Custom Window Sill Trim

March 2, 2019

In the previous post “How to Build a Custom Window Sill from Scratch,” we took a look at a DIY video on fabricating and installing a new window complete with a custom window sill package. In part II of the series will look at the final steps to add the finishing touches to your new window with custom apron and trim package.

Building the Apron for Your Custom Window Sill Trim Package

Measure the width of the window sill and subtract about 1” to ensure that the new apron will neatly tuck under the window sill. Use this dimension to cut the molding to length using a 45-degree miter saw as shown on the video. Cut two mating pieces about 4 inches long which will be used to finish off the ends of the apron trim section.

Use a box or wood vise to secure the 4-inch pieces in place. Apply a generous amount of wood glue to the apron pieces and clamp them in place overnight so the glue can fully cure. Carefully remove the clamps, then trim the 4” section flush with the backside of the apron. The best method to remove the excess is by using a table saw, to ensure a nice, square cut. Let the saw do the work, don’t push too hard or you may break off the mitered extension pieces.

Adding the Vertical Trim

With the new apron and custom window sill in place, its time to add the vertical trim to finish off the project. You have a couple of options here. You can miter the vertical trim and top board for a clean look. Or, optionally, you can add Rosettes in the upper corners for a more ornate look.

Miter Cut Trim: If you want the simple look, measure from the top of the sill to the underside of the top of the window frame. If your trim is 2 “, add 2-1/4” to this dimension. Cut one end of the trim square and the other end at a 45-degree angle. Prepare an exact opposite of this board. For the top section of trim, measure the inside width of the window frame, then add 4-1/2” and cut both ends at a 45-degree angle.

Rosette Style: If you prefer to add Rosette’s in the upper corners of your trim package then you will cut both ends of all trim piece’s square. Assuming the trim is 2” and the Rosettes are 2-1/2”, then you only add ¼” to the vertical length of the trim. While adding ½” to the length of the horizontal board.

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