Start by closing the door on the rebate of the casing to measure the distance marking out the inside

It’s very important to measure the doorframe in a number of places across its width and height.
Make a note of any discrepancies in the measurements as they will help when you’re fitting the door.

Internal doors are normally divided into four categories panel, moulded, flush or fireproof doors and they are normally thinner than exterior doors as they are built for interior use.

Panel doors
There are different panel designs available ranging from 6 wooden panel doors or glazed panel doors.
Finishes are usually in pine, interior hardwood or primed white.
They consist of two vertical stiles and horizontal rails enclosing the panels.
The panels could be made of plywood, solid timber or glass.
There are gennerally two kinds of glass panel doors that you can buy. The cheaper one has a sheet of glass covered by the horizontal and vertical rails. This means the paneled door must be fitted when it contains the glass.
The more expensive type of glass panel door is fitted to the frame first and then the glass panels are added individually, this makes the door lighter to handle when hanging and cheaper to replace if the glass gets broken, personally I think they look better too.

Depending on the location and usage of the door, putty and beading may be used, but if the door has a wood finish, beading alone will suffice. Silicone sealants are also available in a variety of finishes.

Moulded doors
Moulded doors are shaped from a sheet of wood, plastic or fibreboard and then bonded to a timber frame, they are every popular at the moment for both new and refurbish jobs, they are often classic in design with moulded panels and you gen even buy them ready finished for a small extra charge.
NOTE: most of these type of doors have a thicker frame core on one side for the door handle and latch or as it is more properly called, door furniture.

Flush doors
Flush doors are lighter and cheaper than other types, consisting of a narrow timber framework completely covered with either hardboard or plywood on both sides. Inside the frame it can be solid chipboard, semi-solid filled with strips of timber or a cellular core of paper honeycomb. They often have narrow lippings, although some are reinforced in positions where hinges or locks can be fitted.

If the doorway is a particularly awkward size and there is a large section to cut off, a flush door may prove unsuitable, as there is only a limited amount that can be cut off the timber framework without leaving the door structure weak. This could lead to the door warping or twisting. You should not remove more than 4mm from each side of a flush door and 9mm from the top and bottom.

Fire resistant doors
A fire resistant door is thicker than other types. They contain heavy grade chipboard or a heavy core of fire resisting compressed straw. They are there for a purpose, giving between 30 – 60 minutes of fire resistance. They are usually flush robust doors that should always be fitted into a hardwood one-piece rebated frame and fitted with an automatic door closer. If a glass panel is preferred, Georgian wired glass must be used. Building regulations require fire resistant doors when a door leads into an adjoining garage and when a door opens into the loft room.

Existing Hinges
If you intend to use the existing hinge recesses in the frame, the positions need to be marked on the door.
Position and support the door in the opening at an angle to the upright and mark on it where the tops and bottoms
of the hinge recesses are.

New Hinges
The positions need to be marked on both the frame and the door.
Interior doors should be hinged about 125-150mm or 5 to 6 inches from the top and 175-230mm or 7 to 9 inches from the bottom of the door.
Use either brass or steel butt hinges either 75mm (3 inches) or 90mm (3½ inches).

Chisel out the recess carefully and fit the hinges to the door. Make pilot holes first for the screws to avoid splitting the wood. Make sure they fit flush and lie neatly in position.

You will need to support the door in position in the doorway and fix the hinges to the frame, use one screw in each hinge. Before completing the job, check that the door opens and closes correctly. If there are problems, remove the two screws and correct by making minor adjustments where necessary to where the hinge sits in the frame.

Repeat the procedure, test open and closing and secure the hinges completely. Decide on the lock or latch most suitable for your door, perhaps with locking in mind or a match to the other internal doors.

Position the new door against the frame to determine the fit. Use a pencil to mark any overlap if the door is larger and requires cutting. To retain the pattern in the door, remember to cut from the top and bottom and from each side when cutting. This is particularly important on a panel door to keep the panels central.

Use a plane to smooth the edges, working from the ends towards the middle to avoid any splintering at the edges. If a larger section needs cutting off, use a circular or panel saw to make the cut. The door should fit the frame with a gap of approximately 2mm all round the edge.