Always by choosing the correct size of door for the opening so that the minimum amount is removed to make it fit, as taking excessive amounts off any door is never a good idea as can present problems with every type of door. However, situations can arise where more than the normal amount will need to be removed from the bottom of the door. With a flush door this can present special problems in two areas; the bottom rail and the Lock Block.
The bottom rail of the Flush Door is part of the simple framework that makes-up the door. In most manufactured doors this framework is no more than about 25mm. Therefore if more than 25mm is removed from the bottom the door will lose its rigidity by not having the rail in place so that it must be reinstated by insert a completely new piece. If you have saw off the complete rail, you could simply reinstate it, in which case, you must first remove the covering of plywood or hardboard from both sides, as well as the two short pieces of the stile from each end. Simply apply some PVA adhesive to both edges, and tap it into place, using panel pins to secure it in place.
If you are reducing a flush door to fit a smaller opening such as an airing cupboard, you will have to remove a large amount from the door height for it to fit. Because the lock block does not run the full length of the door, consideration must be given as to its position before you proceed, otherwise you will not be able to fit the handles and latch at a sensible position. If you were removing 250 mm from the height of a door as in the case of an airing cupboard and still need the handles to be fitted one metre up from the bottom of the door, the 250 mm would need to be removed from the top, and would also mean reinstating the top rail.
If the door was at a higher level as in the case of a space above the cylinder cupboard, the height of the door may be as little as 600 mm, but because it is high level, the lock block would need to be located at the bottom of the door, so that the cupboard catch would be within easy reach. In which case, the door would first need to be cut off about 30 mm below the lock block, which enables the bottom rail to be reinstalled below the block. The door height would then be measured from the bottom to the required height. The top rail would also need to be reinstated. There are a number of combinations that you could encounter in this respect and should be approached in accordance to the situation.
Pressed Hardboard Door
Pressed Hardboard Doors which creates the panel door 'look', is constructed in the same way as the 'flush' door and is made-up using the same simple narrow framework. If it wasn't for this style, you could follow the same procedure in reducing them. However, a Pressed Hardboard Door gives the impression of having a top and bottom rail complete with panels, just like a genuine panel door. The top (imitation) rails measures about 100 mm, the bottom can vary between 150 mm and 230 mm.
Removing 50 mm off the 150 mm bottom rail would ruin the effect of the door because the top and bottom rails would be equal in size and the door would look odd because of it. For a bottom rail that is greater than 150 mm, this could be reduced as long as its overall finished size is not less than 150 mm. You should also consider the effect of altering such doors in relation to other doors of this type fitted in the same vicinity, where a comparison could be made between them.
Panel doors have top and bottom rails with joints which are mortice and tenon or doweling. Even though this is the best type of door, the amount that can be removed is also limited, as removing excess amounts weaken the joints as well as have an an adverse effect on the symmetry of the door.