A foot of light, fluffy snow weighs far less than a foot of wet, packed snow or ice. Most modern roofs handle normal accumulation fine. The concern comes after several storms in a row, a heavy wet snowfall, or rain on top of existing snow, all common in Western New York.
Warning signs include doors that suddenly stick on interior walls, new cracks in drywall near the ceiling, creaking from the attic, and visible sagging. Flat and low-slope roofs on garages, barns, and additions are the most at risk.
Ice dams forming along the eaves are another reason to clear the lower edge of the roof, even when the total load isn't dangerous.
Roof snow removal is risky work. Snow-covered roofs are slick, edges are hard to see, and metal tools can tear shingles and gutters. If the job needs a ladder, it's worth calling a crew that works on roofs year-round and has the right equipment.