Have you recently purchased or do you plan to purchase one of our superb PVC Doors or Composite Doors through our site? This type of door is best fitted by a Carpenter or Joiner but regardless of this here is some info to look out for.
It is essential that you measure the inside of the overall opening at the top, the middle and the bottom and then check the height, this type of measuring is then to be repeated from the outside (we normally remove the existing timber architrave/facings to allow us to see behind the timber door frame, this is assuming you are replacing a timber door with a PVC equivalent) and only then can you make an informed judgement on which combination of sizes will allow the PVC door, which comes with its own frame, to fit.
At this point, we would advise that you take a total of 6mm from the width and height to give some fitting tolerances. it is not uncommon to have to make the PVC door/frame smaller to ensure it can fit. Timber packers can be fitted behind the frame to take up any clearance if you make this choice, they can then be concealed with facings.
Note: a PVC door will need to be "toed & heeled", this means that you will (no matter what a spirit level may tell you) have to adjust the hinges on the Horizontal, Vertical and Lateral planes, the lateral plane is that which allows you to move the door sash (the opening part of the door) away from or nearer to the fixed door frame.
When fitting PVC doors always try and fit a small timber sub frame, this gives them a firm base to be secured from. If this is not possible there are numerous suppliers of frame fixings on the market that will ensure that they are fixed firmly
There really is some tricky technical requirements to do this type of job, not least the tools, enough joinery equipment to remove any existing timber frame, electric drill and masonry drill bits, large rawl type plugs, silicone and or mastic etc.
Any doubts, ask for advice.