For those of us in the colder parts of the US, winter brings some risks. Snow and ice are dangerous for pedestrians and drivers, but they also pose a structural risk to buildings. Too much snow and ice can cause otherwise stable roofs to fail, but they aren’t the only things that impact structural integrity. Here’s what you need to know to keep your home and offices safe this winter and beyond.
What is structural integrity, and what makes a structure “stable”?
Structural integrity means that a structure is able to withstand a reasonable weight load without breaking or deforming. Structures need to be designed to handle code-required loads, including the weight of all building materials, and need to be able to last under standard operations. Code-required design loads accommodate for the additional weight of furniture, HVAC equipment and occupancy, depending on intended use of the facility. The stability of any structure is highly dependent on the design loads not being exceeded.
What causes structures to fail?
While there are many things that can impact structural integrity, common threats include:
- Deterioration, where things like water damage or pest infestations will cause structures to weaken and eventually fail.
- Improper use, where structures are used for an unintended purpose. For buildings, this could be removing a load bearing wall, or adding rooftop equipment without recalculating or compensating for the change in structural integrity.
- Construction errors or defective materials. If materials are improperly installed or aren’t as strong as a non-defective material would be, structures won’t be as strong as they were estimated to be. On paper, a roof may be strong enough to say, add photovoltaic panels – but it could be disastrous if an otherwise weakened roof is burdened with extra weight.
- And, of course, damage from natural forces – like blizzards, high winds, or falling trees.
When it comes to roofs, which type is the best for snowy regions?
Ultimately, it comes down to pitch. Steeper inclines are harder for snow to cling to, so snow does not accumulate as fast or for as long as flatter roofs. Most of the time, snow will melt and slide off a pitched roof before structural integrity becomes a concern. This is why most homes in places like New England and Upstate New York have gable-style roofs. Generally, for snow-prone areas, a roof pitch of at least 4:12 is recommended.
If pitched roofs are stronger, why do many buildings in snowy areas still have flat roofs?
While flat roofs are not ideal for snow, they are better suited for commercial style facilities due to the facility’s larger footprint and flat roofs being more cost effective to build. They are also ideal for buildings that need rooftop equipment as they are more easily accessible for maintenance. And typically, snow will melt before it poses a risk to structural integrity.
How much snow is too much? What should home and business owners know to keep their facilities safe this winter?
The maximum amount of snow a roof can hold will differ depending on the size and condition of the roof as well as the type of snow. Fresh snow is lighter than settled snow, and ice is heavier than even very wet, tightly packed snow. One blizzard probably won’t affect your roof, but frequent snowfall and sustained cold temperatures will build up fast.
BL should know – in January of 2011, Connecticut and Massachusetts had over 50″ of snowfall in one month, putting many roofs at risk for failing. BL was tasked with providing emergency roof inspections for over 90 buildings within a three-week span to determine which buildings needed to be evacuated and roofs repaired, and which were still stable.
During the visits, visual observations were made to first determine whether there were apparent indications of structural stress. The building structures were then measured and evaluated, or, if as-built drawings were available, the structures were verified against the drawings and then analyzed. Our engineers then climbed to the roof and evaluated a cross-section of the snow to determine its depth and weight for comparison to the structural integrity of the building. Based on the resulting determinations, recommendations were made to the building owners regarding whether snow removal or no additional work was needed. In some instances, the analysis also generated discretionary recommendations for the owners’ consideration. Where required, a return visit was made to confirm that remedial measures were sufficient for continued occupancy of the buildings.
BL evaluated everything from schools and municipal buildings to retail stores to medical clinics, working from early morning through the late evening, seven days a week to make sure all buildings were evaluated quickly, repairs or snow removal could begin, and owners could get back to their normal operations.