Beware Of Falling Ice And Snow: A winter perspective on building design

by Elaina Adams | January 1, 2012 3:16 pm

Photo courtesy Northern Microclimate Inc.[1]
Photo courtesy Northern Microclimate Inc.

By Mike Carter, CET, and Roman Stangl, CET
On a cold February morning, as people walk to their new downtown office buildings (in Toronto, Montréal, Winnipeg, Calgary, or Ottawa), a loud crash is heard, sending them scattering outside the main entrance. Ice shards rain down onto the sidewalk, with a large ice chunk smashing the ground directly in front of the main doors. This is not a Hollywood disaster movie blockbuster, but an actual, all-too-often occurrence.

Design and construction practices are changing in many ways—trends toward energy conservation, unique architecture, taller buildings, and use of advanced technologies and materials—and are having an impact on the performance of a building within its environment. Granted, many projects go through significant scrutiny before being commissioned, but the issue of falling ice and snow is seemingly not in the forefront.

Understanding how the completed building performs under the adverse conditions of a winter storm event seems to be absent from the current design process, or at least low on the priority list. Façades of high-rises are commonly tested during the design stage to investigate their performance for water penetration, air infiltration, thermal performance, and structural capabilities. However, only a few are ever tested for ice and snow collection and release. Consequently, the reported incidents of falling ice and snow in urban areas is increasing throughout North America.

Physical testing of façade components for tall buildings in harsh high-altitude climates. Photo courtesy Mount Washington Observatory[2]
Physical testing of façade components for tall buildings in harsh high-altitude climates.
Photo courtesy Mount Washington Observatory

Rise of falling snow consulting
The history of consulting and investigating in this field started with some well-meaning engineers and technical folks who were approached with reports of ice and snow falling from buildings and thought to themselves, “We can fix that.” Experience was gained over time as reputations grew and successful solutions were implemented. Soon, clients came with more challenging questions, such as, “Can you look at my drawings and tell me how to avoid problems in the future?” or, “How small can we make the solution?” These tasks required investigation and research.

This involved quantifying the effectiveness of various mitigation options or design modifications, along with physical testing in cold-room research laboratories or at outdoor venues like Mount Washington Observatory in New Hampshire (Figure 1). There is, needless to say, much dedication required for those who choose to be responsible for conducting and observing a high elevation icing test, intended to replicate the top of a super-tall structure somewhere else in the world, by standing atop the observatory lookout in freezing cold temperatures, with the clouds rushing by below you. Mount Washington, after all, is where a wind gust of 372 km/h (231 mph) was measured in April 1934—the highest wind speed ever observed.

Example of a sliding snow mass on a slippery roof that will fall to the ground below. Photos courtesy Northern Microclimate Inc.[3]
Example of a sliding snow mass on a slippery roof that will fall to the ground below.
Photos courtesy Northern Microclimate Inc.

The gathering of this type of knowledge and experience has led to some interesting realizations regarding industry trends and the need for education within the building profession. First, on review of numerous building designs and incident reports, it is evident the design industry is not properly informed regarding ice and snow on building details. This is particularly the case for sloped roof surfaces, whether they are asphalt, slate, metal, or membrane. The occurrence of ice dams causing leakage, sliding snow causing damage (Figure 2), or the generation of dangerous icicles that fall from roof edges is common (Figure 3).

The realization the design industry as a whole has very little guidance from building codes or standards and little in the form of formal education to address ice and snow on buildings, outside of static structural loads, was surprising. The attitude of some designers is the responsibility for dealing with snow and ice belongs to the structural engineer. Consequently, as ice and snow consultants, this article’s authors find the first task is often the education of the design team regarding the typical microclimate conditions that will exist on their proposed building in the winter months.

In short, we strive to put a picture in the minds of the architects of their proposed building on February 22, rather than August 1 (most of the beautiful project renderings on drawing set covers tend to be in the summer). This education is done through descriptions of typical winter storm events and photographs of buildings with the resultant problematic snow and ice formations. This insight often creates expressions of concern or just utter silence in the meeting room. “That won’t happen on my building…will it?” The answer tends to be, “Yes, it is possible”.

Example of ice and icicles at a roof edge.[4]
Example of ice and icicles at a roof edge.

Technological impacts
A second realization about the design industry is regarding the very positive and progressive trends of advanced technologies and energy conservation. In the days before energy conservation, buildings were constructed as large simple boxes that leaked heat from every façade and surface. Single-pane glazing, double-brick walls, and badly insulated attics created relatively warm exterior surfaces, which would melt ice and snow on contact (or soon after).

One might think Canada’s extreme cold periods would cause a building’s outside skin to dip below freezing. While this does occur, the largest and most problematic ice and snow storms tend to happen when air temperatures are just below or above freezing. Therefore, a majority of the time snow and ice occur, they just melt off the building.

In these modern times of extreme insulation, double- and triple-pane glazing, and very efficient internal heating strategies, the outside skins of buildings are much colder. To further complicate matters, the industry no longer constructs simple boxes. The built environment has progressed to unique curves, slopes, and shapes that gather more snow than would a simple vertical wall topped by a tall parapet at the edge of a flat roof. Adding insult to potential injury, heat-reduction strategies (e.g. solar shading devices) are often specified for the skin’s exterior, with little thought to the implications. The result is Canadian buildings that are in effect ice generators. The accumulated snow can age and harden into ice and icicles that fall great distances to the building’s base, thanks to the influence of wind (Figure 4).

Example of an ice piece that fell from a roof edge onto a sidewalk.[5]
Example of an ice piece that fell from a roof edge onto a sidewalk.

Technology has allowed us to build bigger, taller buildings than ever before. This has led to structures that literally have their heads in the clouds, where it is cold and wet (i.e. ice and snow). This technology has also resulted in large stadiums, arenas, and shopping malls with big curved or sloped roofs where large volumes of ice and snow can slide or be wind-drifted into snow dunes.

Modern design has allowed for unique building shapes with hills, valleys, troughs, and snow shoots. As initially stated, technology and energy efficiency are positive trends, but they are also creating some unintended hazards that are relatively simple to deal with if they can be appropriately identified and addressed.

The renovation or retrofitting of existing buildings with additional insulation can also be problematic, when it comes to ice and snow. This added insulation changes the façade’s performance and, in some cases, leads to hazardous ice and snow conditions in areas that have otherwise done well for many years.

The investigation process
The reality is we have learned from others’ mistakes, as well as from physical testing of proposed façades, and from constantly progressing the experience and science behind falling ice and snow. Thus, as awareness grows around the topic, it is the authors’ hope more designers will be willing to investigate performance.

Certain façade features can pose problematic when it comes to ice and snow collection.[6]
Certain façade features can pose problematic when it comes to ice and snow collection.

To apply the knowledge gained by the misfortune of others and past test results, a consultant with significant experience contributing to the design team is a good beginning. He or she typically provides an evaluation of historical meteorological records to establish the exterior microclimate. These results would then be combined with an experience-based measure of ice and snow accumulations on all exterior surfaces, in the context of the overall façade. Once an estimation of potential is created, this result is expressed in terms of a relative frequency of occurrence and severity of potential hazard.

To place judgment on the result, a project-specific criterion is developed to assess if the predicted results are acceptable. This last point brings light to a significant issue with which designers struggle in the absence of prescribed building code regulations. What is an acceptable result with respect to falling, sliding, or windblown ice and snow from a building? Should a design be evaluated by the size of ice pieces that could fall? Or should the criterion be focused on snow depth and density similar to snow load criteria? Could a light snowfall create a significant hazard on a particular building geometry? (The answer is, invariably, “Yes.”) Or should the criterion focus on other buildings in the region to set the standard? Currently, there is no defined decision or guidance from building codes to answer this question.

Choosing a path
To address concerns or predicted potential hazards from ice and snow, the mediation options are generally expressed in one of three categories.

The design industry generally has very little guidance from building codes or standards when it comes to addressing ice and snow on buildings. Still, a pylon seems like a simplistic response to a real threat.[7]
The design industry generally has very little guidance from building codes or standards when it comes to addressing ice and snow on buildings. Still, a pylon seems like a simplistic response to a real threat.

Design modifications
This category is relatively self-explanatory—recommendations to modify specific details of a building design are made. This is the most cost-effective means to reduce risk and can often mitigate issues with small-scale modifications not having a significant impact on esthetics.

Examples can include:

Another example is in reference to larger building forms that, with slight modifications of shape, size, or location (e.g. mechanical penthouses) can reduce problematic ice or snow formations.

Mitigation measures
This category comprises various measures of which some are off-the-shelf products, and others are project-specific strategies created to match the individual building design. The most common of these are snow guards or cleats that can reduce falling snow, or the use of heat trace to remove melt water and reduce ice and icicle formations.

Heat-reduction strategies, like these solar shading devices, can be problematic in cold climates when not properly specified.[8]
Heat-reduction strategies, like these solar shading devices, can be problematic in cold climates when not properly specified.

However, it has been shown time and time again if items such as snow guards or heat trace are incorrectly used or installed, they can actually create more severe conditions—therefore, expert advice is prudent. Regarding esthetics, the integration of mitigation measures directly into the design is often desirable. This type of solution requires the implementation of ice and snow strategies early in the design stages.

Operational protocol
This last category has two aspects to it. First, it is important to understand the potential for hazardous ice and snow can be reduced, but not eliminated. Therefore, the need to monitor and inspect a building during the winter season is always recommended. This process is best conveyed via the development of ‘winter operational protocols’ that can be provided to building operators upon completion of a building.

A comprehensive protocol can be both proactive as well as reactive, prescribing:

This becomes an excellent way for the design team to convey to the owner how the building is expected to perform in the winter season.

Frozen stalactites hang ominously on this low-rise building. Photo © Gregory Johnston/[9]
Frozen stalactites hang ominously on this low-rise building.
Photo © Gregory Johnston/

Second, the extent of mitigation or design modifications incorporated into the design can neither eliminate all potential hazards nor anticipate all extreme, severe, or unusual weather events. Additionally, the mitigation of falling ice and snow hazards is designed for more typical winter weather conditions and not tailored to these extreme weather events. Therefore, developing a winter operational protocol can further convey to the owner what actions to take before, during, and after these events to maintain a safe operating condition for the completed building.

Moving forward
Once a mitigation strategy has been developed and accepted by a design team, and the respective drawing details have been sketched up, typically one of two avenues toward a final design becomes evident. Either:

Validating and optimizing modifications or mitigation measures typically occurs in a cold-room laboratory using full-scale, thermally correct mockups of the façade that can be manipulated for fine-tuning. A cold room is a controlled environment where a test assembly can be put through warm and cold cycles, and bombarded with ice, snow, sleet, and wind in various combinations to learn key performance aspects of the assembly.

It is the authors’ hope this article has created awareness over the potential issue of falling ice and snow from buildings, along with the significant influence the industry trends of technology and energy reduction are having on façade performance. It is also our hope design professionals become more aware of the experts available with direct experience who can review proposed building designs and identify early in the process possible concerns, and then bring a best practice approach to reduce risk. The application and documentation of industry best practices, during design or immediately after an incident, is an excellent way to demonstrate due diligence with respect to falling ice and snow.

Mike Carter, CET, is a director and the lead technical consultant of Northern Microclimate, an architectural consulting firm that focuses on the prediction, evaluation, and mitigation of falling ice and snow. A member of the Council on Tall Buildings and Urban Habitat (CTBUH), he has 16 years of experience assessing the performance of the built environment with respect to the exterior microclimate, specifically in relation to cold climates. Carter gained this experience by conducting a significant number of scale model wind tunnel studies, water flume snow simulations, and full-scale laboratory and in-the-field research projects over his career. He has consulted on projects throughout the world, ranging from hospitals, sport stadiums, and museums, to super-tall buildings and structures. Carter can be contacted at 

Roman Stangl, CET, is a director and the lead project manager of Northern Microclimate. A CTBUH member with international experience, he has spent more than six years assessing issues that exist in the built environment and surrounding local microclimate, specifically in relation to cold climates. Stangl can be reached via e-mail at

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