Not enough information given.

Span and pitch all come into play with sizing rafters, along with spacing.

Without a building code a builder might have used anything available regardless of what was commonly used. The live load on a roof is essentially snow and wind which are considered temporary so with no interior finish the deflection of the rafters might be reduced to an acceptable amount by rafter/collar ties as close to the mid point of the rafters as possible.

If the dormer is larger than three rafter bays, I'm surprised the inspector allowed the front rafters to be left alone especially if the bearing wall top plates are higher than the floor. I hope some kind of rafter ties were kept in place at all rafters including the ones at the dormer.

i thought all 1-1/2 story houses had 12/12 pitch roofs? or close to that. we were talking about insulation and he mentioned the 2x4 rafters. the dormer is full width of the rear of house. foundation is 30x30'. builder put in a ridge beam and added support columns. they are supported on 1st floor and they added a post in basement. all was inspected.

A ridge beam changes everything but the front raters are still undersized for a 15 ft span so rafter/collar ties would be needed to reduce their deflection especially if insulation was added between the rafters and a finish was added to their bottoms further overloading them.

Since the pitch is so steep it is likely that the ceiling joists of the new dormer act as ties from the top plate of the new dormer exterior wall to about the mid point of the old rafters.

Where I live an engineer would have been needed to get a permit for a ridge beam.

We are nearing the end of a half-house remodel in which we ripped off (and replaced in a difference configuration) the 1/2 story and straightened a sloping roof on the part of the roof that remained (pulled out the ceiling and did it from the inside). The rafters were all 2x6s, the pitch was 10/12 (or 12/10 ... I forget the order those numbers go in). All rafters were sistered with 2x8s after jacking up the roof to straight. In some places it had sloped more than three inches.

Anyway, I'm not sure if any of this pertains to your circumstances, but the 2x6s (house was built in 1938) were grossly undersized in that case as evidenced by the sloping roof. There was a ridge beam.