How to Replace Interior Doors Without the Guesswork

A door that drags across the floor, will not latch, or has seen one too many coats of paint can make an otherwise well-kept room feel unfinished. Learning how to replace interior doors starts with one decision that affects the whole project: are you replacing only the door slab, or the complete door and frame?

A slab replacement can be a practical update when the existing jamb is square, secure, and in good condition. Replacing the full unit with a prehung door is usually the better answer when the frame is damaged, out of square, or part of the reason the old door does not operate correctly. The right choice depends on what is behind the trim, how well the current door fits, and how much finish work you want to take on.

Choose Between a Slab Door and a Prehung Door

A slab door is just the door itself. It does not include hinges, a jamb, or casing. It can save time and cost when you are matching an existing opening, but it requires precise measuring and careful transfer of hinge and lock locations. A small error in hinge placement can leave the door rubbing, sagging, or sitting unevenly in the opening.

A prehung door arrives mounted in its own jamb. This is often the more dependable option for older homes, rental properties, or openings with worn jambs. You remove the old door, frame, and usually the casing, then install a complete new assembly. There is more demolition and trim work involved, but the door, hinges, and frame are designed to work together.

Before ordering either option, inspect the existing door. If it has an even gap around all sides and closes properly, a new slab may be a reasonable fit. If the top gap is wide on one side, the latch misses the strike plate, or the jamb has cracks and loose fasteners, plan on replacing the frame as well.

Measure Before You Buy

Interior doors are often sold in standard widths, including 24, 28, 30, 32, and 36 inches. Do not assume your opening matches the size of the old door. Measure the actual slab width and height, then measure the opening and jamb depth in several places.

For a slab, record the door width, height, thickness, hinge size, number of hinges, and the distance from the top of the door to each hinge. Also measure the backset, which is the distance from the door edge to the center of the knob or lever hole. Most residential hardware uses a 2-3/8-inch or 2-3/4-inch backset.

For a prehung door, measure the rough opening after the old unit is removed if possible. The rough opening needs extra room around the new jamb for shimming and adjustment. Check the wall thickness, too. A jamb that is too narrow or too deep creates unnecessary trim and drywall repair work.

Handing matters. Stand on the side where the door swings toward you. If the hinges are on the right, it is right-hand hinged; if they are on the left, it is left-hand hinged. Confirm the swing direction so the new door does not interfere with a closet, bathroom fixture, light switch, or adjacent door.

Tools and Materials That Make the Job Go Smoothly

The exact list depends on the type of replacement, but most interior door jobs call for a tape measure, level, drill, driver bits, hammer, utility knife, pry bar, shims, finish nails, and wood filler. A reciprocating saw can help remove a prehung unit, while a circular saw or power planer may be needed to trim a slab door.

You will also need the right finish materials. Keep matching hinge screws, knob or lever hardware, caulk, paintable filler, and touch-up paint on hand. If you are installing new casing, buy enough trim for both sides of the opening and allow for miter cuts. Taking time to stage materials before removing the old door prevents a simple project from turning into a half-finished hallway repair.

How to Replace an Interior Door Slab

Start by removing the existing door pins and lifting the slab away from the jamb. Place the old and new doors on sawhorses with their hinge edges lined up. If the new slab is not pre-bored or pre-mortised, use the old door as a template to mark the hinge locations, latch bore, and edge bore.

Transfer measurements carefully. Mark each hinge with a sharp pencil, then cut the shallow hinge mortises so the hinge leaves sit flush with the door edge. A chisel works for careful hand work, while a router and hinge template can produce cleaner, more consistent results. Do not cut deeper than the hinge leaf, or the door may bind against the jamb.

Hang the door temporarily before installing the knob. Check for even reveals, which are the gaps around the door. A typical gap is about 1/8 inch at the sides and top, with enough clearance at the bottom for flooring, rugs, and airflow. If the door is too tall or wide, trim only the amount necessary and seal any freshly cut edge before painting.

Install the latch and hardware after the door swings freely. Close it slowly to check whether the latch enters the strike plate without lifting, pushing, or pulling the door. Minor strike plate adjustments are common, but a major mismatch usually points to a hinge or sizing issue that should be corrected first.

Installing a Prehung Interior Door

Begin by protecting nearby flooring and cutting any paint or caulk where the old casing meets the wall. Remove the casing carefully if you plan to reuse it. Then take out the old door and jamb, watching for hidden fasteners and protecting nearby drywall from excessive prying.

Clean the rough opening and check the subfloor. If the floor is noticeably uneven, the new jamb may need to be trimmed or shimmed at the bottom. Place the prehung unit in the opening and use shims behind the hinge-side jamb first. That side must be plumb because it controls how the door hangs.

Fasten through the jamb near each hinge, checking plumb after every adjustment. Replace one short hinge screw with a long screw that reaches the framing to provide added strength, especially on heavier solid-core doors. Next, shim the latch side until the reveal is even and the door latches without pressure.

Do not overdrive fasteners. Pulling the jamb too tightly against the shims can bow it inward and cause the door to stick. Once the unit operates correctly, trim the shims flush, install casing, fill nail holes, and caulk the casing edges for a high-finish appearance.

Finish Work Is What Makes the Replacement Look Professional

A door can be level and functional yet still look unfinished if the details are rushed. Fill fastener holes, sand smooth, and use a paintable caulk line where trim meets the wall. Prime bare wood, cut edges, and patched areas before applying paint.

For bedrooms, bathrooms, and closets, consider how the door will be used before selecting hardware and construction. Hollow-core doors are affordable and lightweight, but they do little to reduce sound. Solid-core doors cost more and are heavier, yet they offer a quieter, more substantial feel. A bathroom door may need a privacy lock, while a utility closet may work better with a vented or louvered design.

If you are updating several doors, install matching styles and hardware throughout the visible areas of the home. Consistent panel profiles, hinge finishes, and lever shapes can make a hallway or main floor feel more intentional without a full renovation.

When It Makes Sense to Call a Professional

Door replacement becomes more involved when an opening is not square, a wall has water damage, new flooring changes the finished floor height, or the trim requires drywall repair. Exterior walls, load-bearing framing, electrical wiring near the opening, and custom-sized doors also call for experienced help.

ALF Quality Remodeling Inc. handles door installation along with carpentry, drywall repair, painting, flooring, and complete remodeling work, so homeowners do not have to coordinate several trades to finish one opening properly. A professional installation can be especially worthwhile when the goal is a clean fit, durable operation, and trim that blends with the rest of the home.

A new interior door should close quietly, latch easily, and look like it has always belonged there. Measure twice, choose the replacement method that fits the condition of the opening, and do not let a small alignment problem become a recurring daily frustration.