As the year comes to an end, it’s time to think about winter’s potential threats to your building’s roof. If we lived in a perfect world, we would never have any roof collapses, because: – Roofs would be properly designed to meet code requirements for snow loading, including any sliding or drifting conditions; – Buildings […]
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As the year comes to an end, it’s time to think about winter’s potential threats to your building’s roof.
If we lived in a perfect world, we would never have any roof collapses, because:
- Roofs would be properly designed to meet code requirements for snow loading, including any sliding or drifting conditions;
- Buildings would be properly built and inspected during construction to ensure there are no significant structural flaws;
- The roof membrane and drainage systems would be properly maintained, and replaced at the end of their service lives;
- The impacts of changes would be structurally assessed, and roofs reinforced or modified as needed, so as to not create any fragile conditions;
- Roofs would see no more snow-load than that prescribed by the building code.
However, the world is not perfect, and roofs do collapse. It has been our experience that roofs not meeting the first four conditions above represent nearly all of the collapses that happen. In the dozens of collapses (full or partial) that we have investigated, not one has occurred as the result of a roof experiencing snow loading that exceeds that specified by the building code of New York state for new structures.
Buildings that are more than a few years old have had their roofs load-tested by Central New York winters, and so the likelihood of any undetected major design or construction defect is reduced. However, this effect is erased if the roof’s loading conditions have changed. Such is the case when old roofing is replaced by a new roof system with more (or more effective) insulation, or a more effective air barrier. This has been an explicit requirement of New York state’s Energy Conservation Construction Code since 2010.
Obviously, less snow melt can result in more snow buildup. Coupled with the fact that older roofs were designed under older codes that may have had lower snow-load requirements, especially at drift conditions, it becomes clear why the majority of collapses that we have seen have occurred in the years following a roof-replacement project.
So what can building owners and facilities managers do to minimize the risk and reduce the worry and stress caused by not knowing whether they can trust their roofs? Here are a few suggestions:
- Look at your roof from above and below. Is it leaking and causing structural deterioration? Is the drainage system working properly? Is there any damage visible? Are there large hung loads or rooftop loads that don’t seem to be well-supported?
- Has the roof been replaced within the past few years? If so, was the structure given a review by a structural engineer?
- Have there been any additions — large rooftop mechanical units, penthouses, or adjacent building additions — that may have created new drift conditions?
It’s much better and easier to assess a roof’s structural capacity before it’s covered with snow. However, if you find yourself concerned about a snow-loaded roof, here are some things you can do:
- Make sure the roof drains are cleared to allow water from melting snow to drain away, and not get absorbed by the adjacent snow.
- Compile any existing drawings of the building, especially structural, so that a structural engineer can quickly assess the design capacity, if need be.
- If you decide to remove snow from the roof, be extremely careful, on several levels. In general, such work involves issues of personal safety. In addition, damage to membranes can occur unless extreme care is exercised with plastic shovels. Also, develop a structurally sensible plan for removal. Roofs have collapsed during removals due to the inadvertent creation of unbalanced loading conditions.
- Be aware of unusual noises, ceiling or roof sagging, or tilting or cracking of interior partitions. Most roof collapses are preceded by warning signs that can serve as a signal for evacuation. Make sure egress paths are clear and functional, including non-snow-blocked exit doors (which is required of all buildings, per the Property Management Code of NYS).
Finally, if planning a roof-replacement project, plan for success. Ensure that the new roof complies with the energy code for insulation and air-barrier requirements, and conduct at least a “sanity check” of the structure for potential load increases. We can — and should — make our buildings more energy efficient, and do it in such a way as to not cause any dangerous or debilitating conditions, and stop unnecessary worry. That way, we can all enjoy the winter.
Jim D’Aloisio is a principal with Klepper, Hahn & Hyatt, a structural engineering, landscape architecture, and building-envelope services firm based in DeWitt. Contact him at jad@khhpc.com