Last updated: November 2, 2026
How to Install a Prehung Door: Steps, Shims & Tolerances
Quick answer: A prehung door goes in well when the hinge jamb is set plumb first and the leaf is used as its own template. Prepare a rough opening to the unit manufacturer's dimensions, support the sill or floor condition correctly, shim at hinge and fixing points, then set heads and lock jamb to an even reveal. Do not force the frame square by racking it against a twisted opening.
Confirm rough opening and unit size
Prehung units are sold by nominal leaf size, overall frame size or rough-opening size depending on market, so use the submittal drawing rather than a universal formula. As an initial check, many framed openings need room around the unit for shimming and insulation, but the exact allowance belongs to the manufacturer. Verify the opening is plumb, the floor datum is known and the swing clears skirting, radiators, switches and adjoining doors.
Set the hinge side plumb
Remove only the packaging that the instructions permit and keep cross-bracing in place if it is part of the shipping system. Set the hinge jamb on firm packers at floor level. Plumb it in both planes and temporarily fix near hinge locations. Shims should be paired and placed where screws can clamp them; a shim floating between fixings will allow the frame to move when architraves are fitted.
Align head and lock jamb to the leaf
Open and close the leaf during installation. Level the head by reference to the closed leaf reveal, then bring the lock jamb to a consistent margin and latch contact. Fix behind hinges and strike areas first because those points carry operating loads. Recheck diagonals and the door face against the stops. If the reveal changes when screws are tightened, add or adjust a firm shim at that fixing instead of pulling the jamb into a bow.
Seal, insulate and hand over
Fill only the void specified by the wall and door system; expanding products can bow light jambs if over-applied. Fit hardware, check latch and closer action, confirm the threshold condition and protect finished faces before wet trades continue. For rated units, frame anchors, grout or seal systems and gaps are part of the tested assembly and should be inspected before architraves hide them.
Prehung or slab for a project order?
Prehung sets shift hinge/latch machining and leaf-to-frame fitting into the factory and usually reduce site labour, but they ship with more air and need careful handling. Slabs pack much more densely and can suit markets with separate frame supply. Compare total installed cost—labour, damage, frame purchase and container quantity—not leaf price alone. The loading examples in our 20ft container guide show why pack format changes the comparison.
Specification checklist: what to confirm before ordering
- Finished-floor datum, structural opening and frame dimensions measured before machining.
- Leaf size, thickness, hand, swing and clearance targets frozen on the door schedule.
- Hinge template, handle height, backset and lock/hole positions stated from the same datum.
- One approved reference opening checked before repetitive installation.
- Cut edges sealed and rated-door machining kept inside the approved scope.
Related guides
- How to Hang an Interior Door: Step-by-Step With Tolerances
- Door Handle Height Standard by Country (US, UK, EU, AU)
- Door Hinge Spacing Rule: Positions by Door Height & Weight
Send the door schedule, finished-floor datum and hardware templates so hinge and lock preparations can be checked before production.
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FAQs
How much bigger should a rough opening be than a prehung door?
Use the unit manufacturer's rough-opening table. North American products often publish an opening allowance for shimming, while metric project frames may be scheduled by structural opening; there is no reliable universal add-on.
Which side do I shim first?
Set and plumb the hinge jamb first, with shims and fixings at hinge points. Then use the closed leaf to align the head and lock jamb.
Should I remove the door from the frame to install it?
Only if the instructions call for it. Keeping the leaf in place helps hold reveals, but heavy units may be easier and safer to set as a frame first with temporary spreaders.
