Man injured as roof collapses while clearing snow
February 9, 2010 – 2:46pm

UPPER MARLBORO, Md. (AP) – Fire officials say a man attempting to clear off snow from his garage roof in Upper Marlboro was thrown from a ladder when the roof collapsed.

Prince George’s fire department officials say the man, who was in his 60s, fell about 20 feet Tuesday and sustained potentially serious injuries. The man was taken to a regional trauma center for treatment.

Officials are urging residents not to venture onto their roofs to clear off snow. They say a person’s weight can be enough to cause a collapse.

Instead, officials are urging residents to call roofing contractors to remove the snow. Fire department officials say residents should check for sagging roofs or the sound of breaking wood. If the roof structure seems weak, they say residents should evacuate and call 911.

http://en.wikipedia.org/wiki/Knickerbocker_Storm

captcha = Poland sunbelt

Mr. Green’s discussion are really on point, and should put at ease building owners, and discourage dangerous roof visits. Added reasons to not worry are that occupied structures are built with an extra margin of safety beyond the code-required 25 lbs/sq ft design snow load for the DC area. That margin occurs whether calculated by professionals or by the intuition and tradition of builders going back centuries. Many older structures are stronger than newer calculated-designs.

Data on snow depth is usually the fresh-fallen depth, and fresh-fallen snow is usually under 10 lbs per cubic ft; it usually compacts with time, up to 15 and even 30 lbs per cu ft. (but then it has less depth). So, don’t be so sure that 30 inches of snowfall equals 30 inches on your roof.

Yes, be watchful for new sagging, cracks, groaning. Most winter roof collapses I’ve seen result from 1] ice buildup (like on flat roofs with plugged drains), 2] weak construction (like on porch roofs, DIY garages and sheds, or low-cost buildings using wrong standards e.g. a pre-fab bldg designed for more southern climates), and 3]decrepit old buildings. My advice– maintain your building well before a storm, inspecting drains and poking for wood structure rot or metal structure corrosion. If you have a flat roof, look for ponding on a rainy day. Best candidates for winter roof collapse– an old flat-roof garage with rotted framing and leaves from a big shade tree nearby.

And, yes, a flat roof is often at more risk for several reasons. One is that lots of ice or water can accumulate if a drain is plugged. On a sloped roof, the gutters simply overflow. Secondly, a pitched roof will have more triangular framing (strong) as compared to the trusses or beams for a flat roof.

We can all take comfort in the fact that our roofs have withstood major snows in the past. I also take comfort after seeing what my own roof can take. I own a condo in a row house that had an insanely dangerous deck. The supports sat on top of the roof, and not even the joists. We reconfigured it so that the supports are now on top of the brick wall. That deck supported people for years before it was fixed.