Interior Door Replacement: Slab Fit Versus Replacing the Frame

A slab is the door panel only; a prehung unit includes the slab and a frame with hinges attached. Masonite makes that distinction in its measuring guide (Masonite door measuring). Replacing only a slab can preserve sound casing and jambs, but it requires matching thickness, height, swing, hinge locations, bore, and finish. A prehung replacement resets the jamb and can correct a damaged or badly out-of-square opening, but adds casing removal, shimming, trim, and wall touch-up. Quote those as different jobs.

The sources reviewed here do not establish a useful national price; door type, opening condition, hardware, trim, and finish are required inputs. Have an installer measure the existing door and opening before you buy a slab or prehung unit.

Choose from the opening, not the catalog photo

Condition	Slab replacement	Prehung replacement
Existing jamb is straight and undamaged	Usually the simpler fit	May add unnecessary trim and adjustment
Hinges and bore match the new slab	Strong candidate	Still possible
Jamb is split, warped, loose, or wrong size	Poor candidate	Better scope
Opening needs a new swing or clearances	Limited by existing frame	Easier to reconfigure
Casing is damaged or style must change	Separate casing work	Often removed and replaced
Door needs a new finish	Shop or on-site slab finish	Finish slab and jamb as a unit

Masonite’s measuring guide says a prehung door includes the slab and hinges attached to the frame, while a slab is just the door. Measure the existing slab width, height, thickness, hinge positions, bore centerline, backset, swing, and undercut. For a prehung unit, measure the rough opening, wall thickness, jamb depth, floor condition, and casing. Do not assume a nominal “30-inch door” is the exact slab or rough-opening size. Masonite’s installation guidance provides the frame-alignment context for the prehung option.

Slab work is precise carpentry

A slab swap may require hinge mortises, a new bore, edge trimming, a latch adjustment, and painting all six surfaces. Ask who fills old hinge pockets, how much trimming is allowed by the door manufacturer, and whether the contractor will match the existing jamb finish. If the new slab is heavier or has glass, ask whether the old hinges and screws are suitable. Hardware can be reused, supplied by the owner, or included as a new allowance; spell that out.

The quote should include door delivery, protection, disposal of the old slab, knob or lever installation, strike alignment, and final swing test. A door that closes only after force is not a finished install. If the floor is uneven, record the desired undercut and confirm it will not conflict with ventilation or a threshold.

Prehung work exposes the frame and wall

Masonite’s installation guidance directs the installer to align the door so the jamb is flush with the wall. A prehung quote should therefore cover shimming, fastening, plumb and level checks, reveal consistency, casing, nail-hole filling, caulk, and paint. Ask what happens if the rough opening is out of square or the wall thickness differs from the ordered jamb. “Install door” is not enough to allocate that risk.

Removing casing can pull paint or drywall paper. Price drywall repair and repainting separately, particularly in a finished hallway. If the jamb intersects baseboard or crown, identify the trim cut and return detail. For older painted homes, the EPA says renovation work that disturbs painted surfaces can trigger lead-safe requirements; ask whether the contractor is covered when removing casing or sanding (EPA RRP contractor guidance).

Send two installers the same photos and measurements. Require proposals to identify slab or prehung unit, handing, jamb, casing, hardware, finish, wall repair, disposal, and warranty. Approve the swing and reveal at final inspection. That comparison tells you whether the lower slab quote is truly a fit or simply transfers frame and finishing work to you.