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I'm rehabbing a structure from the 1850's. As you can imagine, nothing is square, level or plumb. The walls were originally plaster on top of true 2x4's.

I've brought the house down to the studs, and am going back with exterior sheathing (0.5") on top of true 2x4's (4.0") plus drywall inside (0.5"). That gives me an actual wall thickness of 5.0".

That tells me I should order my new windows for a wall depth of 5.0"

Problem is, the walls are not straight or consistent. Should I order windows with a little extra? Like 5.5"? I had experience doing this to my first house, which was also from the 1850's, and it took me forever to get the interior trim around the windows right. I had to cut shims, and wedges, because the walls were wavy. It was a huge pain.

Specifically: If my wall has a nominal thickness of 5.0", what is a sane amount of expected deviation? Maybe 0.25"? Should I order windows for a wall depth of 5.25" and just shim out the interior trim?

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The sanest thing is to order new construction flanged windows that are a bit smaller than the rough opening. Then you integrate the flange with the WRB that you're installing over the new sheathing (right?) and bob's your uncle. On the inside, you can cover up the lack of square-ness with new drywall or wood window boxes, shimmed to be square with the window, not the rough opening.

Since you're taking this house down to the studs and redoing the exterior sheathing, let me mention that you have a golden opportunity to add extra insulation outboard of the new sheathing. The marginal cost will be fairly low, especially if you're doing the work yourself and buy reclaimed foam.

Resources:

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Window standard depth is 4 9/16".

The rule with windows and doors is that you block them out to fit the window/door not the other way around. (Every time I see somebody trying to shave a door I chuckle.) What you did before is correct. The skill is learning how to trim the window as efficiently and securely as possible.

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If you're just planning on painting the window jambs, it'd likely be easier for you to order them without trim, and do it yourself. It's easy and you might spend an hour or more on the first one, maybe a half hour on the second, and by the 10th one you'll be knocking them out in 5 minutes.

This is especially helpful when, as you said, the walls aren't going to be straight or consistent, and the rough openings aren't square. You can rip the pieces to the right width for each window without having to fuss with them too much.

What you'll do is put a straightedge along the wall and find the widest point, which will be your measurement. Put the cut side against the window and leave the factory edge facing out (into the room) where you'll see a bit of it. Do the top first, then the sides, shimming as needed to make it square with the window. Then go to the next window and come back later to install the sill and apron while you're putting on the casing.

If the walls are really bad then yeah, you'll end up having to shim the casing out in some areas... Unless you want to fix the framing while you've got it all torn apart. Run string lines from end to end of a room (at the top and bottom of the wall) and sister the existing studs to come out even with the furthest out ones. This is a huge pain, but it will make your life easier when installing the drywall and trimming the windows and doorways.

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