I’d like to replace this concertina “door” that separates my bungalow’s porch from the rest of the house with a real internal door.

The reason is to reduce drafts and so I can a fit a cat flap in this door. The cat can then leave through a open window in the porch.

The frame is 750mm/29.5in wide.

Would a 726mm/28.6in wide door fit into this gap and open properly?

The frame is 60mm deep. The door I was considering is 35mm deep.

I’ve checked other advice sites. Some said doors need to be 6mm smaller than their frame, others said 50mm.

concertina door

  • Fitting the cat flap in the porch isn’t cost-effective. It has double-glazed French doors and tall double-glazed windows. With a cat flap in a draft-proof and lockable door here, she can go in the porch and leave via a window, and the neighbourhood cats won’t be able to get in. Commented Jan 9, 2022 at 22:13
  • 1
    If nothing done to adjust the size, then almost an inch open space between door and frame. Can add about 3/4 inch to one side of door(glue and screws), add to door frame, or add to frame opening(rough) and make frame smaller to fit door. If cat can get out, other cats can get in.
    – crip659
    Commented Jan 9, 2022 at 22:51
  • By way of explaining that 6 vs 50mm… those two numbers represent a door slab in a finished opening (6mm) or a prehung door in a rough (ie, bare timbers) opening Commented Jan 10, 2022 at 3:48
  • Thanks. @crip659 It’s a microchip-reading cat flap. It unlocks for my cat, but not others. Commented Jan 10, 2022 at 14:41
  • It might be easier to get a 30 inch door and trim it down. The side edges of most wooden doors have enough solid wood for a half inch trim(or two 1/4")
    – crip659
    Commented Jan 10, 2022 at 14:54


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