Window-Trim Rules of Thumb
Tom Silva gives a tutorial on installing inside window trim
The double-hung windows in my new house don’t have any inside trim. How should I go about installing it?
—Scott Hicks, Sandston, Va
Have you decided what style you want the trim to be? There’s Colonial, Craftsman, and Victorian, to name a few, with lots of variation within each category. With a little research, you should be able to find a look that suits your windows and the rest of the house.
Whichever style you choose, the basics of installing window trim remain the same. Here are the rules of thumb and the sequence that I use.
Start with the stool. This part, which many people mistakenly call the sill, is glued and nailed to the bottom of the window frame using 8d nails. It should be long enough to span the width of the window, the width of both side casings, plus twice the distance you want the stool to extend out from the face of the casing. In other words, the horizontal distance from the edge of the stool to the face of the casing should equal the distance the stool will extend past the outside edges of both side casings. If the stool has a profile, highlight it by mitering each end and attaching returns. Make sure the stool sits level and square to the jambs; that will help the rest of the trim go in easier.
Side casings come next. These pieces cover the vertical gap between the window’s side jambs and the wall. Rest one end of the casing on the stool and line up its inside edge with the jamb’s inside edge. Mark where the casing meets the horizontal head jamb, and cut the casing to length. Do the same on the opposite side. Apply wood glue to the edge of the jamb and the end of the casing, then nail the casing to the jamb with 6d nails. Nail the other side of the casing to the wall with 8d nails. To keep the casing’s lower end in place, drive an 8d nail up through the bottom of the stool and into each casing’s end grain.
Install head casing next. If the stool is level and the window is square, cutting the head casing should be as simple as holding the stock in place atop the side casings and marking where to make the cut or cuts. If the side casings are mitered, you can find where to miter the ends of the head casing by turning it upside down, resting it on both side casings’ mitered points, and marking where they touch.
Finish with the apron. The apron, which sits under the stool, is cut and installed so each end lines up vertically with the outside edges of side casings. If it has a profile, miter the ends and glue on a return. Glue the apron’s top edge to the underside of the stool and nail the piece in place with 8d nails.