Reddit reviews Carpentry and Building Construction

75+ year old houses are going to have, in most cases, lathe and plaster walls. So when someone says you'll just need dryway and patching compound, that's not technically true (unless you want to do it slap-dash). You can use drywall as a backer but most use lathe still or expanded metal mesh.

Having said that, heh, here's what's "inside" most houses walls that have been built after the 1950s in the US.

Example 1
Example 2

And since you are thinking about potentially remodelling (I'm guessing other stuff besides doors, as well) you should get and read through this book (that's where the above images come from, btw).

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