External Door Sizing
One thing that a lot of people don’t realise when they are shopping for a new front or back door is that there is more than one “standard” external door size. There are requirements for the minimum width and height of the main entryway to a house, only the main door has those restrictions, and as long as the door meets or exceeds those restrictions, it can be any size.
The good news is that there are a handful of standard door sizes for external doors, including:
28, 30, 32 and 36 inches wide
80 inches tall, up to 8 ft tall.
The International Residential Code requires that primary doors be at least 36 inches wide and 80 inches tall. When you buy standard-sized doors, the standard door width is 36 inches wide and the height is 80 inches, and these are designed to fit in a rough opening that is 38 inches by 82 ½ inches. This extra space is intended for the jamb and the floor covering. You will need to allow for a small amount of space around the outside of the door for smooth opening and closing.
However there is a lot more to it than standard door sizes. If you have a period property, for example, you might find that the doors are lower, or an unusual shape. So here's how to make sure you get the right-sized door for your property.
Buying Made to Measure External Doors
If you live in a period property, or your home is newer but was made with doors that were an unusual size, then you may want to buy made-to-measure doors instead of trying to cut normal doors down to size. In fact, depending on the material you want your doors to be made from, made-to-measure doors may be the only option. It is not possible to trim UPVC or metal doors, for example.
In addition, while you can cut wood doors down to size, if you want extra wide doors, you will need to get them custom made. It can take 10-12 weeks to get an extra-tall or extra-wide door made, depending on the size and the type of material.
Adjusting the Size of An External Door
If you want something that is just slightly smaller than the ‘standard’ 3 foot exterior doors, then trimming down a standard-sized door to fit is a good option.
Before trimming down the door, make sure that it is a door with a solid core. Some wooden doors are hollow – and these should not be used as external doors because they are not strong enough to provide good security. Make sure that when you adjust the door you do so in a way that ensures that the lock box lines up securely with the jamb, and that the door lines up perfectly in the frame.
External doors should be a minimum of 44mm thick, but thicker is better – assuming the opening is deep enough. When cutting the door, you will need to remove it from the jamb, and trim the jamb to match. Use a shooting board to ensure that your cut is accurate, and be sure to measure carefully. Err on the side of removing too little and re-measuring if there is any doubt, because if you remove too much wood the door will be unusable.