Every room needs one. Fortunately, framing a door is pretty simple. Here are the basics. (Assume the wall is framed of 2x4s.)
As you can see, a door has it’s own little post and beam construction. The beam — or the header — is a piece of wood the width of the wall. This can be either a solid 4x or two pieces of 2x lumber nailed together with plywood spacers between them to bring them up to the right width.
The header does a couple jobs: ensures a sturdy “roof” that won’t sag, and helps to spread the load when the wall is load-bearing. If the wall is supporting the floor above it, you’ll need bigger stock. For example a 4×4 header would be good on a 4′ opening in a non-load-bearing wall, but you’d need a 4×6 header in a load-bearing wall.
The header is supported by trimmer studs on either side, which are in turn nailed into full-length “king studs”. The cripple studs above the opening keep to the standard 16″ on-center..
When planning, remember the rough opening should be about 2″ wider and higher than the door itself. This gives you room to install and shim the jambs to plumb and level, as well as clearance around the door. We’ll get into that next time.