The weight of 12 inches of snow does not have a single, fixed answer. Snow is measured by volume, but its weight depends entirely on its density—the amount of water contained within that volume. This variability is important because the load placed on structures like roofs and decks changes dramatically based on the type of snow that falls. A foot of light, fluffy powder poses a vastly different safety concern than a foot of heavy, wet snow.
The Critical Role of Snow Density
The true weight of snow is determined by its snow-to-liquid ratio, which compares the depth of accumulated snow to the depth of water that results when the snow melts. This ratio directly measures the snow’s density. The temperature of the air column through which the snow falls is the primary factor influencing this density.
“Dry, light snow,” associated with very cold temperatures, can have a ratio of 15:1 to 20:1. This means 15 to 20 inches of snow contains just one inch of water. This type of snow forms with more air space between the ice crystals, making it easy to shovel.
Conversely, “wet, heavy snow” falls when air temperatures are closer to the freezing point, often resulting in a ratio as low as 5:1 or 7:1. Warmer conditions cause the snowflakes to be wetter and pack together more tightly, resulting in greater mass per volume.
Meteorologists use a 10:1 ratio as the standard for “average” snow, serving as a baseline for general forecasting and load estimations. Factors like wind can also increase density by battering the flakes together, breaking the crystals, and causing them to settle more tightly.
Calculating the Weight of 12 Inches
To determine the weight of a foot of snow, the snow-to-liquid ratio is used to find the water weight. One inch of water weighs approximately 5.2 pounds per square foot (PSF). Average snow at a 10:1 ratio means 12 inches of snow contains 1.2 inches of water. This results in a load of about 6.2 PSF for 12 inches of average snow (1.2 inches of water multiplied by 5.2 PSF).
The contrast in weight is clear when comparing density extremes. Twelve inches of light, dry snow with a 20:1 ratio weighs around 3.1 PSF. This is because the volume contains only 0.6 inches of water.
A foot of heavy, wet snow with a 5:1 ratio contains 2.4 inches of water, leading to a weight of approximately 12.5 PSF. For example, 12 inches of this heavy snow on a 1,000 square foot section of roof places a total load of about 12,500 pounds on that area.
Structural and Safety Implications of Snow Load
Knowing the weight of accumulated snow is important because structures are designed to tolerate only a specific amount of weight, known as the design snow load. Most residential roofs in the United States are engineered to withstand a load of 20 pounds per square foot (PSF) before experiencing significant stress. The amount of snow required to reach this threshold varies widely depending on its density.
A roof could safely hold roughly 40 inches of light, dry snow (3.1 PSF per foot) before reaching the 20 PSF limit. In contrast, it only takes about 19 inches of the heaviest, wettest snow (12.5 PSF per foot) to reach that limit. Once the load exceeds 20 to 25 PSF, the roof enters a danger zone where structural failure is a serious risk.
The weight of heavy, wet snow also creates a physical strain on individuals, earning it the nickname “heart attack snow” due to the risk of overexertion while shoveling. The combination of snow weight and melt/refreeze cycles contributes to the formation of ice dams. These dams occur when melting snow refreezes at the colder roof edge, causing water to back up under the shingles and potentially leak into the structure.