Casing is a multi-purpose feature in your home. See all it does and how to install it with our expert advice.

January 26, 2019

Casing is the molding that frames a door or window opening. In addition to dressing up the opening, casings cover the gaps between the walls and the jambs and hide the raw edge of the drywall. Before wrapping casing around a window or exterior door, add insulation. Loosely fill the gaps with shreds of fiberglass insulation poked in place with a drywall knife or similar tool or use nonexpanding spray foam insulation.

Casings usually are the same throughout a room, if not throughout a house, but that isn't a hard and fast rule. In fact, creating a hierarchy of casing details adds visual interest and richness to a room or home. Consider making the casings for exterior doors wider than those for interior doors and windows. Or adapt the casing size to suit the size of the opening: Larger openings get larger casings. Use your imagination.

What You Need

  • Tape measure
  • Miter box and backsaw or mitersaw
  • Hammer
  • Nail set,
  • Drill/driver
  • Molding
  • 4d, 6d, or 8d finishing nails (depending on molding thickness) 
  • 2-inch trimhead screws

Design Option: Cutting Butt Joints

If your casings are flat boards, it's traditional — and easier — to use butt joints (left). In a butt joint the ends are cut square and the pieces butt together. Most often the head casing sits on top of the side casings, but occasionally the head casing is fitted between the side casings — it's a matter of preference.

Corner blocks (right) came into use during the Victorian era. They add a decorative element and allow butt joints to be used with ornate molded casings, which otherwise would have to be miter-cut. The blocks are slightly wider and thicker than the casing, making them the most forgiving way to wrap trim around a window or door opening.

Step 1: Draw Line for Jamb Edge

Casings are usually positioned to leave 1/8 inch of the jamb's edge visible. This is called the reveal. To lay out the reveal, set a combination square to 1/8 inch or 1/4 inch, hold a pencil in the notch on the edge, and draw a line along the head jamb and both side jambs.

Step 2: Attach Side Casings

Measure from the floor to the head casing reveal on both sides and cut side casings to length. Attach the side casings with five pairs of nails from top to bottom. Allow the nails to protrude in case you have to pull them to trim the casing or adjust its position when you fit the head casing.

Step 3: Drive Nails

Most casings are backcut; that is, they have a shallow channel (or channels) cut in their backs. These channels allow for irregularities in the wall so the molding can fit tightly against the wall and jamb. When you install casing drive the nails through the solid edges.

Step 4: Cut Head Casing

Cut the head casing roughly to length. If the molding is mitered as shown here, start with a piece that's a bit longer than needed and carefully trim it to fit. For butt-joined head casing cut one end square and hold it in place to mark for an exact cut on the other side.

Step 5: Set Head Casing

Nail the head casing to the wall and head jamb with three pairs of nails. As insurance against the miters opening, drill holes in the casing and drive 2-inch trimhead screws through the head casing into the side casings as shown. Set all the nailheads when you are happy with the fit.

Casing a Window

Step 1: Decide on Casing Type

Casing a window is just like casing a door, except the casings don't run all the way to the floor. Choose from two options: The traditional style has a sill that protrudes slightly into the room at the bottom of the window. The sill, which is technically called a stool, is the first piece of trim installed. The side casings then butt to the top of the sill. A piece of casing called an apron is applied under the sill as a finishing touch.

In less traditional construction the sill is eliminated and the casing is wrapped around the window like a picture frame. This technique demands a little more joint-making skill. No clear starting point exists; just pick one of the sides and go from there.

Traditional window trim begins with the stool. Use a jigsaw to cut the horns on either end so they fit tightly against the drywall and the sides of the jambs.

Step 2: Install Apron

The apron is attached under the stool as the final piece of window trim. Measure between the outsides of the side casings to determine the apron length. If the apron has a molded profile, miter the ends of the apron toward the wall. Then glue a tiny piece of molding as a return to the wall.


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