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Written by Doug Fell, Minnesota licensed professional engineer (structural)
January 10, 2023
It is the time of year where we always get the question “do I have too much snow on my roof” and “are these icicles and ice dams a problem”?
The answer is not to ignore the issue and not to run to get your roof cleared of snow and ice dams. The answer lies somewhere in the middle. There is a time to have it cleared and there is a time to be patient.
Without holding a long instructional class in Minnesota residential roof loads and systems, let’s start with the basics of Minnesota residential roofs:
1. The current general roof snow design load is 35 PSF (pounds per square foot).
2. Snow density (how much the snow weighs per inch of depth) can vary greatly depending upon if it is wet (heavy) snow, a light (fluffy) snow or if it has been accumulating and compressing causing the lower snow levels to be denser. Generally, the longer the snow is on the roof the more that it compacts and becomes denser and the more that it weighs per inch of depth. A 1’ depth of snow on the roof can vary from about 2 PSF for very light snow to about 12 PSF for wet snow or a compressed snowpack.
3. The snow water content ratio for snow varies from about 5:1 (5” of snow = about 1” of rainwater depth) for a very heavy wet snow to about 20-40:1 (where 20” to 40” of snow = about 1” of rainwater) for a very light snow.
4. How much snow is too much snow? Based upon the above snow water contents, it would take almost 3’ of wet heavy snow depth or a compacted icy snowpack depth to approach the current roof snow design load and much deeper snow if the snow if a fluffy light snow.
5. Drifts can form on roofs especially at varying heights of roofs that can cause the height of the snow to increase over the normal depth of snow on the roofs. This should be considered since it can cause additional concentrations of snow which can lead to more loading.
6. Icicles and Ice Dams are typically created when the warmth from under the roof melts the layer of snow directly on the roof and the melted snow water runs down the roof and when it gets to the roof edge, that is typically colder than the part of the roof over the enclosed house, it freezes causing icicles and a thick layer of ice on the edge of the roof. As the thick layer of ice on the edge of the roof continues to increase in height, it ultimately gets high enough to stop the melted snow water from reaching the roof edge and the water can back up and flow under the shingles. This is an ice dam. Without getting into the deeper issues of “why” this happens, it typically is related to the amount of insulation and ventilation in the attic area. More insulation and more ventilation of the roof area reduces the potential for the snow on the roof to melt, causing icicles and ice dams.
7. Stay off the roof unless necessary! It is dangerous and it can cause damage to the roofing system that may not show up for years. Only those that are trained and properly equipped should be on the roof to clear ice and snow, and only when it is necessary due to ice dams or heavy snow loads.
8. Don’t rake or shovel the roof unless it is necessary due to ice dams or heavy snow loads.
9. Only trained and experienced people should remove snow and ice from the roofs and then only if it is necessary due to ice dams or heavy snow loads.
10. Take care if using heating elements and/or chemicals to remove ice from the edges of the roof as this can cause severe damage to the roofing system.
11. Snow load upon decks should be monitored also as the decks are typically designed for only a slightly higher live load (typically of occupants not of snow) than typical roofs.
12. Older roofs and damaged roofs may not be able to support the snow loads as well as the newer structures.
13. Storms have been increasing in frequency and duration and can lead to excessively high snowfall in smaller areas that could cause excessively high roof loading.
In summary, each case is different and if you have any questions or concerns it is always good to consult with a professional engineer that is licensed in the state of Minnesota and is experienced in the design and evaluation of residential structures if you have any questions or concerns about snow loads upon your residential structure.
*This is given as a generalization only and is not to be used as instruction of when to clear the roof or not.