whatever they put on the package is considered a requirement,
legally binding in court. Any local building official would then be
permitted to make a full installation mandatory. As there is no need to
overbuild anything, there is no need to make such practice a recommendation.



If this is true, then why do they include hips, ridges, and rakes in
their instructions? Ice and water shield on ridges and rakes is not
code in my state. But you are saying that it is considered a
requirement, legally binding in court because GAF puts it on their
instructions?

Regarding ventilation, in the case of new construction, you are
probably right. But it is highly irresponsible to be recommending ice
and water shield over the whole roof to everyone. Lots of old houses
have roofs that are marginally ventilated and have marginal vapor
barriers, as you must know.