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I have a 1930s internal door frame which measures 810x2020mm. The door is missing so I'm looking to match it to other doors in the house which are 800x2010.

The closest commonly available door sizes are 762x1981 (too small) or 813x2032 (too large).

I cannot use the larger size as it's a fire door and I'm limited to trimming 3mm maximum off each edge.

That leaves me the option of the smaller door which I assume means reducing the size of the doorway by about 20mm all the way around.

How would I go about doing this?

The wall is an original (I think brick) internal supporting wall. The original timber frame is scrappy, but sound. I'm happy to replace it, or not. I'm not so sure about what's underneath though.

old door frame

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    "How to reduce the size of a door opening in a brick wall?" - "The problem is that all the matching doors I've found are limited to how much they can be trimmed." Trim the door or make the opening smaller.
    – Alaska Man
    Feb 16, 2021 at 16:43
  • Yes, my question is how to make the opening smaller to fit a smaller door, rather than try to fit the next size up which is too large and can't be trimmed. If I've not been clear, please suggest how I can improve my question.
    – Tim
    Feb 16, 2021 at 16:53
  • Is a custom-ordered door not an option? By the time you put in the work to fix the framing you will realize that paying the extra few hundred dollars for a custom sized door would have saved you a lot of headache.
    – MonkeyZeus
    Feb 16, 2021 at 17:28
  • I only spoke to one firm that does bespoke fire doors and they couldn't match the style. I'm not done shopping around, but weighing up options
    – Tim
    Feb 16, 2021 at 17:31
  • So an odd-sized door is preferable compared to the wrong style?
    – MonkeyZeus
    Feb 16, 2021 at 17:36

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If you cannot find an adequate door to cut down your only option is to build out the frame to accommodate a smaller door.

Once you select the door that works for you build out the framing on top and sides using whatever sized material that will get you to the proper dimensions. 1x4 or 1x6 or even 2x. The casing can be installed over it and trimmed out shimming where necessary..

You may have to use extra wide trim around the casing to get the look you want. Since it's a fire door be sure to check your local codes concerning the type of door required and compliance requirements regarding the framing and method of closure.

It would be a lot easier if you can find a door that you can trim down or order one custom made..

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    Well, not the only option. The other is to dismount the jamb, reduce it, and reinstall it.
    – isherwood
    Feb 16, 2021 at 17:21
  • Good point. But with the dimensions mentioned by the OP it didn't seem viable.
    – HoneyDo
    Feb 16, 2021 at 17:27
  • If I follow this correctly @HoneyDo, you're suggesting to build out the existing wooden lining rather than removing it, adding material to the wall aperture and then fitting a new lining set?
    – Tim
    Feb 16, 2021 at 17:28
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    You can do it either way depending on the structural integrity of the jamb that's in place. Since the jamb is solid and you don't know what's under it - why not?
    – HoneyDo
    Feb 16, 2021 at 17:38
  • @Tim, please pay attention to the admonition to ensure your new framing material meets fire rating compliance, as well. Also, be aware that a custom ordered door, especially a fire rated door is unlikely to be a cheap option, but may be the best option.
    – FreeMan
    Feb 16, 2021 at 18:25

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