The secret to a good wall-framing job is to get everything straight, and that's especially true when you're framing a door. While you can compensate for a small amount of slope by using shims when you install the jamb, any appreciable tilt prevents the door from working properly. Making careful measurements and checking them twice may take time, but it prevents your door from sticking or swinging open when left ajar.
The Parts of a Door Frame
Carpenters use a particular nomenclature to refer to the various pieces of lumber that make a door frame. The two studs on either side of the frame are the trimmers or jack studs; they are cut to the height of the door and support the header, which is the lumber the spans the top. In a non-load bearing wall, the header is usually a piece of four-by-four lumber or two, two-by-fours nailed together. The two wall studs to which the trimmers are attached are the king studs, and between the top of the header and the top plate of the wall are short lengths of two-by-four lumber called cripples.
Placing the King Studs
The placement of the king studs is critical, because the distance between them defines the width of the door. Before installing them, you need to know the distance between the outside edges of the jamb for the pre-hung door. Take this measurement at the top of the jamb. Adding 3 inches to this measurement gives you the minimum spacing between the king studs, because that's the combined thickness of the trimmers that support the header. It's wise to add an extra half inch to this measurement for wiggle room; you'll compensate for this extra space when you shim out the jamb.
Framing the Wall
If you're constructing a new wall, an efficient way to do it is to construct a large rectangle on the ground consisting of the top and bottom plates of the wall and the studs that form its end. Hoist the wall into position, level the ends with a 4-foot level, then temporarily secure it to the ceiling. The king studs go in next, and after they're secure and level, you can permanently fix the wall and add studs, spacing them by 16 inches as measured from their centers. This is the standard spacing required for drywall installation.
Finishing the Door Frame
The trimmers go on the bottom plate and must be high enough to support the header. Its height above the subfloor equals the height of the door, the thickness of the top jamb and an extra inch for gaps above and below the door. Don't forget to allow for the floor covering if you're framing on a bare subfloor. The four-by-four header sits on the trimmers and gets attached to the king studs. Finally, the cripples, spaced at the same interval as the wall studs, tie everything together. Double-checking the level of every component as you install it and making adjustments when needed helps avoid complications when the time comes to hang the door.