Preparing the Rough Opening for the New Window

Once you have everything out, the opening will look like this:

This photo shows why the new windows must be measured to fit the opening provided by the brick exterior.  You will slide them in from the outside and they need to fit as close as possible, but if they are too big, you're in trouble!  Caution: the bricks were laid by hand, so measure each window opening carefully at several places to make sure you order windows that will fit.  If you go with a quality supplier, they should offer to come to your house for a consultation and take free measurements.

What you will discover is that there is "extra" space where the weights were located.  This area may now be recessed into the wall cavity. What you want is a flat, square opening all around the window.  In this picture you can see that I used some scrap wood (actually a piece of the casing) to furr out, or extend, the rough opening so it is flush with the brick.  (The old casing happened to be the right dimension, but you may need to cut something to fit.) This will make it easier to insulate, and provide a solid surface for screwing in the new window.

At this time you should also clean up the outside brick.  Chisel off any old caulk or pieces of mortar that stick out far enough to prevent window installation.

Trimming the plaster.  You will go through several blades per window!

Because of those window weights the inside wall board (or in this case plaster wall) is not as wide as the rough window opening.  You have to fix that, so the new window will slide into place.  Once you have made any width adjustments to the newly-revealed studs, trim the wall board flush with your "new" rough opening, i.e., in line with the brick exterior.  This is one dusty, messy process.

Here is a look at the trimmed and cleaned-up opening, from the outside:

Note that everything in the vertical plane is flush, solid and cleaned up.

Note: You may also need to furr (extend) or trim the top of the opening (the header board) and/or the bottom of the opening.  It depends on how it was built originally, but just do whatever is needed to create a solid, level, square opening that lines up with the brick and will accommodate the dimensions of the new window.  That said, it is a rough opening, so don't get carried away.  You are only creating a space for the new window, not doing finish work just yet!