As the snow has fallen these past few weeks, so have an increasing number of roofs. But just what causes a roof to fail? We asked George Tsiatas, chairman of the civil engineering department at the University of Rhode Island.
PBN: It seems like every time we turn around another roof has collapsed because of snow. Is it typical that so many roofs in Rhode Island collapse during the winter?
TSIATAS: This is a particularly tough year for roofs. It started with the collapse of the inflatable roof at the Metrodome in Minneapolis and has continued almost nonstop. Rhode Island has seen its share of roof collapses but I think we have fared better than other neighboring states including Connecticut and Massachusetts.
It seems to me this is an unusual year. We have had a lot of snow in a short time frame without the luxury of warm days and melting in between. So the snow keeps accumulating. Also, I suspect some of the primary drainage systems of roofs may not be functioning due to ice. So, even if the snow melts during the day, there is no place to go, so it refreezes in the night.
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PBN: Obviously, the weight of the snow is causing the roof to collapse, but what specifically is failing in the structure?
TSIATAS: It depends. In case of smaller buildings with multiple roof levels, additions of different heights and in general complex roof geometries, the problem may be snowdrifts. We actually did have some strong winds during our recent snowstorms. Under certain conditions, snow can accumulate fast in localized areas in the roof, exerting heavy loads that can cause structural distress in supporting beams or rafters. Ice built up around the eaves can also cause failure especially in case of long overhangs. Sometimes snow melts but has nowhere to go and may find its way inside a building through a possible weak sealing and then the moisture may weaken supporting panels or beams.
In the case of large flat roofs such as in warehouses, the weight of the snow can be high. In this case, a working primary drainage system is helpful. A reliable secondary drainage system is also important. Good engineering practice is also critical. A flexible roof under heavy snow, or water that does not drain, can deflect thus allowing additional snow or water to accumulate, which in turn causes additional deflection. This is the so-called ponding effect that could lead to problems if the joists are not sufficiently stiff and adequately braced. Also, unbalanced loads or localized snowdrifts can indeed exert high loads on the supporting structural system.
PBN: So what should someone concerned about his roof check? And is this something that anyone can see or should experts be called in?
TSIATAS: I don’t think there is a problem with the typical residential house. And I definitely do not recommend homeowners to climb on their roof to get rid of the snow. Of course, if they are concerned about their roof they should contact some professional to take a look.
PBN: I know there’s a debate in some engineering circles about flat roofs versus pitched roofs and the debate often centers on carrying capacity. What are your thoughts? After this winter, should we all move to pitched roofs?
TSIATAS: A well-designed flat roof should not have any problems with snow or other loads for that matter. I would like though to see flat roofs that do have a slight slope, one-fourth inch per foot, along with good drainage systems. This would solve most of the “ponding” problems.
It may also be that the profession needs to re-evaluate some of the assumptions. For example, snow loads have been determined for a 50-year mean recurrence interval. It seems that lately these 50-year storms occur every other year. I don’t know what exactly the effect of climate change is, but we may have to re-calibrate some of our numbers.
PBN: Finally, I know that professors often love real-world examples. Has this winter provided you and your colleagues a chance to impress on students the importance of good design?
TSIATAS: Study of failures, such as roof collapses, is indeed a very good tool to impress on students the importance of good design. Also, we remind them that most codes start by saying “to establish the minimum requirements” and that there are a lot of assumptions such as 50-year return periods. They need to use the their engineering judgment and decide what would be best in each case considering local conditions.
