Not specifically addressed under the IRC so I would use
R104.11 Alternative materials, design and methods of construction and equipment.
Compliance with the specific performance-based provisions of the International Codes in lieu of specific requirements of this code shall also be permitted as an alternate.
Now go to the IBC
705.5 Fire-resistance ratings.
Exterior walls shall be fire-resistance rated in accordance with Tables 601 and 602 and this section. The required fire-resistance rating of exterior walls with a fire separation distance of greater than 10 feet (3048 mm) shall be rated for exposure to fire from the inside. The required fire-resistance rating of exterior walls with a fire separation distance of less than or equal to 10 feet (3048 mm) shall be rated for exposure to fire from both sides
The supporting construction is more likely to be exposed to fire from the inside the building then another building 40 ft away.