Design & DIY

Easy window trim installation

Window trim project

I admit it: I’m no carpenter—but with the help of my friend Rosanne Berry (an accomplished woodworker) and a simplified, DIY-friendly design, I recently added pine trim to the windows of my not-quite-completed cottage near Apsley, Ont. Our approach combines butt-joint construction, making the measuring and cutting easy, with 
professional-looking details, such as reveals along the top and sides, and a variation on bead moulding along the head casing.

We used a mitre saw, an air nailer, and a jig saw. (If we’d needed to rip boards, we’d have used a table saw too.) Rosanne and I stained the wood before installing, then finished with clear semigloss polyurethane after everything was in place.

you’ll need

  • 1 x4 pine for head and side jambs
  • 1 x6 pine for head 
and side casings, 
and apron
  • 1 x1 pine trim plus 3/4″ pine half-round for bead moulding
  • 2 x 8 pine for stool
  • wood conditioner, 
wood stain, and polyurethane 


1. Measure each window’s jambs, since their width and length may vary. In our case, there was so little difference between the widest and narrowest jambs, we had all the jamb boards ripped at the lumberyard to the narrowest width.

Once you’ve ripped the boards, start by cutting and installing the head jamb. Measure the inside width of the window frame at the top, then cut and dry-fit the jamb, adding shims, if needed, to level. Sand, condition, and stain (do this for all the pieces as you work), and then nail in place. Measure for each side jamb from the installed head jamb to the bottom of the window frame; cut and dry-fit, shimming if necessary. Nail in place.

2. Calculate the stool length by measuring between the side jambs and adding 13″ 
(for two 1/4″ reveals, plus two 5!/2″ casings, plus two 3/4″ extensions). Cut to length.
Cut notches out of the stool so it will fit snugly in place. Don’t count on any two windows being the same; measure each one before you cut the stool with a jig saw.

3. Add 11  1/2″ (two 1/4″ reveals, plus two 
5 1/2″ casing widths) to the width between the two side jambs to get the length of 
your head casing and apron. Cut both of these pieces, sand and finish, and set aside.

4. To mimic bead moulding, which can be hard to find, cut 1 x 1 trim to the same length as the stool. Centre it on the base of the head casing, then glue, clamp, and nail to secure. Add half-round moulding along the edge of the trim.

5. Nail the stool in place. Figure out the length of the side casings by measuring from the stool to the head jamb, adding 1/4″ for 
a reveal. Cut the casings and nail in place, leaving a 1/4″ reveal along the side jambs.

6. Nail the head casing and the apron in place, fill any nail holes, and finish everything with polyurethane.

Window trim diagram
Illustration by Jacques Perrault


Why do pros use a reveal?

If you offset the inside edge of window and door casing so a thin strip of 
the jamb edge shows, any slight misalignment won’t be apparent. 
This offset edge—called a reveal—is also a traditional decorative feature that adds an attractive shadow detail to the trim.