It's been a snowy winter in the mid-Atlantic and Northeast snowy and cold, with little if any thawing or melting between successive heavy storms. As a result, snow loads on rooftops are piling up in coastal New England. The latest storms brought a rash of roof collapses to Massachusetts, Connecticut, Rhode Island, and New York. The Boston Globe had this report on February 3, complete with video of the failure of a flat-roofed commercial building in Easton, Mass. ( Building collapses continue; more storms on horizon ). Boston TV station WCVB has this additional report ( Roofs Collapse After Storms Pound Bay State, by M. R. F. Buckley. The video of the Easton collapse is also posted on YouTube. Rain and sleet on Saturday, February 5, increased the risk, the Globe reported ( Today's rainfall could worsen the threat of roof collapses, by Janet Walsh). At a Chelmsford, Mass., mobile home park, three trailers were judged uninhabitable by local officials after a resident called about a sagging roof. Rhode Island structural engineer David Grandpr©, P.E., was busy over the weekend chasing down reports of trouble. Grandpr© handles structural investigations for C.A. Pretzer Associates, Inc., based in Cranston, Rhode Island. Grandpr© talked with Coastal Connection on Monday. The most vulnerable buildings, he said, are commercial flat-roofed structures built before the codes began to require engineers to take account of drifting snow. In Rhode Island, he says, that change occurred around 1978. When you have a change in roof elevation, snow can either blow off from the high roof and land on the low roof, or it can pile up against the wall of the higher portion on the lower roof, Grandpr© explains. Both scenarios are considered in current building code requirements for designing for snow. You just add all that weight at that one concentrated area. In the current snow event, says Grandpr©, I would say there are a lot of problems associated with snow drifting. I looked at one case where they built a new addition with a lower roof than the main building, and the people designing the lower part didn't seem to know that there was an existing higher roof. And then the opposite also happens, where there's an existing low roof, and somebody builds an addition higher, and they don't take into consideration reinforcing that existing lower roof. Commercial building collapses are indicative of flaws, says Grandpré: The large majority of buildings that are falling down and touching the ground have some sort of construction or design defect. Even this February's monster Groundhog Day storm should not have collapsed a properly built structure, he says: The snow loads that I've been seeing are not really that much above the typical design criteria of 30 to 35 pounds per square foot. Those roofs should be able to hold that much weight. Engineered design methods involve the application of significant factors of safety, he points out so if we're seeing roofs come down with close to the design snow loads, then there's got to be some kind of construction or design defect. Grandpré has also looked at some houses with snow problems. But in these cases, he says, the trouble has been far short of any structural collapse, and may not signify any construction or design deficiency. Light wood-frame residential buildings, such as a typical ranch house or colonial, can be well built and code-conforming, says Grandpré, and still have trouble. Codes specify an allowable deflection for floor or roof framing under the design loads, he points out; and when there's enough snow on the roof, that amount of deflection could cause finish or performance problems without risking an actual structural failure.\ We use the equation L/180 for deflection, Grandpré explains. So if we have a 180-inch span, that member is allowed to deflect one inch under load. If you have a 360 inch span 30 feet you could have two inches of deflection. But when you have a two-inch deflection, ceiling panels can go out of whack, sprinkler heads can drop, sliding doors can bottom out. And all that can be well within the safe limits of the performance of the structure. But they cause distress, and it's a good idea to have them checked out, just in case a structure really is failing. The house Grandpré inspected this weekend had a wood paneled ceiling and faux wood beams. When the trusses deflected, those faux wood beams got pushed down and started separating. Gaps were opening up by the end of these beams. But it looked like the roof was caving in. So they got a contractor to shovel off the roof, and the trusses rebounded a little bit; but the beams got stuck in the new position. So the ceiling's going to have to be replaced. And it's very frightening for the homeowner, but it's really just an esthetic problem, not a structural safety problem. Besides that, Grandpré said, the home's sliding patio door got stuck because the wood header over the door deflected under the weight of the truss ends. Grandpré assured the homeowners that the house was safe for their Super Bowl party this Sunday. But he says, This speaks to the nature of the building code. It says right at the beginning of the code that the code is a minimum criterion. It's the owner's prerogative to do more than that minimum. And on high-end houses, we often go above the code. You can spend more, and get more, if you intelligently put your money in the right place.
