Interior doors take a beating. Over years of use, hinges loosen, gaps appear at the bottom, and locks stop catching properly. A door that drags on the frame or swings open on its own signals that replacement is overdue. If your toddler can push open a locked door because the gap is too wide, or the bathroom door leaves a privacy-compromising crack, it is time to act. This article covers tools, measuring, hinge mortising, lock installation, and finishing steps. If you have experienced doors that will not stay put, diagnosing the root cause is the first step toward a permanent fix.
Choosing the Right Replacement Door
The first decision is prehung versus slab. A prehung unit arrives with hinges already mounted in the jamb and predrilled lock bores, making installation straightforward if you replace the frame too. A slab door is just the panel itself, requiring you to cut hinge mortises, drill for the lockset, and fit the door to the existing jamb. For most interior replacements where the jamb is in good shape, a slab is the economical choice and gives you full control over fit.
Door construction affects cost and performance. Solid wood doors are heavy, durable, and accept stain well. Hollow-core doors are lightweight and inexpensive but offer minimal sound blocking. Solid-core doors (usually MDF or particleboard with a wood veneer) sit in the middle. To make an informed choice, review the available interior doors construction types and how they compare for your specific rooms.
| Door Type | Weight | Sound Damping | Cost | Best For |
|---|---|---|---|---|
| Hollow-core | Light (15-25 lb) | Low | $ | Closets, utility rooms |
| Solid-core (MDF) | Medium (30-50 lb) | Moderate | $$ | Bedrooms, hallways |
| Solid wood | Heavy (40-70 lb) | High | $$$ | Bathrooms, home offices |
Match the slab thickness (typically 1-3/8 inches for interior doors) to your existing door. Thicker doors require longer hinge screws and different latch hardware. Check the swing direction before buying. Stand in the doorway facing into the room; if the hinges are on your left, you need a left-hand door. A simple mistake here means returning the slab or cutting new hinge mortises on the wrong edge.
Taking Accurate Measurements
Measuring is the step where mistakes cost the most. Remove the old door and set it on a flat surface. Measure the width at the top, middle, and bottom. Do the same for the height down the hinge side and the latch side. Old doors can be out of square by a quarter inch or more, so record the smallest width and height as your baseline. Plan to trim the new door rather than assume the rough opening is square.
Accounting for Crooked Jambs
If the jamb is out of plumb, the new slab inherits the problem unless you shim the hinges or plane the edge. Measure the reveal between the top of the door and the jamb on both sides. An uneven reveal means the jamb is racked. One fix is to remove the casing, shim the hinge-side jamb, then reinstall. Another is to scribe the door to match the frame, trimming the latch edge to follow the jamb line. Transfer the angle using a compass or a scrap piece of wood.
Setting the Bottom Gap
Bottom clearance is often the most debated measurement. Standard practice calls for a 1/2-inch gap under a bedroom door and up to 3/4 inch under a bathroom door for air circulation and flooring clearance. If your home has a central HVAC return in the hallway, the bottom gap is part of the return air path. A gap as narrow as 1/4 inch may restrict airflow. A gap of 1-1/2 inches (common when carpet was removed but the door was not trimmed) looks sloppy and defeats privacy. Measure from the top of the finished floor to the bottom of the existing door, then subtract the desired gap to find your trim target.
Essential Tools and Setup
Replacing a slab door does not require a workshop full of specialty gear, but the right tools make the difference. You need a circular saw or hand plane for trimming, a sharp chisel for hinge mortises, a drill with a 2-1/8-inch hole saw and a 7/8-inch spade bit for the lockset, a combination square, a tape measure, a utility knife, and a hammer. Clamps and a straightedge guide help produce straight cuts.
A door lock installation kit combines the hole saw and spade bit with a guide plate that centers the bit on the door edge. These kits are inexpensive and prevent the bit from wandering. For sanding and finishing, you need 80- through 220-grit sandpaper, primer, and paint or stain. The pro tips for painting interior doors cover the sequence that avoids drips and brush marks. If you plan to paint before hanging, set up sawhorses in a dust-free area and apply thin, even coats.
Door supports (door jacks or simple wood wedges) let a single person hold the slab in position while transferring hinge locations. For cleaner hinge mortises, a router with a template guide and a flush-trim bit removes most of the material before you clean up with a chisel. Professional painting techniques also apply to the door edges and panel recesses, which are easy to miss with a brush.
- Circular saw or pull saw for trimming door height and width
- Block plane or belt sander for fine edge adjustments
- Door lock installation kit with hole saw and spade bit
- Sharp 1-inch and 1-1/2-inch chisels for hinge mortises
- Router with hinge template (optional, speeds up mortising)
- Combination square, tape measure, and pencil
- Door jack or shims for solo installation
- Sanding equipment and finishing supplies
Hinge Mortising: Router Template vs. Hand Chisel
Hinge mortises are the shallow recesses cut into the door edge that let the hinge sit flush. A mortise that is too deep forces the hinge to bend; one that is too shallow prevents the door from closing. The goal is a snug fit where the hinge leaf sits exactly level with the surrounding wood.
Using a Router Template
A hinge router template clamps to the door edge and guides a router to cut all three mortises in about two minutes. This is the fastest method and produces consistent depth. The template positions the mortise at the correct distance from the top and bottom. Set the router depth to match the hinge leaf thickness (typically 1/8 inch). After routing, square up the rounded corners with a chisel. The template is worth the investment if you are replacing multiple doors.
Hand Chisel Method
For a single door, a sharp chisel and hammer are adequate. Mark the hinge by holding it in position and tracing around it with a utility knife. Score the outline, then make parallel cuts 1/8 inch apart across the mortise area. Tap the chisel bevel-down, levering out waste chips. Pare the bottom flat with the chisel bevel-up, checking depth with a straightedge. The hinge should press in without force. If it sits too high, deepen the mortise in even passes.
Regardless of the method, install hinges onto the new door with screws, then hold the door in the opening and transfer hinge locations to the jamb using a knife. Mortise the jamb locations the same way. Use 2-1/2-inch or 3-inch screws on at least one hole per hinge to reach the wall stud behind the jamb. This prevents the door from sagging over time.
Drilling for Locks and Latches
Modern interior locksets have two holes: a 2-1/8-inch diameter bore through the door face for the knob or lever, and a 7/8-inch diameter hole through the door edge for the latch bolt. The center of the bore is typically 2-3/8 inches or 2-3/4 inches from the edge, depending on the backset. Measure the backset of your existing hardware or check the lock specifications before drilling.
Mark the center height (usually 36 inches from the bottom) on the edge and on the face. Use a combination square to extend the line across the face. Drill the edge hole first with a stop collar on the spade bit to avoid drilling through. Then drill the face bore using the hole saw. A door lock installation kit includes a guide plate that keeps the hole saw perpendicular. Drill from one side until the pilot bit pokes through, then finish from the opposite side to avoid splintering the veneer.
Insert the latch mechanism and ensure it slides freely. Install the strike plate on the jamb by closing the door and marking where the latch hits. Mortise the strike plate flush with the jamb surface using the same chisel technique. If the latch does not engage smoothly, file the strike plate opening slightly or deepen the mortise. For those considering more elaborate projects, the process of crafting laminated custom interior doors follows similar mortising and hardware principles on a larger scale.
- Mark the cross-bore center on the door face at the correct height.
- Mark the edge-bore center on the door edge, aligned with the face mark.
- Drill the edge bore first using a 7/8-inch spade bit with depth stop.
- Drill the face bore using a 2-1/8-inch hole saw from both sides.
- Install the latch and test the fit before mounting the door.
- Transfer the strike plate location and mortise the jamb.
Finishing and Adjusting the Fit
Finish the door before hanging. Painting or staining on sawhorses produces a smoother result than working around hinges. Sand with 120-grit, then 180-grit, wiping dust between grits. Apply primer to bare wood or MDF edges, especially the bottom. Two coats of paint or stain followed by a clear topcoat protect the wood. The methods covered in repainting interior trim apply equally to doors: use a quality brush for panel recesses and a small foam roller for flat sections to eliminate brush lines.
Once the finish is dry, hang the door and check the gaps. The reveal around the top and sides should be a consistent 1/8 inch. If the door binds at the top, remove it and plane the hinge-side edge. If the latch side hits before closing, sand or plane that edge. For minor sticking, rub a bar of soap along the contact edge as a temporary lubricant. Tighten all hinge screws and confirm the latch operates smoothly.
Final adjustments are small but critical. A door that rubs against the jamb at the top corner on the latch side indicates the hinge-side edge was planed unevenly. Check the hinge mortise depths; if one hinge is set deeper than the others, the door will twist. Shim the shallow mortise with cardboard or veneer. The same principles of measurement and adjustment apply to other carpentry projects, including replacing siding on your home, where fitting and gap consistency determine the quality of the finished job. Interior door replacement is a weekend project that rewards precision at every step.