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External Back Doors: Size Guide

One thing many people don’t realise is that doors come in many different sizes. Every external door on your street will probably be the same size, so it’s easy to assume that all doors are that size – but there are several different “standard” exterior door sizes to choose from.

 

UK building regulations set a requirement for the minimum width and height of the entryway to your home – but as long as the main door to the property either meets or exceeds that requirement, then it can be any size at all.  Standard external door sizes include:

  • Widths of 27, 30, 33 and 36 inches
  • Heights from 6.6ft to 8ft

The International Residential code requires that the main door to a property be at least 36 inches wide and 80 inches tall. Because of this international standard, new builds usually feature a standard external door width of 36 inches (3ft) and a height of 80 inches (6.6ft). These will fit a rough opening of 38 inches by 82.5 inches. The additional space is to take into account the floor covering and the door jamb. There should be a very small amount of space around the door to allow for smooth opening and closing, and the small amount of swelling that may occur in humid conditions.

Aspect Oak External Back Door

How to Measure a Back Door

To measure the size of a rear door opening, you should take three measurements for both the height and the width:

  • Measure the height from the ground to the top of the opening at the left hand side of the door. Measure the same distance again on the right, and then again in the middle of the door. If there is a difference, take the smallest of the three measurements.
  • To measure the width, take the measurement from edge to edge at the top, again at the bottom, and at the middle, and take the smallest of each of the three.
  • Deduct 10mm from both the width, and the height. This is to give some clearance to make the door’s installation process run smoothly, and allow the door to move easily in the opening.
  • Measure diagonally from the top left to the bottom right, and again from the top right to the bottom left. Both measurements should be very similar –within 5mm of each other. IF they are not, the opening will need shimmed to make it square.

Make sure you use a metric tape measure that is intended for DIY use – not a dressmaker’s tape measure. This is because these are made from fabric and may stretch, giving you inaccurate measurements.

Unusual Sized Doors

Not all properties have doors that fit these standard sizes. In this case you might look at getting doors made to measure. Unfortunately made to measure doors can be very expensive. A more cost-effective solution is to purchase solid timber doors in the closest standard size and have your carpenter trim them to fit on site.

That said, the best option for very large openings may be French doors, or to get a standard sized door and add sidelights. This will make up some of the difference in width or height. If you don’t like that idea and want an extra-wide or extra-tall door, then you will need to order a bespoke back door.

Bear in mind that there is a lead time on the manufacture of bespoke doors. Depending on the material you choose for your door, this could be between 10 and 12 weeks. External doors need to be robust for security and insulation purposes, so the materials will be carefully selected to ensure the door is as strong and solid as possible.

Adjusting the Size of a Door

As discussed earlier, there are many standard door sizes, and if none of these suffice, you can buy made-to-measure doors instead. External doors should be solid core, allowing you to trim them down to size quite easily.  Some wooden doors are hollow; these should not be used for external doors because they are not strong enough to prevent someone breaking them open.

Some cheaper external doors have an engineered wood core, and come with hardwood lippings which can be sanded or trimmed to fit slightly smaller openings. The lippings are usually around 20mm thick and run around the door, so if the difference in size is minimal you can sand or plane a little off both sides of the door to improve the fit.

Back Door Security

With front and back doors, it’s important that you choose a thick, strong door. Ideally your back door should be at least 44mm thick – and thicker is better, up to the depth of the opening. If you need to trim the door, make sure that you remove it from the jamb, and that you trim the jamb to match the door. Work slowly and carefully, and remove small amounts at a time, re-testing the fit frequently. It’s better to remove too little, and then have to cut a little more off, than to ruin the door by removing too much on your first try.

If you are buying a back door with some glass windows in it, make sure that the windows are double glazed (or triple glazed) for extra security. Some doors have small, narrow windows at the top, and even they should be toughened glass, to reduce the risk of the window being broken and a would-be intruder trying to operate any latch or handle from the inside.

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