What each unit actually is
A slab is the door only. It hangs on the hinges you already have, or on new hinges in the existing jamb, and it has to match hinge locations and lock bore. A pre-hung unit is the door already hung in a new frame, with a threshold gap and a strike, ready to shim into the rough opening. If the jamb is sound, the slab is the smaller job. If the jamb is twisted, split, or out of square from settlement, hanging a slab into it just moves the problem onto a new door.
How Wasatch Front houses fail at the jamb
Newer Lehi and South Jordan production jambs are often thin and under-fastened. Older Sandy and American Fork jambs have moved with the house. In both cases a door that will not latch is sometimes a 20-minute strike adjustment and sometimes a pre-hung swap. We measure diagonal square, look at hinge screws (short drywall screws are a tell), and swing the existing slab before we order anything.
Casing, paint, and the rest of the opening
A pre-hung swap usually means new casing. Matching a colonial or craftsman profile already in the house is a material run, not a leftover. Paint or primer on the new unit should be in the same scope or the opening will read as a replacement forever. A slab into existing casing can be the quieter path in a finished hall, if the jamb deserves to stay.
Bottom line
Read the jamb first. Slab into a sound frame. Pre-hung into a failed frame. Price casing and paint with the unit or the hall will still look unfinished.
Questions
Is a slab interior door cheaper than a pre-hung door?
Usually yes, because you are not replacing the frame and often not the casing. It is only cheaper if the existing jamb is actually reusable.
When must I use a pre-hung interior door?
When the jamb is split, racked, or too far out of square to hang a slab with even reveals. A new door in a bad frame will still drag.
Can you hang a slab on my existing hinges?
Yes, when hinge locations match. Many replacements need new hinge mortises. That is checked with a tape, not assumed from the old door.
Do I need new casing with a pre-hung unit?
Usually yes, because the new frame will not land on the old miters. Matching the house profile is part of the opening, not an extra flourish.
