How to Install Interior Doors Step by Step
updated 11 July 2026
Quick answer
To install an interior door, measure the opening and choose a frame that matches the wall thickness and the opening direction. Set the frame perfectly plumb and level, wedge it in place, brace it with spreaders, and fill the gap with low-expansion foam. Once the foam cures, hang the door, adjust the hinges, and fit the handle and strike plate so the door closes lightly and without play.
Step by step
- 1
Measure the opening and choose the frame
Measure the width and height of the rough opening in several spots, plus the wall thickness. An adjustable frame fits a range of wall thicknesses, for example 75-95 or 95-115 mm, and wraps the wall with trim on both sides, which makes it convenient for finished walls. A fixed frame is stiffer and cheaper but needs cover strips. Also check the opening direction, meaning whether you need a left-hand or right-hand door.
- 2
Assemble the frame and place it in the opening
Screw the frame parts together flat on the floor following the manufacturer's instructions, paying attention to the hinge side. Place the assembled frame in the opening and support it initially with wedges from below. Don't fix anything permanently at this stage, because the frame needs to move freely for adjustment. Check that the door fits in the opening with some room to spare for adjustments.
- 3
Set plumb, level, and diagonals
This is the most important moment of the whole job. Use a spirit level to set the frame jambs perfectly plumb and the head perfectly level. Wedge the frame at the hinges, at the lock, and at the top. Measure the diagonals - they should be equal, which means the frame isn't twisted. Only move on once the geometry checks out.
- 4
Brace the frame with spreaders
Insert spreaders, meaning bracing bars, into the frame opening - at least three: at the top, in the middle, and at the bottom. The spreaders keep the jambs the right distance apart once the foam starts to expand, and they're what protects the frame from bowing inward. Check plumb, level, and diagonals once more, because bracing can subtly shift the alignment.
- 5
Fill the gap with low-expansion foam
Use only low-expansion foam intended for door installation, because regular high-expansion foam would bow the jambs. Apply it in strips or spots and don't fill the gap all the way. Leave the foam to cure for the time the manufacturer specifies, usually a few hours, and only then remove the wedges and spreaders and trim off the excess.
- 6
Hang the door and adjust the hinges
Set or screw the door onto the hinges. Check the gaps around the door - they should be even, about 3-4 mm on each side. Modern adjustable hinges let you shift the door vertically and horizontally and change the compression. Adjust in small increments until the door stops swinging open or shut on its own and sits evenly all around.
- 7
Fit the handle, lock, and strike plate
Push the handle spindle through the hole in the lock, fit the handles or rosettes on both sides, and screw them together. Check that the latch extends freely. On the frame, mount the strike plate opposite the latch and adjust its position so the door closes firmly, without rattling and without resistance.
The ventilation gap under the door - what to remember
Interior doors shouldn't sit tight against the floor, because that blocks airflow between rooms. The standard is to leave a gap of a few millimeters under the door. Rooms with ventilation, above all the bathroom, kitchen, and boiler room, need a bigger cross-section, so the door gets a ventilation undercut, usually about 2 cm (3/4 in) above the floor, or ventilation sleeves or grilles set into its lower part.
Without it, the bathroom exhaust grille has nowhere to draw air from, steam lingers, and damp and mold show up on the walls. When planning the door height, factor in the final thickness of the flooring, threshold, or carpet so the gap comes out just right.
How to fit hinges on interior doors and avoid mistakes
In ready-made sets, the hinges usually come factory-seated in the frame and door, so all you do is hang the door and adjust. If you're fitting hinges yourself, their recesses must sit flush with the surface, because a hinge standing proud of the wood pushes the gap apart and the door won't close. Transfer the hinge spacing from the door to the frame precisely, ideally by holding the door up and tracing the positions.
The most common installation mistakes are too much foam, which bows the jambs inward, and skipping the spreaders while the foam expands. Add to that installing without checking the diagonals and pulling the wedges before the foam hardens. The result is always similar: the door rubs, won't close, or swings open on its own. Calm, precise work at the frame-setting stage saves hours of fixes later.
Frequently asked questions
›How do I install a door if the opening is too big?
A small excess, up to a few centimeters per side, can be filled with foam and wedges, as long as you use spreaders. A larger opening needs to be reduced first: brick it in, or add profiles or structural blocks you can screw the frame to. Don't count on foam alone holding the frame in an oversized opening - it's not a load-bearing material.
›How do I tell whether I need a left-hand or right-hand door?
The direction is determined from the side the door opens toward. The simplest way: stand on the side the door swings toward - if the hinges are on your left, it's a left-hand door; if on your right, a right-hand one. Labeling varies between manufacturers, so confirm the direction against the store's description to be sure.
›What foam should I use for installing interior doors?
Always low-expansion foam, sold as installation foam for windows and doors. Regular foam expands strongly and can bow the frame jambs, and then the door stops closing. Even with low-expansion foam, don't fill the gap completely and keep the spreaders in while you work.
›How do I fit the hinges so the door doesn't sag?
The hinges must sit in one line and be recessed flush with the surface, with the screws driven firmly into solid material. If the door sags or rubs, check the frame's plumb first, then adjust the hinges. For heavy doors, a third hinge in the middle helps carry the load.
›How soon after installation can I use the door?
Wait until the installation foam has fully cured. The curing time is set by the manufacturer, usually from a few hours up to a dozen or more. Only then remove the spreaders and wedges and trim off the excess foam. Loading the frame too early, before the foam hardens, can throw the whole alignment off.