What’s Wet, White and a Worry for 10 Months Out of 12? Snow Loads, Say Climatologists at Colorado State’s Climate Center

Except for July and August, there’s always the possibility that you could have a 15-ton dump truck parked on top of your modest ranch house.

It comes, says the current issue of Colorado Climate, from the sometimes incredible water content of spring snows, and while wet snow doesn’t resemble a truck, it can weigh the same.

Staff at the Colorado Climatology Center at Colorado State University say because of what can happen during snowstorms, especially those in April and May, Colorado buildings, including modest ones like homes and stores, are built with an eye to holding up an amazing load of white stuff.

"Snow loads" are the weight of snow that accumulates on roofs. The key to snow load, said Nolan Doesken, assistant state climatologist at Colorado State, is how much water the snow contains. A foot of fresh snow in Colorado typically melts into something between seven-tenths and an inch of water. Really fluffy powder, he points out, might contain only three-tenths of an inch per foot of snow.

However, wet autumn or spring snowfalls like the one of March 6, 1990, dropped 17 inches that added up to a remarkable 3.48 inches of water. Recent storms in April and May dropped very wet snows in parts of Colorado.

About that dump truck: a cubic foot of water weighs just over 62 pounds, so a square foot of a flat roof covered with 17 inches of snow with a water content of 3.48 inches has about 18.1 pounds pressing on it, Doesken calculates.

Roofs, including porches and eaves, typically have an area about 10 percent greater than the area of the house itself. Multiply pounds of snow per square foot by the horizontal square footage of the roof of, say, a modest, 1,500 square foot ranch house (1,700 square feet times 18.1 pounds per square feet), and the result is 30,763 pounds of weight on the roof.

"That’s equivalent to having a dump truck parked up on your roof," Doesken said, "or a whole herd of horses for people who are animal lovers."

In some resort areas, he said, renovation costs for buildings, especially historic ones, are so high that it’s cheaper to just hire a crew to go up and shovel off flat or slightly sloped building roofs.

Doesken writes on snow load and other wintry topics in the current issue of Colorado Climate, a quarterly magazine about the state’s long-term weather behavior published by the Colorado Climate Center at Colorado State.

For more information on the magazine or Colorado Climate Center resources, call (970) 491-8545.