Ok, I got to the bottom of this one. This was an attached home… a “townhome.” This wall was the common wall, with a valley between the two units. Basically two gable roofs with a horizontal valley in between. Not a very BRIGHT way to construct a roof in Alaska. They then installed a inverted crickets on the front and rear to create two additional valleys to cover that one long horizontal valley.
The bottom line is, they installed fire resistant sheathing in the roof on both sides of the 2 hr rated wall so they would not have to install a parapet. However, the two units were not aligned with each other. This unit extended past the front wall of the other unit. The builder did not install fire resistant sheathing where the roof extended past the other unit. The city inspector decided that this did not meet the requirements of IRCR3220.127.116.11 So, in accordance with the same section, they mitigated it with “one layer of 5/8-inch (15.9 mm) Type X gypsum board is installed directly beneath the roof decking or sheathing for a distance of 4 feet (1219 mm) on each side of the wall.”
So, it is code. Not good code, and not even the wy I would have interpreted the code, but I am not a code inspector. I did note in the picture that the adhesive looked red. Well it is. It is fire rated. Thank you city inspectors.