January 21, 2016 at 4:45 PM
(Editor's note: This post was originally published in February 2015 and has been adapted given the blizzard forecast for the D.C. area Jan. 22-24, 2016)
More than two feet of snow could fall in parts of the D.C. region between Friday and Sunday, which could severely stress some roofs. We are concerned about the potential for structural collapses.
In order to best answer the title question of how much is too much, a number of factors must be considered: the weight of the snow; the strength of the roof's trusses and rafters; the slope of the roof; structures on the roof, such as HVAC systems; outside conditions; sun angle and exposure, etc. In addition, certain types of roofs, especially those with dormers, are particularly prone to ice dams which can do great harm, acting to tear away gutters and shingles and causing interior leaks. I write this with some experience, as my last house was badly damaged from ice dams during the blizzard of 1996.
Theoretically, if a roof is built to code (i.e., built to withstand the maximum weight of snow that might accumulate where you live) then you needn't worry about shoveling or raking it off. But the key point there is the maximum weight — not the maximum depth — because, according to FEMA's snow load safety guide, the weight of one foot of fresh snow ranges from three psf (pounds per square foot) for light dry snow to 21 psf for wet heavy snow.
Based on the limited available information that I've uncovered, in eastern Maryland, and probably the surrounding jurisdictions as well, it appears that most newer roofs are built (or are supposed to be built) to withstand snow loads of 15 to 30 psf.
Accordingly, most local roofs should be able (other things being equal) to withstand a one-foot snowfall — wet or dry. Even here, however, when we get an unusually large amount of snow in a short time, we do see roof collapses, as we did during the Snowmageddon winter of 2009-10.
The most famous of all roof collapses in the Washington area, of course, was that of the Knickerbocker Theater on Jan. 28-29, 1922, when 28 inches of snow (D.C.'s biggest single storm to date) caused a catastrophic roof failure, killing almost 100 people. Although the snowfall was extreme, the five year old roof might still have held had it not been for one factor: the roof was flat, the worst kind to have in a heavy snowstorm.
*The term "blizzard," although it seems like it's been around forever to describe a wind-blown snowstorm, really took hold back in the 1870's, when a small newspaper in the town of Estherville, Iowa used the word to describe a recent storm of snow and wind. Formerly, the term had not been used in connection with the weather.
The former U.S. Weather Bureau — the predecessor to the National Weather Service — had long reserved the term "blizzard" to describe a snowstorm attended by high winds, unusually low temperatures (below 20 degrees) and near zero visibilities caused by blowing an/or drifting snow. In other words, falling snow was not even required to produce blizzard conditions.
Today, the National Weather Service has modified its description of what constitutes a blizzard. It now describes a blizzard as a winter storm with sustained winds or frequent gusts of 35 mph or more that cause major blowing and drifting of snow, frequently reducing visibility to less than ¼ mile for three or more hours. Rather than being a prerequisite, it now says that extremely cold temperatures often are associated with dangerous blizzard conditions.
In recent years, there's been a tendency to describe any heavy snowstorm as a blizzard, particularly in those areas unaccustomed to seeing them.