How much snow can a roof support?
City of Ketchikan Building Code specifies new construction be capable of withstanding a Ground Snow Load of 55 lbs per horizontal square foot.
To provide a response to "How Much" requires a little bit of science, math and knowledge of the roof structure and type. A flat roof, gable roof, shed roof gambrels, and roofs with multiple valleys and snow gathering points will vary the weight differential factors as will the number and type of roof coverings effect the value. For rule of thumb example to offer some type of response to the property owner questions, we have to presume that the structure has been constructed to sustain the minimum snow load requirements of the code. The weight of snow/ice, not the depth, is critical in assessing a roof's vulnerability. The water content of snow may range from 3% for very dry snow to 33% for wet, heavy snow to nearly 100% for ice. An inch of water depth weighs 5.2 psf. Thus, a roof designed to a carry a snow load of 20 Ibs. per horizontal square foot is expected to support nearly 12 inches of wet, heavy snow. (University of Arkansas Division of Agriculture)
If using the heaviest wet snow value for roof designed to sustain 40 psf of snow load then it can be viewed that it should be capable of supporting nearly 24" inches of wet heavy snow. If the snow moisture content is less, the depth of snow for sustaining can be greater.
The science aspect: measure the moisture content of the snow to determine its weight factor. This can be done simply by taking a 3 pound coffee can and pushing it down into the snow to the roof. As you fill the can with snow empty it and keep filling until you reach the surface, melt the snow then pour it back into the can and measure it. Depth of inches multiplied by 5.2 = psf.
When rain falls on top of existing snow it can greatly increase the water content and thus the weight of the snow making it very difficult to calculate in advance.
Please use this information as tool to assist you and others in reducing danger to their self that could have been avoided. Catastrophic roof failure may not be solely attributed to built up snow, there may be other actions that occur that affect the roof structural stability and sustainability.