Increased Snow Load Due to Reroofing

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Improving roof insulation reduces snow melt and could mean serious problems for your roof.

In 2015, the federal government of Canada decided to invest heavily in its aging fleet of patrimonial estates in Montreal. These buildings had been built in the early 20th century and had seen their fair share of winter storms. To usher them into the 21st century, the government had decided to focus their efforts on upgrading their aging roofs and, more precisely, improve their thermal efficiency. These old roofing systems had limited insulative properties as energy efficiency was an afterthought in those days.

Unfortunately, what initially seems like a harmless idea can prove to be costly if some structural concerns are not addressed. The increase of insulation reduces heat loss, which may have been melting the snow on the roof. Therefore, structures which had been designed for lighter snow loads than we tend to receive today and have stood the test of time are now subject to greater snow accumulations and can be prone to failure. For example, a flat roof building constructed in the 1950’s in Montreal had likely been designed for a uniform snow load around 35 lbs/ft2 (approximately 22” of snow) whereas the same building today would be designed for 52 lbs/ft2 (approximately 32” of snow) to respect the current National Building Code.

The National Building Code of Canada briefly mentions this concern in the Structural Commentary (NBCC 2010, Structural Commentary, Appendix L, article 35), but leaves it up to the engineer’s judgement on how to address it. Nonetheless, when such situations arise, the National Building Code requires upgrading the existing building to meet current standards (Commentary L), which can be costly.

In order to treat the various heritage building that the federal government was addressing in 2015, we had to develop risk assessment and eventual reinforcement plans, which catered to the particular challenges of each building. Through these projects, a common process was developed.

1. Consider whether an increase in insulation is necessary
Building owners are motivated to improve their insulation on their roofs for a variation of reasons, which range from the requirement to comply to current codes, ethical concerns with the lack of thermal efficiency and monetary savings on the long-term life of the building. If the decision is mainly economical, it is critical that these additional structural reinforcement costs be known as early as possible to judge the feasibility of the project.

2. Understand the building
Since records are rarely kept for older buildings, a detailed survey is usually required to identify the structure. The engineer should take this opportunity to pay particular attention to signs of wear or abnormal deflections, which may be a sign of weakness. Furthermore, the actual snow load on the roof can be sampled and tested by a qualified lab as well as compared to adjacent properties to verify whether high heat loss is melting the snow as hypothesised.

3. Perform an accurate analysis
Once all the information is gathered, the design team must regroup and evaluate the individual elements composing the structure. As the quality and strength of older construction materials are often unknown, specifically when the original drawings are not available, material sampling and testing should be considered so as not to rely on overly conservative assumptions.

4. Develop a reinforcement plan
As each building is different, a custom reinforcement plan should be developed. Joists and beams are likely the first elements to be reinforced as their loads are not cumulative, but if, the increase in snow load is considerable, the full building structure down to the foundations may need to be upgraded.

By applying this step by step approach to these projects, we were able to develop structured reinforcement programs, which could be integrated within the governments initial renovation projects without aversely affecting the natural flow in the building. The reinforcements ranged from simply upgrading secondary joists and primary frames at the Canadian Grenadier Guard’s Armoury to the installation of 6 impressive 45-meter-long post-tensioned steel arches to relieve various existing structural elements at the Côte-des-Neiges Armoury.

With climate changing more rapidly than ever and building codes lagging, it will become even more important for structural engineers to be involved in these reroofing projects at the onset. Building owners must be sensitive to these challenges and plan their renovations accordingly to avoid dangerous situations.

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