[dad911]

It is common, probably better to 'weave' a valley. Less chance to leak with ice dams.
Are you talking about flashing at the bottom of the roof by the gutter? Or at the top like where a shed roof meets a sidewall?

Are you sure there isn't flashing? I make the roofer cover the flashing with shingles, so you don't see an aluminum band. Ugly, IMHO.

Most common cause (again, IMHO) of roofing failure is not enough attic ventilation. Were vents installed? Did homeowner cover them to prevent drafts?

And 15 years on a 20 year roof isn't so bad....No leaks? So the builder is Criminal?

And why would you not expect some 'growth' on the north side of a roof? Heavily shaded? Hell, I've seen 'growth' on the shady sides of houses quite frequently....... I'd call it a maintenance item.