How to Fit an Internal Door: Step-by-Step UK Guide

Learning how to fit an internal door properly is mainly about accuracy. The door needs to sit square in the opening, move freely, close neatly against the stops and work with the chosen hinges, latch and handles. Rushing the measuring or trimming stage can turn a straightforward replacement into a door that catches, swings open by itself or leaves uneven gaps.

This UK step-by-step guide covers the complete process from checking the existing frame to final adjustment and finishing. If you have not chosen the door yet, start with our internal doors range and compare the exact height, width, thickness, finish and permitted trimming allowance for the product you plan to fit.

Before You Start: Check the Door and Frame

Inspect the existing lining or frame before removing material from a new door. Look for loose joints, split timber, damaged stops, excessive twisting and signs that the frame has moved. Use a spirit level on both jambs and the head. Older frames are rarely perfect, but a badly distorted lining can make a correctly sized door difficult to hang.

If the lining is damaged or the rebate does not suit the new leaf thickness, replacing it may give a better result than trying to adapt the door heavily. Browse our door linings if the frame forms part of the replacement project.

Tools You May Need to Fit an Internal Door

• Tape measure and pencil.
• Long spirit level and combination square.
• Sharp chisels.
• Hand plane or suitable power plane.
• Drill/driver and pilot bits.
• Screwdrivers.
• Door wedges or packers.
• Suitable saw where a permitted larger reduction is required.
• Hinge gauge or marking knife if preferred.
• Eye, hearing and hand protection when using power tools.

A full-size timber or fire-rated door can be heavy. Use a second person for lifting and support the leaf securely while trimming or drilling.

Step 1: Measure the Opening Again

Before cutting anything, measure the frame width at the top, middle and bottom and the height at several points. Measure the new leaf as well. If you need a complete measuring process, use our how to measure for an internal door guide.

Do not rely on the phrase standard door size. A nominal 1981 x 762mm leaf may still need minor adjustment for an older opening, and two frames that appear identical can differ by several millimetres.

Step 2: Check the Door Thickness and Orientation

Confirm that the leaf thickness works with the frame rebate, stops and intended hardware. Many domestic internal doors are around 35mm thick, while other products can be thicker. Use the actual product specification rather than assuming thickness from appearance.

Decide which face will be visible from each room, which edge will carry the hinges and where the latch will sit. If the design is directional, lightly mark the top and hinge edge. Some engineered doors also have specific lock blocks or lippings, so read the manufacturer instructions before drilling.

Step 3: Offer the Door Up to the Frame

Place wedges beneath the new door and stand it in the opening. This lets you compare the leaf with the frame and mark where material needs to be removed. Aim for a balanced fit rather than copying every irregularity from an old door.

If the difference is large, stop and confirm that you have the correct size. It is normally better to choose the nearest appropriate leaf than to remove excessive material from a larger door.

Step 4: Mark Any Trimming Carefully

Use a straightedge and pencil to mark the areas that need adjustment. Where only a small amount is required, planing gives good control. For a larger permitted reduction, a saw may be appropriate, but only when the product instructions allow it.

There is no universal safe trimming allowance. The amount varies by construction, lipping, glazing and fire performance. Our internal door trimming guide explains what to check before removing material.

Step 5: Plane or Cut the Door to Size

Remove material gradually and recheck the fit frequently. When planing an edge, work in a direction that reduces the chance of tearing the veneer or timber fibres. If using a power plane, take shallow passes because it is easy to remove too much material quickly.

Pay particular attention to the bottom edge if new carpet, tiles or timber flooring will be installed. The correct clearance should be based on the finished floor level, not an unfinished subfloor.

Step 6: Mark the Hinge Positions

If the old frame and existing hinge positions are sound, they can provide a useful reference. Otherwise, position suitable hinges according to the door weight, dimensions and manufacturer guidance. Heavy doors and fire-rated assemblies can need different hinge specifications from lightweight standard leaves.

Choose suitable hardware from our door hinges range. Hold each hinge square to the edge, mark around it accurately and identify the screw positions before cutting the recess.

Step 7: Chisel the Hinge Recesses

Use a sharp chisel and remove small amounts of material until the hinge leaf sits flush with the door edge. A recess that is too deep can pull the door toward the frame and alter the gaps; a hinge left proud can push the leaf away and make it bind on the latch side.

Drill pilot holes for the hinge screws, particularly near the edge of timber or engineered doors. Pilot holes help keep the screws centred and reduce the chance of splitting.

Step 8: Fix the Hinges to the Door

Fit the hinges firmly without overtightening the screws. Check that the hinge leaves sit flat and square. If an unfinished door has been trimmed or chiselled, follow the manufacturer's instructions for sealing exposed timber before final installation.

For a fire door, use only hardware that is suitable for the tested assembly. Do not treat a fire-rated leaf as a standard door with heavier hinges added afterwards.

Step 9: Hang the Door

Support the leaf on wedges at the correct height and line the hinges up with the frame. Fit one or two screws initially, then test the movement before installing every fixing. This gives you a chance to correct alignment without repeatedly removing screws.

Open the door slowly and watch the gaps. If it catches near the top or bottom, determine whether the cause is hinge position, a frame that is out of square or the leaf size before removing more material.

Step 10: Check Gaps and Movement

The door should open freely and close without needing to be forced. The perimeter gaps should look reasonably consistent for a normal domestic door, but do not apply a generic gap rule to specialist or fire-rated doors. Those installations must follow the relevant certification and manufacturer instructions.

If the door swings open or closed by itself, check whether the frame and hinge line are plumb. Planing the leaf will not fix a frame that is leaning.

Step 11: Fit the Latch

Choose a handle height that suits the project or matches other doors in the property. Mark the latch centre on the edge and check that the chosen position is suitable for the construction of the door. Engineered doors can have designated lock areas.

Drill the latch hole carefully, recess the faceplate until it sits flush and secure the latch. Do not drill into a decorative face without checking the required backset and handle position first.

Step 12: Fit the Door Handles

Browse our door handles and choose a set compatible with the latch and leaf. Mark both faces accurately and drill cleanly. Where practical, drilling from each side toward the middle can reduce breakout on a finished surface.

Fit the spindle and handles, tighten the fixings evenly and test the return action. The latch should retract fully and spring back without the handle binding.

Step 13: Fit the Strike Plate

Close the door gently and mark where the latch meets the frame. Recess the strike plate so it sits flush and test the closure. The latch should engage without needing to lift, push or slam the door.

If the latch misses after the door has been hung, check for hinge movement or sag before enlarging the strike opening.

Step 14: Finish the Door Correctly

Unfinished and primed doors may require a complete coating system, while pre-finished doors may need only permitted cut edges to be sealed. Follow the exact manufacturer's guidance for faces, sides, top and bottom. Do not assume primed means no further finishing is required.

Keep newly fitted timber-based doors away from wet plaster, excessive moisture and harsh site conditions. Store and finish them in line with the product instructions.

Fitting an Internal Fire Door

Fire doors need a complete-system approach. The leaf, frame or lining, suitable hinges, intumescent door seals, closer and other components can all form part of the required tested assembly. Permitted trimming and installation gaps can also be critical.

Use our internal fire door guide for buying checks and follow the relevant certification and manufacturer instructions. If you are not competent to install a fire-rated assembly, use a suitably experienced installer.

Fitting Several Matching Internal Doors

For a whole-house renovation, create a simple door schedule before installation starts. Record each room, leaf size, finish, hinge requirement, handle set, latch, glazing and any fire requirement. This reduces the risk of putting the wrong size or hardware into the wrong opening.

Fit and check one representative door before trimming every leaf in the batch. Frames can vary even when the specified door size is the same.

Common Internal Door Fitting Problems

The Door Rubs at the Top

Check the top hinge, frame square and head level before planing the top edge. A loose hinge can make the lock corner drop.

The Door Catches on the Floor

Confirm the finished floor height and permitted bottom trimming. If the floor was changed after installation, the clearance may need careful adjustment.

The Door Springs Open

This often points to a hinge line or frame that is out of plumb. Check alignment before altering the latch or stops.

The Latch Does Not Line Up

Look for sagging hinges, loose screws or leaf movement. Move the strike plate only after the door geometry is correct.

Internal Door Fitting Checklist

• Correct leaf size and thickness confirmed.
• Frame checked for damage and square.
• Trimming allowance read before cutting.
• Finished floor level considered.
• Hinges matched to the door.
• Door opens freely before latch fitting.
• Handles and latch aligned.
• Strike plate engages without force.
• Cut edges finished or sealed as required.
• Fire-door components checked where relevant.

Frequently Asked Questions

Can I fit an internal door into an old frame?

Yes, if the frame is structurally sound and suitable for the new leaf. Measure carefully and check for twisting, damaged rebates and incompatible thickness.

How long does it take to fit an internal door?

The time varies with the condition of the frame, amount of trimming and hardware work required. An experienced joiner can complete a straightforward replacement far faster than a first-time DIY installation.

Do I need three hinges on an internal door?

The correct number and grade depend on the door's weight, size, construction and any fire-rating requirements. Follow the door and hinge manufacturers' guidance.

Can I reuse old hinges and handles?

Only if they are in good condition and suitable for the new door. Fire-rated or heavier leaves can require different hardware.

Should I paint the door before hanging it?

Follow the manufacturer's finishing instructions. Many projects benefit from completing most finishing before final hanging, while still treating any site-cut edges as required.

What if my frame is not square?

Minor variation can often be managed during fitting, but a badly distorted lining may be better repaired or replaced rather than heavily trimming the new door.

Where can I buy the door and hardware together?

Browse our internal doors for sale, then match the chosen leaf with suitable door handles, hinges and linings from JJ Building Materials.

Shop Internal Doors for Your Project

Successful fitting starts with the correct product. Compare sizes, finishes and designs in our internal door collection, and use our internal doors buying guide if you are still choosing between oak, white, glazed and specialist options.