In this post, I will provide a brief overview of determining thermal factors, Ct, for calculating flat roof snow loads on structures per ASCE 7-10, Table 7-3. The thermal factors are based on the thermal condition of the roof.
Thermal Factors for Structures Based on Thermal Conditions
Structures kept just above freezing and other with cold, ventilated roofs in which the thermal resistance (R-value) between the ventilated space and the heated space exceeds 25 °F x h x ft2/Btu (4.4 K x m2/W), Ct = 1.1
Unheated and open air structures, Ct = 1.2
Structures intentionally kept below freezing, Ct = 1.3
Continuously heated greenhouses with a roof having a thermal resistance (R-value) less than 2.0 °F x h x ft2/Btu (0.4 K x m2/W), Ct = 0.85
All other structures, Ct = 1.0
ASCE 7-10, Table 7-3