The question is when framing a gable dormer where there will not be a
supporting wall under the joists (at least not under the entire run), how
wide can these be made? Asked a different way, is there a limit to the
number of joists that can be removed?

I understand the concept of replacing each removed joist with one on each
side when cutting in a dormer.

The entire run is 10 foot from ridge to OS wall.
The roof (very old construction), is currently SYP 2x4 24" OC. I realize I
will need to use 2x6 to meet current codes.
There will be no header. The dormer ridge will run from the existing ridge
to the OS wall.

I will check with the local code people. I just don't want to look like a
fool when I go in there so I am gathering a little information first.

Colbyt