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For most interior-door replacements, a prehung unit is the most dependable route: verify the opening and swing, set the hinge jamb plumb on shims, adjust the latch jamb until the reveal is even, fasten only after the door operates freely, and install trim last. Exact rough-opening allowances, clearances and fasteners belong to the instructions supplied with your door, because published guides describe different products and methods.
Choose the installation method before buying materials
“Hanging a door” can mean installing a complete prehung assembly (slab, jambs and hinges together) or fitting a slab into an existing frame. The two jobs require different measurements and tools. Decide which condition you have before removing trim or ordering a door.
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| Approach | What you install | Best fit | Main skills and risks |
|---|---|---|---|
| Prehung replacement | Door slab already attached to a new jamb frame | An opening with a damaged frame, a new partition, or a homeowner who wants the hinges aligned at the factory | Measuring the rough opening, shimming, plumbing the jambs, and supporting fasteners; a solid-core unit may require a helper |
| Slab replacement | Door slab only, fitted to the existing frame | A sound, square frame whose casing and jambs can remain | Matching hinge locations, cutting hinge mortises, drilling for the latch, and fitting the strike; small measurement errors can make the door bind |
Existing hinge mortises can guide a slab replacement, but they do not guarantee that a new slab has the same thickness, bevel, hinge spacing or lockset position. Measure the frame and follow the slab and hardware instructions rather than assuming a standard layout.
Inspect and measure the opening first
Measure the opening, not just the old slab
Remove enough casing to see the jambs and studs. Measure width at the top, middle and bottom, then measure height at both sides and near the center. Record the smallest dimensions. Check the depth of the wall and whether the jamb width matches the finished wall. A replacement frame that is too wide or too narrow will not sit correctly against the wall surface.
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MMI Door’s installation guide describes a rough opening about 1/2 inch larger than the door unit in both width and height for the method it documents. That is a specification for that guide, not a universal allowance. Your manufacturer may require a different gap for shimming, insulation, flooring or a particular frame design.
Check plumb, level and square
- Hold a long level against each side of the opening and check the head. Out-of-plumb framing can make a door swing open or closed by itself.
- Use a framing square at the corners and compare diagonal measurements. Unequal diagonals indicate a racked opening.
- Check the subfloor and finished floor. A floor that rises toward the hinge side changes the reveal and can force the bottom of the slab to scrape.
- Look for bowed studs, loose framing, protruding drywall fasteners and old shims that would prevent the new jamb from sitting flat.
Correct major framing defects before placing the door. Shims can fine-tune alignment; they cannot reliably straighten a badly twisted opening.
Confirm handing, swing and floor height
Stand on the side from which the door will normally be pulled toward you. Confirm which side receives the hinges and whether the slab swings into or out of the room. Verify this before fastening anything: reversing a prehung unit after it is secured usually means removing it and starting over.
Account for the finished floor, including tile, underlayment or carpet that has not yet been installed. Masonite’s 2023 guide specifies at least 7/8 inch between the bottom of the door and the top of the flooring for the installation it describes. That figure is not a universal code requirement or a clearance for every door; use the supplied instructions and local requirements for your assembly.
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Tools, shims and fasteners
Gather the door-specific instructions before opening the package. A practical kit includes:
- Tape measure, pencil and a long construction level
- Framing square
- Wood door shims (use pairs where you need firm, adjustable support)
- Hammer and screwdriver or drill/driver
- Finish nails or other door-approved finish fasteners
- Fasteners long enough to pass through the jamb and shims into the framing, selected according to the manufacturer
- Utility knife, pry bar and safety glasses for removal and trimming
Masonite lists a level, shims, screwdriver, hammer and door-specific finish fasteners. MMI Door lists a long construction level, framing square, hammer, screwdriver, finish nails and wood shims. These lists overlap, but neither establishes a fastener size or spacing for every product. Do not substitute drywall screws or random nails when the instructions call for a particular structural or finish fastener.
Have a helper for a heavy solid-core prehung door. Masonite specifically notes that such a unit can be difficult for one person to maneuver. Keep the slab closed while moving the frame so the assembly stays rigid, and protect finished edges from the floor.
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Step-by-step: install a prehung interior door
1. Prepare the opening
- Remove the old casing carefully if the trim will be reused; otherwise remove it and the old frame.
- Scrape away protruding caulk, nails and drywall. Repair loose or damaged framing.
- Dry-fit the new unit. Check that the frame can sit fully in the opening, that the intended handle side is correct and that the bottom of the slab clears the existing or planned floor.
- Read the door maker’s sequence for shipping clips, temporary braces and hinge screws. Leave protective packaging in place until it is safe to remove.
2. Set and support the hinge jamb
- Place the prehung unit in the opening with the hinge side aligned to the intended wall face. Keep the slab closed while you position it.
- Insert pairs of wood shims between the hinge jamb and the stud behind each hinge location. The shims should support the jamb without bowing it.
- Check the hinge jamb for plumb from top to bottom. Masonite recommends checking both the back and interior faces of the jamb; a jamb can appear plumb on one face while twisting on the other.
- Adjust the shim pairs until the jamb is straight in both directions. Recheck the head and the floor clearance.
- Drive the specified fasteners through the jamb and shims into the framing. Keep the jamb tight to the shims, but do not overtighten and pull it out of alignment.
Shim locations matter because a fastener driven through unsupported jamb material can flex the jamb when the door is used. Supporting the jamb at the hinges transfers the load into the studs.
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3. Align the head and latch jamb
- Place shims above the head and behind the latch-side jamb as needed. Keep the head straight rather than forcing it to follow a bowed stud.
- Close the door gently and inspect the reveal—the visible gap between the slab and jamb. Adjust the latch-side shims until the gap is consistent from top to bottom and the door face sits evenly within the frame.
- Check that the latch edge is not rubbing and that the head gap remains even after each adjustment.
- Fasten the latch jamb through supported shim locations, following the manufacturer’s fastener type and spacing.
- Open the door through its full swing. It should move without scraping, springing open or drifting closed. Correct alignment now, before adding casing.
An even reveal is a result of plumb jambs and a square relationship between the jambs and head; it is not achieved by making one side of the gap arbitrarily wide. If the reveal changes after a fastener is driven, back that fastener off, reset the shim and try again.
4. Make the final checks
- Confirm the hinge screws are tight and the jamb remains plumb.
- Close the door until the latch engages. The slab should meet the weatherless interior stop without excessive pressure.
- Check the bottom clearance over the actual flooring and verify that the handle and strike are at the intended height.
- Trim protruding shims flush only after the door passes the swing, latch and reveal tests.
Install the lockset, strike plate and casing according to their instructions. Casing hides the installation gap; it must not be used to pull a misaligned jamb into place.
Step-by-step: fit a slab into an existing frame
A slab-only job is economical when the existing jamb is solid, square and worth keeping, but it demands more layout work.
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- Place the new slab in the opening with temporary wedges. Check the desired side gaps, head gap and bottom clearance against the door and hardware instructions.
- Transfer the existing hinge positions only after confirming that the new slab’s height and hinge spacing match. If they do not, lay out new mortises with a marking knife and cut them to the hinge leaf depth.
- Attach the hinges to the slab, hang it temporarily and test the swing. Remove small amounts of material rather than cutting a deep mortise in one pass.
- Mark and bore the latch hole and edge bore using the lockset template. Mortise the strike plate into the jamb so the latch enters cleanly without lifting or pushing the door.
- Install the hardware, tighten all screws and test the door repeatedly. Correct hinge alignment or a rubbing edge before painting or replacing trim.
RONA’s guide covers hinge notches and strike-plate mortising, while This Old House notes that existing mortises can guide a replacement slab. Neither source establishes one hinge layout or clearance that fits every slab, so measure the actual frame and use the supplied templates.
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How to troubleshoot a door that will not hang correctly
The door swings open or closed by itself
The hinge jamb is likely out of plumb, or the opening is twisted. Remove casing if necessary, check both faces of the jamb with a level and reset the hinge-side shims. Do not compensate by bending hinges until the frame alignment is verified.
The reveal is wider at one end
Check the latch jamb and head for plumb and square, then inspect whether a fastener has pulled the jamb toward an unsupported spot. Loosen that fastener, add or reposition a shim, close the door to set the reveal and refasten.
The latch misses the strike
First confirm that the jambs are parallel and the door is not sagging on loose hinge screws. If alignment is correct, adjust the strike position in small increments or follow the lockset maker’s adjustment method. Enlarging the strike opening before correcting a crooked jamb can hide the actual problem.
The bottom rubs the floor
Check finished-floor height and the frame’s level. A new floor covering may have consumed the original clearance. If the door was supplied for a different floor condition, follow the manufacturer’s trimming limits; never remove material beyond the permitted amount without confirming that the door construction allows it.
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The jamb bows after fastening
Fasteners may have been driven without solid shim support or overtightened. Remove the offending fastener, straighten the jamb with paired shims, and reinstall using the specified fastener. Keep shims at the load points rather than relying on a single shim at the top or bottom.
Prehung or slab: a practical decision checklist
- Choose prehung when the old frame is damaged, the opening is new, or you want hinges and stops supplied as one assembly.
- Choose a slab when the existing jamb is square and sound, the replacement matches its dimensions, and you are prepared to cut mortises and fit hardware.
- Plan for help when the door is solid core or the opening is narrow enough that one person cannot hold the frame and level at once.
- Allow extra time for an opening that is out of plumb, a floor that is not level, or a slab whose hinge locations differ from the existing frame.
Published installation guides agree on checking fit, using shims, aligning the jambs and testing operation. They differ on opening allowances, nail spacing, fasteners and floor-clearance figures because they describe different products. Treat each number as product-specific rather than combining figures from separate guides.
Final quality check before trim
- The handing and swing match the room plan.
- Both jambs are plumb, the head is straight and the frame is supported at fastener locations.
- The reveal is visually even on the hinge, latch and head sides.
- The slab opens, closes and latches without scraping, springing or excessive force.
- The bottom clears the finished floor by the amount required for this door and installation.
- Shims are trimmed only after testing, and casing is installed without pulling the frame out of alignment.
Taking the time to measure first and reset shims before final fastening prevents most callbacks. The door should prove that it is aligned every time it swings; trim and paint cannot correct a frame that is not.
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