Buying a new door
When selecting a new door, you will need to make a careful note of the overall size of the opening into which it is to fit. As you will no doubt find, door openings are rarely square or perfectly upright. The new door will need to be trimmed to suit.
Doors come in standard sizes, so you will need to choose one which suits, making allowance for trimming. Most interior doors can be trimmed to fit. Be sure to check the thickness of the door as this will also need to suit your frame.
Preparing the new door
Many new doors come with the stiles untrimmed. These ‘horns’ as they are called should be cut off square. Lay the door on your workbench and use a carpenter's try square to mark the cut line. Hold the handle section against the edge of the door and line the blade up with the bottom/top of the door. Mark the line and trim with a reasonably fine-toothed saw. You should hold the saw so that it cuts cleanly along the line, but also pay attention to the angle of the blade in relation to the face of the door so that the bottom/top edge is also square.
Measure the width of the opening at the top and bottom but deduct 4mm, to allow 2mm clearance at both sides. . Transfer these measurements to the door. If the door is only a little wider than the opening, measure from one edge and mark the cut line. If, however, it is a good deal larger, measure and mark the cut lines so that an equal amount will be trimmed from both sides. This is essential when fitting panelled or framed doors to ensure they appear symmetrical.
Now, measure the height and transfer the measurements in a similar fashion, allowing 2mm for clearance at the top and 6mm clearance for the floor. The latter may well need to be more if the floor is carpeted or uneven. If in doubt, take off the minimum. It’s always possible to take off more, but a little difficult to add more on!
Use a vice on your workbench to steady the door. Pack both sides with cardboard to prevent damage to the door. Trim to the marked lines using a sharp plane. The trick is to hold the plane firmly in contact with the surface, square with the edge, and run it along smoothly. Work your way gradually down to the trim line over the entire length. Avoid trimming a short section at a time as this will lead to an undulating surface.
When trimming the top and bottom, you must only work from the outer edge, in. If you plane towards the outer edge, the end of the stiles will split.
Where more than 6mm or so needs to be trimmed from the door, it makes sense to use a saw to cut the majority of the excess before finally planing down to the trim line.
Offer the door up to the opening and check the fit. If necessary, mark any additional trimming and plane as before. To help with this process, use a couple of small cheese shaped wooden wedges to hold the door off the floor by the appropriate amount. At this stage, any minor adjustments can be made from one edge rather than both without noticeably upsetting the symmetry.