[Kent Whitten]

I was waiting for a builder one day and noticed this on the house next door. I have seen this occasionally on some houses and wonder what causes it?

 

[loneframer]

Possibly a venting issue with the rafters in the sloped ceiling?

 

[2ndGen]

I was waiting for a builder one day and noticed this on the house next door. I have seen this occasionally on some houses and wonder what causes it?

Excessive condensation build up?

Bad sheet of plywood that's delaminating?

My first impression was that they tarred the roof in that area to stop some sort of leak.

Or, a particular area where snow builds up and rots the shingles from sitting there (again, another area where there might be hot air escaping through the deck especially at a point where valley rafters meet).

Ultimately, if I had to guess from the pic (which is extremely difficult to do), I'd say it was faulty decking that's contributing to the shingles premature failure.

 

[Slyfox]

I would say it's a ventilation issue, if they blocked off access to the main roof cavity with the raised ceiling over that round top window.
Could also be an insulation issue tho, if they did not properly insulate that ceiling area it could be staying cooler longer in that area.
Maybe a combination of both.