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Installing new windows on an out-of-plumb wall

The oldest part of my house was built in the late 1700's. To say that the walls are out-of-plumb would be an understatement.

We recently replaced a rotten bay window and now it's time to tackle the hard part -- the trim. The window was installed plumb and level and as flush as it could be to the outside of the house. The inside wall is neither plumb nor square to the footprint of the house.

Based on everything I've read, here's how I believe I'm supposed to trim out the inside of the window.

The casing should be flush to the wall, meaning that the casing will square and level, but not plumb. The jamb extensions should be custom fit -- i.e., the edge of the jamb extension that's attached to the window will be straight and plumb, while the edge of the extension that's attached to the back of the casing will be straight and out-of-plumb.

To be specific, on one side of the window the jamb extension at the top corner will be 1 5/8" inch wide, but only 5/8" wide at the bottom. To minimize the discrepancy, my plan was to recess the jamb extension by 1/16" on both sides and at the top of the window. At the bottom, the extension would be flush with the top edge of the casing.

Does this sound right? Are there any other ways to do this and make it look reasonable?

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