Install interior pre-hung doors before new finished flooring in most cases, but calculate the jamb height from the final floor elevation. Leave enough clearance for the door slab to swing freely over the completed floor, and install or complete the casing trim after the flooring is installed. If the floor height is unknown, do not permanently fasten the door until the floor assembly, underlayment, and transition details are confirmed.
Recommended installation sequence
For a typical remodel, the practical order is to prepare the opening, confirm the finished floor height, set the pre-hung door, install the flooring, and then install or finish the casing. The door frame is established before the floor reaches the opening, while the trim is delayed so it does not interfere with flooring installation.
- Confirm the finished floor assembly, including underlayment, adhesive, tile, wood, laminate, or other finish materials.
- Determine the required gap beneath the door slab from the flooring manufacturer, door manufacturer, and local practice.
- Set the pre-hung jamb so the door will have that clearance above the completed floor.
- Plumb, square, and temporarily secure the frame without trapping flooring that still needs to expand or move.
- Install the flooring and any required underlayment or perimeter spacing.
- Test the door swing, trim the slab only if appropriate, and install the casing after the floor is complete.
This sequence is not a universal code requirement. It is a common remodeling practice that must be adjusted for the floor product, door manufacturer’s instructions, moisture movement, and the conditions at the opening.
Why the door usually goes in first
Installing the pre-hung unit before the floor gives the installer control over the frame’s height and alignment. The jamb can be positioned so the door slab clears the final floor without forcing the frame too high or leaving an excessive gap at the bottom.
It also allows the flooring to run cleanly to the jamb. With the door frame already in place, flooring cuts can be made around the jamb legs rather than leaving a large unfinished opening that must be concealed later.
There are important exceptions. If the existing floor is staying, the door may be set directly in relation to that surface. If the new floor is unusually thick, such as tile over a substantial underlayment, the frame height may need to be planned before the door is delivered. A door opening that was suitable for carpet may require different preparation for wood, luxury vinyl, or tile.
Calculating jamb height and door undercut
The key measurement is the finished floor elevation at the door opening. Add every layer that will remain under the door, including underlayment, leveling material, mortar or adhesive, and the finished flooring itself. Then provide the bottom clearance required for the door to operate over that assembly.
A useful planning formula is:
Required rough door position = existing substrate elevation + floor assembly thickness + planned door clearance
The door slab and jamb are related but are not always adjusted in the same way. The jamb legs establish the frame position, while the bottom of the slab may be undercut later if the final clearance is slightly different from the initial estimate. Do not assume that cutting the slab is always acceptable. Hollow-core, solid-core, fire-rated, acoustical, and specialty doors can have limits on how much material may be removed.
| Condition | Planning concern | What to verify |
|---|---|---|
| Carpet or thin resilient flooring | The final surface may be relatively close to the subfloor, but padding or underlayment can change the height. | Flooring assembly thickness and door manufacturer limits. |
| Luxury vinyl or laminate | Plank thickness, pad, and floating-floor movement affect the opening detail. | Required perimeter gap and whether the floor may run beneath the jamb. |
| Engineered or solid wood | The floor may be nailed, glued, or floated, and expansion space is usually part of the installation design. | Manufacturer’s installation instructions and transition details. |
| Ceramic or stone tile | Tile, mortar or adhesive, and underlayment can raise the finished floor substantially. | Total buildup, doorway transition, and whether the jamb should sit over or beside the tile. |
There is no single undercut size that works for every room. A bathroom, laundry room, bedroom, and mechanical room may have different air-transfer, moisture, threshold, or accessibility considerations. Where a door is part of a rated assembly or serves a special ventilation function, verify the required detail with the authority having jurisdiction and the relevant product documentation.
Setting the pre-hung jamb for the finished floor
Before fastening the unit, mark the expected finished floor line on both sides of the opening. Use a sample or documented thickness of the actual floor assembly rather than relying on a nominal product name. For example, a flooring product described as 8 millimeters may still be installed with a separate pad, adhesive, or underlayment that changes the completed height.
- Measure from the structural or subfloor surface to the planned top of the finished floor.
- Transfer that elevation to both jamb legs and the door side of the opening.
- Position the jamb so the door slab will clear the completed floor after the frame is plumb and level.
- Check the hinge-side reveal, latch-side reveal, and head jamb before permanent fastening.
- Secure the frame with shims at the hinge and strike areas, keeping the opening square.
Do not use the jamb as a substitute for a missing or poorly prepared subfloor. Large variations in the substrate should be corrected before the door is installed. A frame that is twisted to follow an uneven floor can create latch problems, uneven reveals, or a door that swings open or closed on its own.
How to size the door undercut
The undercut is the space between the bottom of the door slab and the completed floor. Measure the clearance after accounting for the floor surface, not just the subfloor. The correct amount depends on the door, floor, room function, and any applicable manufacturer or project requirements.
Use this process:
- Confirm the finished floor height at the hinge side and latch side of the opening.
- Check whether the floor is level. A sloping floor may require the clearance to be checked at several points.
- Review the door manufacturer’s permitted trimming range and any label or assembly restrictions.
- Mark the slab with the door supported securely and use a cutting method appropriate for the door construction.
- Seal or finish an exposed cut edge when the door manufacturer calls for it, especially on wood-based products.
- Rehang the slab and check the swing through its full range before installing final trim.
A floating floor needs room to move at its perimeter and should not be wedged tightly against the jamb or casing. Follow the flooring manufacturer’s instructions for the perimeter gap and doorway treatment. In some installations, the flooring is slid beneath a previously installed jamb; in others, the jamb is undercut so the flooring can pass beneath it. The correct approach depends on the product and the installation method.
Installing flooring around the jamb
After the pre-hung unit is stable, install the finished floor toward the doorway. The cleanest appearance usually comes from undercutting the jamb or casing enough for the flooring to slide beneath it, rather than making a precise cut that stops visibly against the trim.
Protect the door and jamb from adhesive, grout, finish, and tools. Do not use the door slab as a brace while forcing flooring into position. Leave any required movement space at the edges, then cover it with approved trim or a transition detail rather than filling it with a rigid material that could restrict a floating floor.
At a transition between different floor materials, check the elevation before the door is permanently adjusted. A reducer, saddle, or other transition can change the clearance and may affect whether the door can swing in both directions. A transition should not create a trip hazard or prevent the door from closing as intended.
When to install the casing trim
Install final casing after the finished flooring is in place. This lets the casing end at a consistent height above the floor and allows the installer to hide the required flooring gap without guessing where the final surface will land.
For a clean casing sequence:
- Complete the floor installation and remove temporary spacers only as directed by the flooring system.
- Check that the door operates and that the frame remains plumb.
- Mark the casing legs for the desired reveal at the jamb.
- Cut the legs to follow the floor or transition while maintaining a consistent visual relationship.
- Install the head casing after the legs are positioned, then fill and finish nail holes as appropriate.
If casing must be installed before the floor for scheduling reasons, leave enough room for the actual flooring and protect the trim. That approach is more prone to gaps, damaged trim, or a casing leg that sits too low after the floor is installed.
Common problems and practical fixes
| Problem | Likely cause | Better response |
|---|---|---|
| Door rubs after flooring installation | The final floor is higher than planned or the slab was not undercut for the completed assembly. | Confirm the floor elevation, check manufacturer trimming limits, and adjust the slab only by an approved method. |
| Large gap beneath the door | The jamb was raised for an estimated floor thickness that was not installed. | Measure the actual clearance and consult the door manufacturer before changing the slab or jamb. |
| Casing has an uneven gap at the floor | Trim was installed before the final floor or the substrate is out of level. | Remove and recut the casing, or use a compatible transition detail where appropriate. |
| Floating floor buckles near the doorway | The floor was trapped against the jamb, casing, or transition. | Restore the required movement space according to the flooring manufacturer’s instructions. |
| Door frame is twisted | Shims or fasteners were placed without checking the opening for plumb and square. | Reset the frame before casing hides the problem. |
Pre-installation checklist
- Confirm the exact floor product and all layers beneath it.
- Measure the floor buildup at the doorway, not only in the center of the room.
- Check the door manufacturer’s trimming limits and any special assembly restrictions.
- Determine whether the floor is floating, glued, nailed, or bonded to the substrate.
- Plan the doorway transition before setting the jamb.
- Keep the frame plumb, square, and temporarily protected during flooring work.
- Test the door after flooring installation and before final casing.
- Verify special requirements with the local building department when the door is rated, accessible, part of a required separation, or otherwise regulated.
Frequently asked questions
Can I install the floor before the pre-hung door?
Yes, especially when the floor is already finished or the project sequence requires it. Protect the surface, account for the completed floor elevation, and do not rely on a standard jamb length without checking the opening. The frame must remain stable without damaging or restricting the floor.
How much should I undercut an interior door?
There is no universal number. Base the undercut on the final floor assembly, the door manufacturer’s permitted trimming range, and the clearance needed for the room and door type. Check the clearance across the full swing, particularly where the floor is not level.
Should flooring go under the door jamb?
Often, flooring is detailed to run beneath an undercut jamb or terminate at an approved transition. The correct method depends on the flooring product, its movement requirements, and the manufacturer’s instructions. Do not trap a floating floor tightly against the frame.
Should casing be installed before or after flooring?
Install casing after the finished flooring in most remodels. The floor then determines the final casing cut and height, which reduces visible gaps and makes it easier to protect the trim during flooring work.
What if I do not know the final floor height yet?
Delay permanent door adjustment until the floor assembly is confirmed. You can prepare the opening and temporarily position the unit, but setting the jamb from an assumed thickness may cause rubbing, an excessive undercut, or a transition problem.