Do You Need to Shovel a Flat Roof?

Snow Load

• Most flat roofs can safely support about 30 inches of fresh snow, up to 17 inches of hardened snow or 7 to 8 inches of solid ice, according to the Canada Mortgage and Housing Corporation. Many older flat roofs are not designed to withstand as much added weight as newer structures, however, so overall weight limits can vary from roof to roof. Older flat roofs typically were designed to support snow loads of up to 30 pounds per square foot. While the building permits of some municipalities require new flat roofs to withstand up to 40 pounds of snow weight per square foot, others require new structures to be able to support as much as 60 pounds per square foot.

Estimated Weight

• Light, fluffy snow weighs less than dense, saturated snow and can greatly affect its accumulated weight. Snow’s moisture content ranges from 1 percent to 33 percent, according to the building department of the city of Coeur d’Alene in Idaho. Consequently, 1 cubic foot of snow can weigh anywhere from 1 to 21 pounds. A basic formula to approximate snow weight requires you to use ground snow depth as a reference, or — if you can safely measure it — roof snow depth. Multiply the number of inches of snow by a factor of 1.25 for the approximate pounds per square foot. A flat roof covered in 16 inches of snow, for example, is supporting approximately 20 pounds of added weight per square foot, assuming the snow has a density of 25 percent.

Considerations

• Fresh, saturated snow generally weighs approximately 20 pounds per cubic foot, but ice buildup and hardened snow can add a considerable amount of weight. For every inch of ice buildup, each square foot of flat roof supports an additional 5.2 pounds of weight. Consequently, a flat roof covered in 2 inches of ice and 16 inches of moderately heavy snow is supporting a load of approximately 30.4 pounds per square foot. Depending on how well the roof was constructed, this amount may be more than its maximum threshold, or may fall well below it. If your home is manufactured, it should have an affixed metal certification label that lists the roof’s snow load capacity.

Signs of Stress

• Flat roofs tend to hold more snow moisture because their lack of pitch doesn’t allow for adequate drainage. Water leaks usually indicate roof damage, but don’t necessarily mandate a total clearing of snow, which can be expensive and dangerous, according to the Canada Mortgage and Housing Corporation. Jammed doors and new cracks in the drywall or plaster of interior rooms, however, may point to structural damage that requires you to clear the snow off your roof. The Fire/EMS Department of Maryland's Prince George County recommends calling snow removal professionals to handle the job, because you could potentially fall off — or through — the roof. An article in the "New Haven Register" also reports that, for safety reasons, experts recommend that homeowners not climb on their roofs to shovel the snow. Cracking, popping or other sounds of stressed wood and metal may indicate an impending structural collapse. If you hear such sounds, leave your house and call 911 from a safe location.