A split jamb pre-hung door, also called an adjustable jamb door, comes with the frame split into two overlapping pieces that slide together to match the finished wall thickness. That design removes the need to build a separate jamb or cut trim to odd widths, which is why these doors dominate new construction and remodels. The installation is straightforward: set the door in the rough opening, plumb and shim the hinge side, fasten through the jamb, and slide the second jamb piece into place. If the opening does not exist yet, the work starts earlier; our step-by-step guide to cutting a brick wall to install a new door opening covers making the rough opening itself.
Tools and Materials
The tool list for a split jamb door is short, and most homeowners already own everything on it:
- Split jamb pre-hung door unit
- 4-foot level and a 6-inch torpedo level
- Shim stock or cedar shims
- Hammer or nail gun with 8d and finish nails
- Drill/driver with 3-inch and 1-1/4-inch screws
- Tape measure, utility knife, and pencil
- Circular saw or handsaw for trimming shims and casing
- Construction adhesive and paintable caulk
The shimming and leveling discipline matters more than the tool brand. The same sequence used in sliding glass door installation, level the sill, plumb the frame, square the head, applies here. Skipping any of the three shows up as a door that binds or a latch that misses.
Order the unit with the wall thickness in hand. Split jamb assemblies come in a range, commonly 4-9/16 to 6-9/16 inches for interior walls, and the two halves slide to cover the difference. Measure the finished wall thickness, including drywall on both sides, before you buy.
Casing style is a separate decision. Split jamb units accept flat casing, colonial casing, or a custom profile, and the casing width should cover the jamb joint plus 1/4 inch on each side. Order the casing with the door so the finish matches the jamb factory finish.
Preparing the Rough Opening
A split jamb door needs a rough opening about 2 inches wider and 2-1/2 inches taller than the door slab. The table below gives the common sizes for standard interior doors.
| Door size | Rough opening width | Rough opening height |
|---|---|---|
| 24 in. x 80 in. | 26 in. | 82-1/2 in. |
| 28 in. x 80 in. | 30 in. | 82-1/2 in. |
| 30 in. x 80 in. | 32 in. | 82-1/2 in. |
| 32 in. x 80 in. | 34 in. | 82-1/2 in. |
| 36 in. x 80 in. | 38 in. | 82-1/2 in. |
Check the opening for plumb, level, and square before you unwrap the door. This Old House’s guide to installing a pre-hung interior door walks through the same pre-check routine: mark the finished floor height, confirm the opening is square within 1/8 inch, and fix bowed studs before the unit goes in.
Confirm the opening width at the top, middle, and bottom. Framing can bow inward at the middle, which is exactly where the door slab needs its clearance. If the narrowest point measures less than the door width plus 1 inch, plane the studs or sister a straight stud alongside before you continue.
Checking the finished floor height
The door sits on the finished floor, so measure from the subfloor to the finished floor height you plan to install. A tile or hardwood floor adds 3/4 inch or more. Set the door on 1/2-inch spacers or temporary blocks so the slab clears the finished floor by the required gap, usually 1/2 to 3/4 inch for interior doors.
Dry Fitting and Shimming the Door
Set the unit in the opening without fastening it. The hinge side goes in first. Place shims behind the hinge jamb at each hinge location and at the bottom, then check the hinge jamb for plumb with the 4-foot level. A door that hangs on a plumb hinge side closes straight even if the rest of the opening is imperfect.
Work in this order:
- Stand the unit in the opening and center it
- Shim the hinge side at the top, middle, and bottom hinges
- Plumb the hinge jamb with the level, tapping shims to adjust
- Tack the hinge jamb with two 8d nails per shim
- Check the head jamb for level across the top
- Re-check the gap between the door slab and the jamb
Two people make the dry fit noticeably easier. One holds the unit in place while the other shims and checks the level. If you work alone, drive two temporary screws through the hinge jamb into the top plate and bottom plate so the unit cannot shift while you adjust.
The dry-fit step is the same idea as test-fitting before committing, the approach we use in our guide to installing a new storm door in an old opening. Old houses rarely have square openings, so the dry fit tells you where the shims need to go before you drive the first nail.
Fastening the Jamb and Setting the Split
With the door plumb and level, drive screws through the hinge jamb and into the framing. Use 3-inch screws at the hinge locations and at the strike plate side. Nails work, but screws pull the jamb tight against the shims without bending, and they hold better in framing that moves with the seasons.
On exterior entry doors, the same fastening sequence appears in weather-sensitive projects like installing an awning over an entry door, where the frame must stay square while the exterior work goes in around it. For the split jamb unit, fasten the hinge side first, then the lock side, then slide the second jamb piece in from the opposite side until it meets the first.
Sliding the split jamb into place
The second jamb half overlaps the first and slides left or right to fit the wall thickness. Start it at the top, align the factory kerf with the first half, and tap it gently with a block to seat it. The joint hides behind the casing, so a slight mismatch does not matter as long as the casing nails reach solid wood.
Hanging the Slab and Adjusting the Gap
Split jamb units usually arrive with the door slab already hinged. If yours ships loose, screw the hinges into the pre-drilled holes on the door, hang the slab on the jamb, and check the reveal: the gap around the slab should be even, about 1/8 inch on the sides and top.
Reveal and strike plate alignment
The reveal is the uniform gap between the slab and the jamb. If it is uneven, loosen the lock-side jamb and re-shim rather than forcing the hinges. The strike plate should catch the latch in the center of its opening; a latch that drags means the jamb is out of plumb.
Adjustment sequence for common problems:
- Door binds at the top: loosen the top hinge screws, add a shim behind the hinge, retighten
- Door binds at the bottom: shim behind the bottom hinge
- Latch misses the strike: move the strike plate or file the opening 1/16 inch
- Uneven reveal: loosen the lock-side jamb and re-shim
Test the door after every adjustment. Close it from a fully open position, then from a partially open position, and listen for a clean latch. A door that closes fine from full swing but drags from a slow close is telling you the jamb is twisted, not the hinges.
Trim, Casing, and Wall Finishing
Once the door operates smoothly, trim the shims flush with a utility knife, then install the casing. Split jamb casing has a factory cut for the jamb joint, so it goes on without the mitered returns that traditional jambs need. Nail through the casing into the jamb and the framing behind it.
Coordinate the door work with the rest of the room’s finishing. If the space includes wall tile, schedule the door first so the casing lands on a clean, flat surface; our technical guide to installing tile murals in shower walls covers the adjacent work that often follows a door install in a bathroom remodel.
Weatherstripping and threshold work belong on exterior units. Interior split jamb doors need only a bead of caulk at the baseboard joint and touch-up paint on the nail heads. A final pass with a vacuum under the threshold keeps the sweep clear.
When the project is a retrofit and the existing door needs hinge work instead, the chisel-based method in our guide to mortising a hinge with a chisel keeps the fit precise without buying new hardware. The same care that makes a split jamb install go fast, measure twice, shim deliberately, fasten square, applies to every hinge you touch.