In ,
trader_4 typed:
On Saturday, June 20, 2015 at 2:27:19 PM UTC-4, TomR wrote:

If I am picturing everything correctly, I am not sure why ice shield
would need to be on the vertical wall anyway. I think of ice shield
as going along a roof edge under the roof shingles in an area where
there is an unheated space along the roof edge. The melting water
from the roof flows down and then hits the cold edge and freezes,
causing an ice dam, which raises the roof shingles and causes water
to back up into the property.

So, maybe the only place that you need the new ice shield would be
on the slope under the soon-to-be-new roof shingles.

It does go over the roof deck like you say, but it also needs to
extend 4 to 6" up any vertical wall surfaces that meet the roof.
Otherwise, with ice damming, water will rise up and flow in there.

You are probably correct about that. But, I assume that is how it is done
initially before the roofing and siding go on. In this case, the siding is
already there, and removing it will be an issue. So, maybe trying to get
new ice shield to go under the old siding is not really a viable option.