Frost on a roof

I feel like I should know the answer to this.
In the morning when I let the dogs out my neighbors roof is half covered with frost. The lay out of the house is the same as mine so I am assuming there is a vaulted ceiling on the side where the frost is. Is this an insulation/ventilation issue or normal?

Normal. There’s typically a lot less insulation in a vaulted ceiling than an attic - limited by the space afforded by the roof rafters. The roof is simply warmer in that area. I see a lot of cape cod-style homes in my area, and they all have that sloped area of the upstairs ceiling formed by the roof rafters that is hard to insulate. When conditions are right, I see the same thing you do - a “stripe” of missing frost across the roof where this ceiling is.

Is it the half that isn’t being hit by the sun?