The applicable building energy code that determines the minimum insulation requirements for commercial roofs with insulation entirely above the deck in Georgia is the 2020 Georgia State Minimum Standard Energy Code (based on the 2015 IECC with state specific amendments). The minimum insulation requirements apply both to new construction and roof replacements on existing buildings.
The minimum requirement for insulation installed entirely above the roof deck is R-25 for climate zones 2 and 3, and R-30 for climate zone 4. This code became effective in January 1, 2020.
Sample roof assembly diagrams and additional information can be found in PIMA’s Georgia code fact sheet.