Prehung Door Unit vs Slab Door: Sizes, Install and Code

Quick answer

A prehung door unit is a door slab that comes already mounted on hinges inside its own frame, with jambs on both sides and the head across the top, ready to set into a rough opening as one piece.

Exterior units usually add a sill, weatherstripping and often exterior trim. The alternative, a slab door, is the bare door alone, fitted to an existing frame.

Quick facts

The key facts about Prehung Door Unit, with sources:

Includes: Door slab, hinges, side jambs, head jamb; exterior units add a sill and weatherstrip ³
Swing: Ordered left-hand or right-hand, inswing or outswing
Installed into: A framed rough opening, shimmed plumb and fastened through the jambs ³
Main egress door (2021 IRC R311.2): Side-hinged, at least 32 inches clear width and 78 inches clear height ¹
Garage-to-house door: Solid wood or steel at least 1-3/8 inches, or 20-minute rated; self-closing ¹
Energy rating: Exterior door U-factor counts as fenestration under the 2021 IECC ²

Key takeaways

A prehung door comes hung in its frame, so the hard part of hanging a door, cutting hinges and fitting gaps, is done at the factory.
Choose prehung when the frame is damaged, the opening is new, or you want a tight exterior seal; choose a slab when the old frame is sound.
Order by rough opening size, swing direction, jamb depth and handing, and measure all of them before you buy.
An exterior prehung unit only performs if the opening is plumb, the sill is sealed or panned and the jamb is shimmed straight.
The 2021 IRC sets size rules for the main exit door and fire rules for the door between the garage and the house.

A prehung door in plain English

A door has two parts: the moving panel you push, called the slab, and the frame it swings in, called the jamb. A slab door is just the panel. A prehung door unit is the panel and frame together, with the hinges already attached and the gaps already set.

That changes the job. Hanging a slab means trimming it to fit an old frame and cutting hinge pockets that line up with the old ones. Setting a prehung unit means setting the whole frame into a framed hole in the wall, called the rough opening, getting it plumb, level and straight, and fastening it.

Exterior prehung units go further. They usually include a sill (the threshold at the bottom), weatherstripping in the jamb, and often brickmold trim on the outside.

That full kit is why many installers prefer prehung units for entry doors: the seal between door and frame is set by the maker. See exterior door threshold for the sill itself.

How a prehung unit goes into the wall

These steps follow one maker's published instructions for a prehung entry unit. Your door's instructions govern ³.

Check the opening. The maker calls for a "string test": run strings diagonally across the opening; if they do not touch in the middle, the opening is out of plane and must be fixed first ³.
Seal the floor. Without a sill pan, lay three large beads of high-quality sealant across the subfloor; with a sill pan, dry-fit it and follow the pan maker's directions ³.
Seal the trim. Run a 1/2 to 5/8 inch bead of sealant on the back of the exterior brickmold around the whole unit ³.
Set and plumb the hinge side. Tilt the unit in, center it, and plumb the hinge jamb both ways with a 6-foot level, screwing through the jamb at the hinges ³.
Shim and fasten. Shim above each screw and behind the strike, then drive long screws through the strike plate into the framing for security ³.
Adjust and finish. Check gaps, adjust the strike and sill, then insulate and air seal the gap between jamb and framing before interior trim.

Process Setting a prehung exterior door

String test the openingConfirm the framing is in one plane
Seal or pan the sill areaCaulk beads or a sill pan
Seal the brickmoldBead on the back of exterior trim
Plumb the hinge jambScrew at hinges, shim behind
Shim the strike sideEven gaps, long screws through the strike
Air seal and trimSeal the shim gap, then case the opening

Most leaks and sticking doors trace back to the first two steps. Source: [3]

Types of prehung door units

Type	Typical use	What comes with it
Interior prehung	Bedrooms, closets, baths	Slab, jambs, hinges; often casing sold separately
Exterior entry prehung	Front and back doors	Slab, jambs, sill, weatherstrip, often brickmold ³
Prehung with sidelites or transom	Wide entries	Door plus glass panels in one frame ³
Double (French) prehung	Patio, dining or office openings	Two slabs in one frame, active and passive
Fire-rated prehung	Garage-to-house door where required	20-minute rated assembly, self-closing hardware ¹
Split-jamb prehung	Interior doors in finished walls	Jamb in two halves that slide together to fit wall thickness

Example: measuring for a 36-inch prehung entry door

A sample measurement for replacing a front door. Door sizes are given as nominal width by height. Rough opening sizes vary by maker, so use the size chart for the unit you buy; these numbers show the method, not a spec.

Measurement	Sample result	Why it matters
Door size (nominal)	36 x 80 inches (called 3/0 x 6/8)	The slab size
Rough opening width	Measured at top, middle and bottom; smallest used	Unit must fit the tightest point
Rough opening height	Measured from subfloor to header at both sides	Includes room for the sill
Wall thickness (jamb depth)	Stud plus sheathing plus drywall	Sets jamb width so trim lands flush
Handing and swing	Hinges on the left, swinging in	Ordered wrong, the unit will not work
Main exit door check	Clear width at least 32 in, clear height at least 78 in	2021 IRC R311.2 ¹
Plane check	Diagonal strings touch in the middle	Out-of-plane openings leak ³

Formula Clear width of a swinging door

Clear width = distance from the face of the open door (at 90°) to the door stop on the latch side

Open door face: The door swung fully to 90 degrees
Stop: The strip the door closes against

The 2021 IRC requires at least 32 inches for the required egress door, measured this way.

Source: [1]

A nominal 36-inch door leaves well over 32 inches of clear width once the door stop and the open door are subtracted, so it easily meets the IRC egress-door rule.

A 30-inch door would not. If your opening is tight, measure clear width with the door open 90 degrees, the way the code does ¹.

Where "prehung" appears on quotes and plans

On a door quote. Look for "prehung" or "PH," the size (for example 3/0 x 6/8), handing (LH or RH, inswing or outswing), jamb width, core (hollow or solid), finish, and for exterior units the sill type and glass. If the quote lists "slab," you are buying only the door.

On a remodel scope. A scope of work should say who removes the old door and frame, who repairs the rough opening, and who installs trim and hardware. Interior door replacement often comes up in a whole-home renovation.

On the label. Exterior doors with glass may carry an NFRC label with U-factor and solar heat gain values. ENERGY STAR certified doors are independently tested and certified through NFRC ⁵.

On the permit. Like-for-like door swaps often need no permit, but a new or widened opening does, since it needs a header. Check with the permit checker.

Handing trick

Stand on the side the door swings toward you. If the hinges are on your left, it is a left-hand door; on your right, right-hand. Confirm with your supplier, since some makers name handing from the other side.

Prehung unit vs slab door

Why choose prehung

Hinges, gaps and weatherstripping set at the factory.
Fixes a damaged, rotted or out-of-square frame in one step.
Better seal for exterior doors, with a matched sill ³.
Faster for new openings and additions.

Why choose a slab

Keeps existing trim and frame, so less wall and paint repair.
Matches old hinge locations in historic frames.
Smaller and lighter to carry in.
Avoids disturbing old painted trim in pre-1978 homes ⁶.

Limits of prehung units

A prehung unit is only as straight as the opening it goes into. The maker's instructions single out out-of-plumb or out-of-plane openings as a common reason doors leak air and water ³. A crooked wall means shims, adjustments or reframing.

Replacing the frame also means removing interior casing and often exterior trim, and patching around them. In houses built before 1978, cutting or prying painted trim can release lead dust, and paid contractors must follow EPA's Renovation, Repair and Painting rule ⁶.

Finally, a prehung unit has a fixed jamb width. If your wall is thicker than standard, as with added exterior insulation, you need jamb extensions or a custom jamb. Order the right depth the first time.

What drives the cost of a prehung door

We do not list door prices. The drivers are size, material (hollow-core, solid-core, wood, steel or fiberglass), glass and sidelites, fire rating, finish, hardware, and whether the unit is a stock size or custom.

Installation cost depends on removing the old frame, repairing the opening, sill pans, trim and paint, and any lead-safe work practices in older homes.

Warranties come from the door maker and often depend on following its installation instructions, including sealant type and sill details ³. Get them in writing. Compare door bids line by line with the quote comparison tool, and see the cost guide for how doors fit a larger budget.

From order to inspection: the prehung door process

Decide prehung or slab based on frame condition and the look you want.
Measure rough opening, wall thickness, handing and swing, and check the exit door size rule if this is your main entry ¹.
Order the unit; allow lead time for custom sizes or finishes.
Remove the old door, frame and trim, following lead-safe practices in pre-1978 homes ⁶.
Repair the rough opening and install flashing or a sill pan per the maker and ASTM E2112 practice ⁴.
Set, plumb, shim and fasten the unit per the maker's instructions ³.
Seal the gap, install trim and hardware, and adjust for even gaps and a smooth latch.

New or wider openings are structural

Making a door opening wider or cutting a new one removes studs and needs a properly sized header, often in a load-bearing or braced wall. Do not cut framing yourself. Get a permit and have a licensed contractor, with an engineer where needed, design the header.

Keeping a prehung door working

Tighten hinge screws once a year; loose top hinges cause sagging and rubbing.
Replace worn weatherstripping and door sweeps on exterior units.
Adjust the sill if your exterior unit has an adjustable threshold and you feel a draft.
Repaint or refinish exterior faces and edges, including the top and bottom, on the maker's schedule.
Re-caulk the exterior trim joints when they crack.

Warning signs of a bad install

The door swings open or shut by itself, a sign the hinge jamb is not plumb.
Uneven gaps around the door, or the latch only catches when you push.
Daylight or drafts at the corners of an exterior door.
Water stains or soft flooring inside the sill, a sign the sill was not sealed or panned ³.
Trim pulling away from the wall as the frame moves.

A loose hinge screw is a homeowner fix. Water at the sill or a frame that is out of plumb needs the installer back, since fixing it may mean pulling the unit.

Code and standards for doors: 2021 IRC R311 and R302, IECC R402

Egress door. 2021 IRC R311.2 requires at least one side-hinged egress door per dwelling, with clear width of at least 32 inches measured between the door face and the stop with the door open 90 degrees, and clear height of at least 78 inches.

It must open from inside without a key or special effort. Other doors do not need these dimensions ¹.

Landings. R311.3.1 limits the landing or floor at the required egress door to no more than 1-1/2 inches below the top of the threshold, with an exception of up to 7-3/4 inches on the outside where the door does not swing over the landing ¹.

Garage doors into the house. R302.5.1 bans openings from a garage directly into a sleeping room and requires other garage-to-house doors to be solid wood or steel at least 1-3/8 inches thick, or 20-minute fire-rated, self-latching and self-closing ¹.

Energy. The 2021 IECC treats door U-factor as fenestration in Table R402.1.2, and R402.3.4 exempts one side-hinged opaque door assembly up to 24 square feet ².

Installation practice. ASTM E2112 is the standard practice for installing exterior windows, doors and skylights, covering new and existing construction ⁴.

Adopted editions and amendments vary; your building department decides what applies.

Prehung door vs slab door vs door kit

Option	What you get	Best for
Prehung unit	Slab hung in a new frame with hinges	New openings, damaged frames, exterior doors
Slab door	Door panel only	Sound existing frames, matching old trim
Knock-down frame kit	Frame parts assembled on site with a slab	Unusual wall depths, some commercial work
Fire-rated prehung	Rated door and frame together	Garage-to-house where 20-minute rating chosen ¹

Prehung door myths

Myth A prehung door fixes a crooked wall.: Reality It needs a plumb, in-plane opening; out-of-plumb openings are a top cause of leaks ³.
Myth Any 32-inch door meets the egress rule.: Reality The IRC measures 32 inches of clear width with the door open, which a 32-inch slab does not give ¹.
Myth Caulk under every sill is always right.: Reality One maker says not to caulk the bottom of the sill when a sill pan is used ³.
Myth Any solid door works from the garage.: Reality It must be solid wood or steel at least 1-3/8 inches, or 20-minute rated, and self-closing ¹.
Myth Exterior doors have no energy rules.: Reality Door U-factor is fenestration under the IECC, with one opaque door exempt ².

Climate and local rules for exterior doors

For exterior prehung units, climate zone sets the energy target: under the 2021 IECC, fenestration U-factor limits run from 0.50 in zones 0 and 1 to 0.30 in zones 3 through 8, and door U-factor counts toward that limit unless it is the one exempt opaque door ².

Wind and rain exposure also change sill and flashing details; the maker's instructions and ASTM E2112 practice cover them ⁴. Some coastal jurisdictions require impact-rated doors. Your building department confirms the zone and adopted code.

When to choose a prehung unit

Choose prehung for exterior doors, new openings in an addition or basement finish, frames that are rotted or racked, and any time you want a factory-set seal. Choose a slab when the frame is sound and you want to keep original trim.

Next step: measure the rough opening, wall thickness and handing, then get two quotes that each name prehung or slab, size and sill type. Our contractor guide covers what to ask installers.

Decision guide Prehung or slab decision guide

If The old frame is rotted or out of square: Then Prehung unit
If The frame is sound and you love the trim: Then Slab door fitted to the frame
If It is an exterior entry door: Then Prehung, with a sill pan or sealed sill
If It is the door from the garage into the house: Then Prehung, solid or fire-rated, self-closing

Questions about Prehung Door Unit

What is the difference between a prehung door and a slab door?

A prehung door comes mounted on hinges in its own frame, ready to set into a rough opening.

A slab door is just the door panel, which must be trimmed and hinged to fit an existing frame.

Prehung is easier when the frame is damaged or the opening is new; a slab keeps the old frame and trim.

How do I measure for a prehung door?

Remove the trim and measure the rough opening width at top, middle and bottom and the height at both sides, using the smallest numbers. Measure wall thickness for jamb depth, and note which side the hinges go on and which way the door swings. Then match those to the maker's rough opening chart.

What size rough opening does a prehung door need?

It depends on the maker and door size. Rough openings are usually a little larger than the unit so it can be shimmed plumb. Always use the rough opening size printed in the maker's catalog or instructions for the exact unit you are buying.

Can I install a prehung door myself?

An interior prehung door is a common skilled DIY job if the opening is sound. Exterior doors are harder because the sill, flashing and sealing must keep water out. Widening an opening or cutting a new one is structural and needs a permit and a contractor.

What is door handing?

Handing describes which side the hinges are on and which way the door swings. A common method is to stand on the side the door opens toward you: hinges on the left means left-hand, on the right means right-hand. Makers differ, so confirm with your supplier before ordering.

What does 3/0 x 6/8 mean on a door?

It is shorthand for door size in feet and inches. 3/0 is 3 feet 0 inches wide, or 36 inches; 6/8 is 6 feet 8 inches tall, or 80 inches. A 2/8 x 6/8 door is 32 by 80 inches. These are nominal slab sizes, not rough opening sizes.

How wide must a front door be by code?

The 2021 IRC requires at least one egress door that is side-hinged with a clear width of at least 32 inches and a clear height of at least 78 inches.

Clear width is measured from the open door's face to the stop, so a 36-inch door is the usual choice. Other doors in the house have no minimum.

What door is required between a garage and a house?

Under the 2021 IRC, a door from an attached garage into the house must be solid wood or solid or honeycomb-core steel at least 1-3/8 inches thick, or a 20-minute fire-rated door, and it must be self-latching and self-closing. No door may open from the garage directly into a bedroom.

Should an exterior prehung door have a sill pan?

A sill pan is good practice wherever water can reach the bottom of the door, and some makers and jurisdictions require one in exposed locations. It catches water that gets past the sill and sends it outside. Follow the door maker's directions, which may change how you caulk the sill.

Do I need a permit to replace a door?

A like-for-like door replacement in the same opening often needs no permit, but rules vary. A new opening, a wider opening, or a change to an exterior wall usually does, because it involves a header. Check with your building department before ordering.

Sources

ICC, 2021 International Residential Code, Chapter 3 Building Planning (R302.5.1, R311.2, R311.3.1), retrieved October 13, 2026.
ICC, 2021 International Energy Conservation Code, Chapter 4 [RE] (Table R402.1.2; R402.3.4 opaque door exemption), retrieved October 13, 2026.
Therma-Tru, Pre-Hung Unit Installation instructions (2022), retrieved October 13, 2026.
ASTM International, E2112-19c Standard Practice for Installation of Exterior Windows, Doors and Skylights, retrieved October 13, 2026.
ENERGY STAR, Residential Windows, Doors and Skylights: What makes it ENERGY STAR?, retrieved October 13, 2026.
U.S. EPA, Lead Renovation, Repair and Painting Program, retrieved October 13, 2026.

Expert review

Written by the HyreRemodel Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.

How the numbers were checked: Door size, landing and garage-door rules were read from 2021 IRC R311.2, R311.3.1 and R302.5.1; energy rules from 2021 IECC Table R402.1.2 and R402.3.4, on ICC Digital Codes. Installation steps come from Therma-Tru's pre-hung unit instructions and are one maker's method.

ASTM E2112 scope comes from the ASTM page; ENERGY STAR certification basis from its criteria page; lead rules from EPA RRP. The measuring example shows method, not a product spec. No prices are given.

Suggest a correction. We fix errors and say what changed.