There’s a Certain Order to Installing Window Trim in Order to Do It Right

By Mark J. Donovan

Installing window trim is the perfect DIY home improvement project to cut your teeth on. It doesn’t require being high up on ladders, there’s no chance of electrical shock, and if you mess up you can start over again, albeit you may spend a little extra money on wasted scrap.

However, to install window trim properly you need to install it in a certain order and you need the right tools. For tools, I highly recommend buying or renting a powered miter saw. A powered miter saw will ensure cleaner cuts at the appropriate angles (e.g. 45 degrees) and make the work go faster.

In addition, a table saw comes in handy if you need to fabricate your own window extension jambs. A pneumatic finishing nail gun is also very handy for fastening the window trim to the walls.

Installation of Extension Jambs

In regards to the right sequence when installing window trim, first make sure that the window extension jambs are flush with the surrounding drywall surface. Extension jambs are thin 1 inch wide strips of lumber that extend outwards from the perimeter of the interior window frame, to the very edge of the surrounding walls. If extension jambs are missing, then you’ll need to install your own. You can fabricate them out of 1”xN” lumber and cutting them to the appropriate dimensions with a table saw.

As with installing window casing trim, it is imperative that the extension jambs be installed square and level. So make sure you have a level and a square handy when installing your window trim. To help square up and level the extension jambs and window trim use small wood shims where necessary.

Fasten the extension jambs to the rough window frame lumber using a finished nail gun, or with hammer and finish nails. Make sure to use finish nails that are 2-1/2 inch in length.

How to install interior window trim

Installation of Window Sill Stool Cap

The next piece of widow trim that needs to be installed is the window sill stool cap. The window stool is the piece of window trim that sits at the bottom of the window and forms the window sill.

A jig saw comes in handy when fabricating the stool cap. What makes this piece particularly tough is that you need to make some interesting cuts. The stool needs to have wings, if you will, that extend outwards beyond the edge of the window. The wings need to be long enough to support the vertical window trim pieces. Typically the wings should extend approximately ½ inch further out than the outer edge of the vertical window trim pieces.

Another complication when installing the window stool cap is positioning it at the proper height. Often shims are necessary and in many cases you may need to use small blocks of lumber along with standard wood shims to get the right height.

Again, fasten the stool cap to the rough window frame lumber using a finished nail gun, or with hammer and finish nails. Make sure to use finish nails that are 2-1/2 inch in length.

Installation of Vertical Window Trim Pieces

On one side of the widow measure from the top surface of the window stool cap to the location of where you want the bottom inside corner of the vertical window trim piece to be. Then cut a length of window case moulding to the measured length. Make sure the flat edge is cut squarely. On the opposite end of the window trim moulding cut it at a 45 degree angle using your miter saw. The lower edge of the 45 degree cut to the flat cut edge of the trim piece should measure the same length as what you measured earlier on the wall.

Use the level to hold the vertical window trim piece straight. Also position it so that it sits approximately 1/8 to 1/4 inch out from the inside edge of the window jamb to create a reveal edge. While holding the trim piece firmly in place fasten it to the wall with one nail at the top outer edge and one at the bottom.

Repeat the process for the other vertical window trim piece.

Install Top Window Trim Piece

Measure the distance between the two lower inside edges of the two vertical window trim pieces. Then cut a piece of window trim with 45 degree angles on both sides. The distance between the inside edges of the window trim should be the same length as what you measured on the wall.

If you installed your vertical members perfectly straight, and you measured and cut the top piece appropriately the top piece should fit like a glove in between the two vertical trim pieces.

While holding it firmly in place fasten the top piece to the wall.

Final Suggestions

While installing the vertical and top horizontal window trim pieces you may need to shim out the corner edges a bit to ensure that the corners come together perfectly.

Go back and install additional nails into the window trim. Use smaller, e.g. 4 penny, finish nails for the inside corners.

Also, if your window trim is to be stained, I highly recommend staining it prior to installing it on the walls. Also, after each cut lightly sand the cut edges and dab them with stain.

Finally, fill all nail holes with a wood filler to complete the project.

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