If you want to have a defined entrance and exit to a room, but don’t want to lose the wall space an open door can take up, the answer might be a simple cased opening. The arrangement is basically the same an interior door, but without the actual door slab. You can use a cased opening to replace an existing door or as a transition from room to room in a new partition that’s just been constructed.

What You Need for the Project

• Hammer or finish nailer
• Miter saw
• Wood shims
• Level – four foot works best
• Measuring tape
• Wood jamb material – available from millwork shops and home improvement stores
• Door casing – also called door trim – it’s available in many different profiles and widths.
• Circular saw

While it’s not absolutely necessary, you may want to match the cased opening’s trim to the other door and window trim in the room.

Tackling the Job

Whether removing an old door or starting with a new partition, there will be a framed opening in the wall that consist of vertical studs on both sides of the opening and a framed header across the top. The width of your new jamb material will depend on the thickness of the wall including the finish material on each side – a standard interior wall is 4 ½ inches thick and consists of a 3 ½ inch stud with a layer of ½ inch sheet rock on each side.

However, many older homes have thicker walls and even some newer residences may have wider walls in places to accommodate ductwork or plumbing drain lines – check your wall thickness prior to purchasing the jamb material. You want the boards to be the same width as the overall thickness of the interior wall. Most millwork shops will rip the boards to your measurements or if you have a table saw and know how to use it safely, it can be done at your home. Here’s how to build your cased opening:

• Install the header – cut a jamb board to length to fit across the top of the opening. Hold it across the bottom of the framing header and use your level to check its alignment. Install wood shims as needed until the jamb board is level, hold it so that it’s flush on each side, and nail it in place with finish nails or a finish nailing gun.
• Install the jamb sides – cut two jamb sides to length and after using your level to ensure they’re plumb; secure them in the same fashion as the jamb header. If you’re using finish nails, leave the heads exposed and set them later with a nail set.
• Cut the trim – the door trim material is basically used to picture frame the opening much the same as the interior trim around your windows. Typically 1/8 to ¼ inch of the inside edge of the jamb material is left as a reveal to add a little definition to the opening so keep this in mind when measuring the trim. The bottom edge of the top piece of trim should extend about that distance from the inside surfaces of the side jamb boards. Cut the corners of the trim at a 45 degree angle using the miter box.
• Install the trim – hold the header piece of trim in place and after ensuring the jamb reveal is the same all the way across the opening, nail it in place. Nail the side trim pieces after double checking that their alignment is correct and reveals are the same. Repeat on the other side of the opening.

If you’re planning on painting, set the nails and add a little filler, caulk all the joints in the trim and jamb boards, do a little touch up sanding, and start the first coat.