Installing Window Casing

Enchance and update the look of your old windows by hanging new window trim.

Related To:

Materials and Tools:

air compressor and hoses
finish nailer
measuring tape
miter saw
utility knife
eight linear feet of window sill trim (pre-milled with a back notch)
24 linear feet of casing trim (The amount of sill and casing trim will depend on the size of your window. It is always a good idea to purchase a little extra.)


After you remove the old casing, you may want to add extra insulation around the window before you hang the new casing.

After you remove the old casing, you may want to add extra insulation around the window before you hang the new casing.


1. Remove the old casing. Use a utility knife to score around the casing then pry it off with a crowbar.

2. Start at the sill. The pre-milled sill has a notch cut out of the back that clips over the bottom of the window. Cut the sill long enough to span the length of the window plus the width of the window trim on each side, plus a 1/2 inch for an overhang.

3. Cut the vertical trim. Measuring from the sill to the top of the window along the inside edge. Then add 3-1/2-inches to allow for the 45-degree angle. Mark the two measurements on the trim and set your miter saw to cut a 45-degree angle between the two measurements.

4. Tack up the vertical pieces with just a few nails at the bottom. Don’t nail securely, you may have to make adjustments later.

5. Measure for the top piece of molding. Cut each end at a 45-degree angle to fit inside your two vertical pieces.

6. Install the top molding. Start at one end, nail up the molding. When you get to the final end, you can squeeze or spread the vertical molding slightly to line up the corner. This is why you left the vertical piece un-nailed at the top.

7. Cut the molding for the piece underneath the sill. Cut this the same length as the sill less the 1/2 inch for the overhang (1/4-inch on each side). Cut the depth at a 45-degree angle (scarf joint) to allow room for the returns.


Fill in nail holes and gaps before painting.

Fill in nail holes and gaps before painting.

8. To make the returns on the bottom piece of molding, cut two small pieces the thickness of the molding (the length of the cut will be the depth of your molding). Cut a scarf joint on one side.

9. Center the bottom piece of molding underneath the sill and nail in place. Nail in the small returns on each side for a clean, finished look.

10. Fill in any gaps and nail holes. Then sand, prime and paint your new window casing.

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