Ceiling darkened at roof beams- I've never seen anything like it

[Summer Dean]

Have any of you seen anything like this? It is throughout the house and the dark lines seem to correspond to the attic/roof beams. The house is only 14 years old. And seller says he replaced the roof due to storm damage last year. At first we just noticed the dark lines in the living room with the fireplace and thought it could be due to using the fireplace, but it is even in the bedrooms. The owners smoke in the house, but I've been in smoker homes and never seen anything like this. 

Could this be some sort of structural problem?