Susan Hartnett shovels snow from the roof of her Beacon Hill home in Boston.
Steven Senne/AP Images Susan Hartnett shovels snow from the roof of her Beacon Hill home in Boston.

This winter has been a particularly tough one for those in the Boston area, which has experienced 31 days and counting of heavy snow, ice, and deep freezes. The severe winter weather has not only claimed several lives, but also a number of the region’s buildings44 in a two-day span. In particular, the unanticipated and extreme snow and ice loads have taken their toll on the region’s roofs, causing them to sag, leak, and collapse. The damage to municipal buildings can be seen through the city’s drone footage.

Principal engineer at United Construction, in Newport, N.H., and editor of Todd Fratzel told The Boston Globe that his general rule of thumb is to clear snow topping 18 inches or more on low-pitch roofs. But a number of other factors related to a building's structural integrity should be considered before removing snow: a roof's shape, its slope, construction materials, maintenance, exposure to wind, and the type of snow that has accumulated. These variables can all cause snow-induced structural failures, according to a FEMA snow safety guide.

One of the great threats is what FEMA calls an “unbalanced snow loading,” the condition in which snow amasses at different depths at different locations on a roof plane, resulting in a differential weight load. This uneven loading, which can be caused by drifting or sliding snow, poses a greater risk to the building’s structural system than a uniform load.

The roof type also factors into its survival during severe winter weather. Steep roofs are more common in New England, which is a good thing since they can shed snow easier than are flat or low-slope roofs. According to the FEMA snow guide, a roof pitch that exceeds snow's angle of repose—30 degrees—will result in snow sliding.

However, tactile, abrasive roofing materials, such as asphalt shingles, will not shed snow as easily as a slippery surface, such as metal panels and single-ply membrane roofing. Snow guards or cleats, required by the Massachusetts building code in some instances, will also inhibit snow from sliding off the roof and damaging to subsequent structures or adjacent roofs.

A roof’s thermal properties will also affect how much snow accumulates. Roofs that are well insulated generally retain more snow because heat from the conditioned building interior cannot reach the snow. Well-ventilated attics also preclude roof snow from melting as quickly because the attic space temperature will be close to ambient air temperature.

Snow Load Safety Guide, FEMA;

Local building codes across the state for residential roof snow loads range from 35 to 65 pounds per square foot (psf). According to the FEMA guide, 1 foot of fresh snow can weigh between 3 psf for light, dry snow and 21 psf for wet, heavy snow. Meanwhile, 1 inch of ice weighs comes in at a little less than 5 psf, and 1 foot of ice weighs around 57 psf. City planning departments have data tables that factor in truss size, pitch, and materials to calculate the roof loads.

Snow Load Safety Guide, FEMA;

Clearing accumulations from a roof can sometimes be more hazardous than beneficial as it poses serious threats to those doing the work and to the roof itself. Snow removal can result in serious and even fatal injuries, as well as roof collapses. Volunteer organizations, such as All Hands, have created immediate disaster-relief projects to help the disabled and elderly with snow removal from roofs. All Hands communications and marketing associate Kaitlin Robbins says that since the rapid response project began two weeks ago, volunteers and staff members have put in a collective 650 hours of work. A big focus has been on clearing roofs, which has proved to be a bigger issue than initially anticipated.

Boston’s worst storm this winter (Jan. 26–27) recorded 24.6 inches of snow, making it the sixth biggest blizzard in the city’s history. Worchester, Mass., experienced nearly a foot more in that particular storm: a record-breaking 34.5 inches. This winter's snowfall in Boston totals about more than 8 feet—and counting.