Exterior Door Installation: Slab or Prehung Replacement?

Choose a slab when the existing jamb is sound, square, weather-tight, and compatible with the new door. Choose a prehung unit when the jamb, threshold, weatherstrip, or rough opening needs replacement. A slab quote can look cheap because it leaves the frame and weatherproofing in place; a prehung quote costs more because it resets the opening and trim. Therma-Tru’s installation guidance treats plumb, level, square, shimming, sill adjustment, flashing, and caulking as separate parts of a complete installation (Therma-Tru door installation).

Scope comparison

Item	Replacement slab	Prehung replacement
Existing jamb	Reused and must fit the slab	Removed or altered with new unit
Threshold	Usually reused unless damaged	Supplied or installed with the unit
Weather seal	Existing kerf and adjustments	New weatherstrip and sill setup
Opening repair	Limited unless defects appear	Rough-opening, flashing, insulation, and trim may be exposed
Hardware	Hinges and bore must align	New hinge prep and lock alignment possible

Inspect the opening before ordering. Measure slab width, height, thickness, hinge locations, backset, swing, and bore. For a prehung door, measure rough opening width, height, depth, sill condition, exterior brickmold, and interior casing. If the opening is out of square, a slab may bind even when its nominal size matches.

Therma-Tru’s alignment guide recommends checking each jamb and sill and comparing the two diagonal measurements to confirm the unit is square (Therma-Tru plumb, level, and square guide). Put those checks in the installer’s completion list. They are especially useful when an old frame is being reused and the new slab must follow existing hinge and strike locations.

Weatherproofing is part of the install

Therma-Tru says to install head flashing, seal the exterior jamb and moulding joints, seal around exterior hardware, and insulate the gap between the jamb and opening. Those details should appear in the scope, with materials and responsibility identified. A door that closes in the showroom can still leak if the sill, flashing, or perimeter seal is wrong.

Ask whether the installer will remove siding or trim, repair water-damaged framing, set the sill, shim the jamb, and restore interior casing. Request photos of the rough opening before it is closed. If the existing frame is rotten, a slab-only quote is not comparable with a prehung replacement.

Hardware and security

List lockset, deadbolt, hinges, strike plates, threshold, sweep, and closer separately. Exterior doors may have code or egress requirements that vary by location. The installer should check local requirements rather than promise compliance from a generic unit description. A multi-point or impact-rated product may need its own preparation and certification.

Compare the complete installed opening, including the sill and trim work, rather than treating a door-unit listing as the project total. Door size, material, jamb condition, rough opening, trim, flashing, hardware, and local labor vary. Ask for two bids with the same slab or prehung product and a separate allowance for hidden framing damage. Approve the proposal only after it states how the opening will be weatherproofed.