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The structural design of roof framing is typically performed to account for the effects of snow loading in accordance with the provisions of the load standard published by the American Society of Civil Engineers titled ASCE 7 “Minimum Design Loads for Buildings and Other Structures". The latest edition, ASCE 7-22, has not yet been adopted by CT or NY State, but it will most likely be included in the next Code cycle and it includes some interesting provisions based on the latest research in snow loading.
One of the more significant findings includes an increase in the Thermal Factor, Ct, a factor that is used to convert ground snow load to roof snow load and accounts for the expected reduction in roof snow loads due to heat flow upward through the roof. Recent trends and requirements for increased levels of roof insulation have led to findings indicating that modern ventilated roofs that meet the minimum energy code requirements result in essentially no heat flow through the snow layer atop the roof. Similarly, the increased levels of insulation for unventilated roofs meeting the minimum energy code requirements result in reduced melting of snow.
Currently, a Thermal Factor, Ct, of 1.1 is used for ventilated roofs and 1.0 for unventilated roofs. The latest edition of ASCE 7-22 has increased the value of Ct to 1.2 for ventilated roofs and has provided a range of Ct values for unventilated roofs between 1.0 and 1.2 depending on the roof R-value and the ground snow load. For a typical unventilated roof with an R-value of 30 and a ground snow load of 30 psf the Ct value will be 1.17, an increase over the current load of approximately 5 psf.
The ASCE 7-22 standard can be purchased from the ASCE Library on the website of the American Society of Civil Engineers at https://ascelibrary.org/doi/book/10.1061/9780784415788
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