Infrared images of the areas that had experienced severe icing the previous winter showed that temperature variations across the surface were within a few degrees of each other, indicating that the roof assembly was uniformly insulated and not subject to "hot spots" that could have caused localized melting.
Given the calculated R-value of the roof and an assumed interior temperature of 74°F, this graph plots melting on the roof surface as a function of snow cover and outside temperature. Below the sloped line, no melting occurs; above it, but below 32°F, melting will occur and ice dams will form at the eaves. Above 32°F (the horizontal line across the top), melting would take place without freezing.
The data was averaged and superimposed on the graph to make the point visually that conditions were ripe for icing at the eaves.