As required by Part 9 of the Canuck Building code (with exceptions, but I ALWAYS spec it).... asphaltic felt or the really expensive blue stuff known by a few names (used to be able to use 6 mil poly too), installed properly, a min of 36" up the roof from the edge of the eave and to a point at least 12" beyond the vertical plane of the interior of the exterior wall.
This is NOT a sbstitute for proper insulation and ventilation. It is a "backup". And some contractors just don't bother, on brand new houses.
Say... I still have 30 "Home Reference Books" by Carson-Dunlop from my old inspection company. I'll check with my accountant to see if I can sell them without getting into trouble with Revenscrue Canada. They retail for close to $100 and they are the best reference book for owning or buying a home that I have ever see. Check out www.carsondunlop.com... say... I'll bet they have a blurb on ice dams.