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How to Install Window Trim

In this how-to video, This Old House general contractor Tom Silva shares his techniques for trimming out the interior of a window

In this video, This Old House general contractor Tom Silva explains how to install window trim.


1 Hold casing against each window side, and make reference marks along the wall.
2 Measure distance between the marks, then add 2 inches to determine the length of the window stool.
3 Cut the stool to length using a power miter saw with the blade set to 7 degrees.
4 Hold stool in place against the windowsill and use scribers or a pencil compass to mark where it meets the window sash and wall.
5 Trim the stool along the scribed lines using a jigsaw.
6 Set the stool in place, then use the scribe to mark the stool a second time.
7 Trim the stool once more with the jigsaw to ensure a tight fit.
8 Attach the stool to the windowsill using 4d finishing nails.
9 Mark and cut the side casings to length with the miter saw set to 45 degrees.
10 Nail casings to the window's side jambs using 4d finishing nails.
11 Lay a piece of casing on top of side casings, upside down, and mark where each end extends past.
12 Miter-cut the head casing to 45 degrees.
13 Glue and nail the head casing to the window's head jamb using 4d finishing nails.
14 Use 8d finishing nails to secure the outer edges of the side and head casings to the wall.
15 With the saw set to 45 degrees, cut the apron to length from a piece of window casing.
16 Cut 45-degree returns from a length of casing.
17 Glue the returns to the ends of the apron.
18 Nail apron to wall beneath the window stool using 8d finishing nails.


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