Cutting existing roof joist

[GlenBill]

Hello - I just discovered that I have a section of an addition that never had insulation installed in the ceiling & walls of a bathroom with a long skylight tower. I do not have access to this attic area where I can crawl through, but I can see it through a small opening. I must add insulation and have access to this.

In order to access this area, I would need to cut a 2 X 8 roof joist and connect it (by header) with 2 other roof joists that were originally cut to connect this addition to the main frame of the house. I could do this by putting in a longer header than exists. I'm concerned about the longer span this would create, which would be 65" or so vs 48" existing. Please advise me as to code limitations for cutting roof rafters and header limitations.

Thanks.
GB