See also Shingles pulled off by Snow

Placer County advises to clear snow from rooftops prior to incoming rains, observe signs of potential structural failure
"A snow load on a rooftop can become even heavier by absorbing rainwater, which makes a structure even more susceptible to collapse in addition to a higher likelihood of snow sliding and flooding. "

"unbalanced unloading of the snow can create unstable conditions and potential building collapse"

There are several problems with snow on roofs.
snow type density [lbs/cu ft]
Fresh snow 3.75
Damp fresh snow 6.87
Settled snow 15.61
Wind-packed snow 23.41
Very wet snow 46.82
Ice 57.25

Public Land Survey System (PLSS) Snow Load Capacity
Placer county building codes snow and wind design 2019
Section R302.2.3 Snow Loasds for 6,000-6,500 elevation snow density 25 p.c.f (pounds per cubic foot) Maximum anticipated snow depth 12'
Ice dam structural damage.

Upper St. Clair Roof Ice Dams finally Explained
Roof Snow Guards, Are they Really Needed? | Roofing Tutor
"They are designed to hold snow on the roof until it melts so that it wonÕt avalanche off the roof, possibly damaging decks, shrubbery, other property, or even causing personal injury."

"On shingled roofs where snow dams are a problem they should NOT be installed. "

Do Snow Guards Cause Ice Dams? The quick answer is no, snow guards do not cause ice dams. A snow guard, also known as ice guards, snow stops, snow cleats, pad style snow guards, and ice cleats hold snow and ice in place to prevent avalanching or sliding off the roof onto walkways, patios, landscaping below. While snow guards and metal roofing systems go hand in hand, asphalt shingles are a different story. Shingled roofs are extremely prone to ice dam formation and the addition of snow brackets only can only worsen the risk. A lot of companies, therefore, do not recommend using this device if your roofing system is primarily comprised of tiles, shakes, or shingles. A snow guard is a device used to retain snow and ice from falling from one surface to a lower one; in contemporary usage, they are installed to prevent snow/ice pack from avalanching and damaging people, plants, and property below.

Unbalanced snow load