Over the past several years, climate change has been tied to a rise in the frequency and intensity of extreme weather events, such as wildfires, high winds, and heavy snowfall. The resulting increased snow loads can cause serious damage to structures, including cracks in the ceiling, sagging roofs, and total roof collapse. The costs of these claims can range anywhere from a few thousand to hundreds of thousands of dollars and present a major concern for insurers. However, while snow loading on roofs may be a contributing factor, these damages are often symptomatic of other preexisting issues, as forensic structural engineers have found.

 Canadian Building Codes

Canadian building codes require buildings to meet two main types of design criteria: Serviceability Limit State (SLS) and Ultimate Limit State (ULS).

The SLS ensures that buildings remain functional and comfortable under normal, everyday conditions. One example of exceeding SLS includes excessive deflection – or sagging – of joists and rafters. While the building may remain structurally sound, excessive sagging in the floor may cause problems like cracked tiles or jammed doors, which hinders the intended functionality of the building.

The ULS ensures that buildings have adequate strength under design loads to protect life safety. For example, ensuring that roof rafters will not break under the anticipated maximum snow load during the building’s lifespan. Exceeding ULS imposes safety risks on occupants as the building becomes susceptible to serious structural failure.

Probabilistic Approach to Structural Design

Failure occurs when the demand placed on a structure – the load – is greater than the structure’s capacity to carry or resist those loads. However, neither the loads on a structure nor the structure’s load-carrying capacity can be predicted with 100% accuracy. Both loads and material properties have some degree of variability. A designer cannot reduce the probability of structural failure all the way to zero, but they can make the chance of failure really small, say less than 1 in 1,000,000, by making sure there’s a wide enough safety margin between the expected load and the expected capacity.

On the load side of the equation, engineers calculate the expected maximum load on the building during its service life, and then increase that load for design so that it is improbably high. On the capacity side of the equation, engineers calculate a low estimate of the nominal capacity. In other words, engineers use a low estimate of the strength of the material, where at least 95% of the time the true strength is higher. Then they reduce that nominal capacity for design so that it’s an improbably low estimate. The total safety margin comes from the combined effect of using a low estimate of the structural capacity and a high estimate of the maximum load on the building when checking the structural design.

Understanding Snow Loads on Roofs

In Canada, snow load design values are set out in the national and provincial building codes and are based on climate data for each geographical location. These design values have been determined by the Meteorological Service of Canada based on historical snowfall records. As more data becomes available the design values for a particular location may occasionally get adjusted, but no matter how many years of weather data we have, there is always going to be some uncertainty when using historical data to predict the future.

The weight of snow on a roof is estimated from the ground snow load, with some adjustments to account for factors like snow sliding off steep roofs or snow drifting around obstructions. Of course, there is variation in snow loads on roofs because the weather is never the same from one year to the next. The amount of snow on a roof is not only affected by the amount of snowfall, but also daily weather conditions like temperature, humidity, wind speed and direction, and cloud cover. The daily weather factors that can influence the snow load simply can’t be accurately considered in a design calculation because that future weather data is unknown, and even if it wasn’t, the high engineering cost of doing such a high-fidelity snow load calculation wouldn’t be justifiable on most building projects. So the way engineers translate ground snow load to roof snow load isn’t perfect, but it’s a reasonable approximation that has been validated by years of research data.

Because there’s such a large degree of variability in snow loads, the nominal snow load is typically increased an extra 50% for design. So even if the actual snow load exceeds what’s specified in the building code by a little bit, if the load is still within the safety margin, a properly designed roof shouldn’t collapse. A roof isn’t expected to collapse unless it sees a truly exceptional load that exceeds even the safety margin.

What to Blame

If the snow load exceeds the design value but is still within the safety margin, any observed signs of failure almost always indicate other underlying issues. In such cases, it becomes necessary to rule out these underlying issues before blaming the snow load. Common issues found during structural investigations of weight-of-snow claims include:

• Unintended and undesired load paths due to the way the roof was framed
• Under-sized structural members
• Improper connections
• Structural alterations that were not designed by a qualified professional
• Long-term deterioration/lack of maintenance

Under normal circumstances, pre-existing conditions like under-sized structural members often remain hidden for years. However, these latent structural problems will manifest themselves under extreme weather conditions such as an unusually heavy snowfall. Under repeated exposure to snowfall over many years, a roof with pre-existing deficiencies may exhibit a gradual progression of cracking and sagging until damage becomes noticeable.

Buildings that were originally designed and constructed to meet code requirements can also sustain damage from snow on the roof. In these cases, during structural investigations, it is common to find there has been some past alteration to the structure that was not properly designed, or that the building has not been maintained properly and the structure has been deteriorating for years. For example, prolonged exposure to moisture from a roof leak could lead to corrosion of steel and decay of wood. Left unchecked, a structure could deteriorate to the point where it can no longer safely carry snow loads until one day it fails. In this scenario, the real cause of the failure is the long-term deterioration of the structure, not the weight of snow.

Here are some examples of preexisting problems discovered during structural investigations that were exposed by heavy snowfalls:

Post-Snow Roof Damage

Figure 1 shows inadequately spliced rafters for a roof that sagged after a heavy snowfall. The snowfall was about 20% above the design value for serviceability but was still well within the safety margin for life safety. However, due to the pre-existing construction deficiencies in the wood framing, the roof experienced large deflections. Rafters generally shouldn’t be spliced, though a situation can sometimes arise where a splice is needed. Rafter splices must always be designed by an engineer. This splice was improvised by a contractor.

Unintended and undesired load paths can also cause ceiling cracks after a heavy snowfall. One such example is shown in Figure 2, where temporary struts were left in place after the completion of the roof construction. These struts are only installed to facilitate the construction of the roof and are not intended to carry or transfer any roof loads. Framers are supposed to remove these temporary struts after the roof framing is completed. In this house, the struts were left in place. As a result, a portion of the roof load was transferred to the ceiling joists below through the struts. Since the ceiling joists were not designed for the unintended transferred loads, they deflected more than expected and the ceiling finishes sustained damage.

Figure 3 shows an example of an aluminum pergola that was originally designed and constructed as an open structure. Without completing a structural assessment of the pergola, clear panels were later installed by the owner to keep the rain off the deck. During winter, snow and ice collected on the new roof panels, but the pergola structure had not been designed to carry the weight of accumulated snow and ice. Shortly after a major snowfall, the pergola collapsed.

Figure 4 shows a fabric-covered arched building in western Ontario. The owner purchased the building from a dealer at a discount. What the dealer had not disclosed was that the building had been designed and fabricated for a customer in the southern U.S. who later cancelled the order. The loads used in the design of the building were significantly lower than the loads prescribed by the Ontario Building Code. The building collapsed after a major snowfall because it had not been designed for use in the region where it was delivered.

Figure 5 shows a barn structure that collapsed due to decay of the wood framing near the bearings of the gambrel frames (Figure 6). Damp conditions are often found in barns. Common sources include roof leaks, livestock, and improper drying and storage of hay. Many barns are also not well-maintained. However, even the most well-constructed buildings will eventually fail if moisture-related deterioration is allowed to progress unchecked.

Like all structures, roofs are designed with safety margins specified in the applicable building codes. Heavy snow loads do occur, sometimes even exceeding the nominal value specified in the code. However, properly designed buildings incorporate safety margins to guard against structural failure even during these rare snowfall events. If structural damage occurs while snow loads remain within the safety margin, there is almost always some other underlying issue, often a problem that went undetected for many years. In these cases, the snow load is only the proverbial last straw that sets the failure off; the pre-existing conditions are the real cause of the failure. It is only by discovering the root cause of the failure, through structural investigation, that you can determine the proper plan of repair and the appropriate amount of insurance coverage.

Nabi Goudarzi, Ph.D., P.Eng.

Nabi specializes in the forensic structural investigation of buildings and the design of retrofitting schemes for damaged or under-designed structures. In the course of his career, Nabi has conducted structural assessments of existing steel, concrete, masonry and wood buildings under gravity loading, vibration and seismic forces. He has also designed steel and concrete structures, including high-rise residential and office towers, commercial retail units, and long-span steel trusses. Nabi holds a MSc. in Earthquake Engineering from Sharif University of Technology and a Ph.D. in Structural Engineering from the University of Alberta.

 

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