How to Install Window Casings and Trim

Install window casings by measuring and cutting trim pieces to fit around your window frame, then nail them in place starting with the top piece and working down the sides.

  1. Measure Twice, Cut Once. Use a tape measure to determine the width of the top of your window frame and the height of each side. Add 1/4 inch to each measurement to account for any slight irregularities in the wall. Write these measurements down clearly as you'll reference them when cutting your trim pieces.
  2. Choose Your Trim Wisely. Choose trim boards that match your home's style, typically 1x4 or 1x6 pine boards work well for most windows. Inspect each piece for knots, warping, or damage. If using raw wood, sand lightly with 120-grit sandpaper and wipe clean with a tack cloth.
  3. Slice the Header First. Mark your top trim piece according to your measurements and cut it to length using a miter saw or hand saw. This piece should span the full width of the window opening. Test fit it against the window to ensure proper length before proceeding.
  4. Shape the Side Rails. Measure from where the bottom of the top piece will sit down to your desired stopping point, usually the windowsill or floor. Cut both side pieces to this length. These pieces will butt up against the underside of the top casing.
  5. Anchor the Header Level. Position the top piece so it overhangs the window frame evenly on both sides. Use a level to ensure it's perfectly horizontal. Nail it in place using 2-inch finish nails, driving them into the wall studs behind the drywall. Countersink the nails slightly below the surface.
  6. Secure Both Sides Flush. Place each side piece so it fits snugly against the underside of the top casing and runs straight down the side of the window. Check that each piece is plumb using a level. Nail in place with 2-inch finish nails every 12-16 inches, again countersinking the heads.
  7. Hide Nails, Seal Seams. Use wood filler to fill all countersunk nail holes. Let dry completely according to manufacturer instructions, then sand smooth with fine-grit sandpaper. Prime and paint or stain the trim to match your desired finish.