CSA S502:21 Managing Changing Snow Load Risks for Buildings in Canada's North

In 2021, the Standards Council of Canada’s Northern Infrastructure Standardization Initiative (NISI) funded the Canadian Standards Association (CSA) Group to update an existing standard on managing changing snow load risks for buildings in Canada’s North.

The objective of this Standard is to inform communities on measures for safe roof snow removal from existing buildings and for protection of building occupants and assets from overloading risks due to increasing accumulations and weights. Procedures that can reduce risks for roof and building collapses are outlined, including procedures for monitoring heavy snow and ice accumulations, safe removal of snow on roofs when needed, and for maintenance and snow removal planning.

As climate conditions continue changing, Northern communities need to be prepared to deal with the impacts arising from increases in snow weights (loads) on existing buildings. When the weight of the snow on the roof of a building approaches or exceeds its original design capacity to withstand heavy snow conditions, a condition of snow “overloading” is reached that poses risks for building roof collapse and the safety of building occupants and critical assets that are part of a building (e.g., communications and health care equipment). In addition to increased risks for the collapse of roofs, snow on the roof can cause water leakage (from ice dams) which can lead to structural concerns and health risks from mould as well as slip-and-fall hazards near the building. An abundance of snow around buildings can also lead to increased snow removal expenditures and hinder access to and egress from buildings, access to water and sewer services, influence permafrost warming and refreezing, and impact building maintenance.

The Standard provides maintenance procedures to reduce snow overload risk on existing buildings; monitoring, detection, and assessment methods for snow load risks on buildings; and procedures for snow removal.

Understanding and Assessing Impacts

The Standard states that planning for adaptation to climate change and overall building resilience requires using the best available and actionable climate science and climate change information, which should be followed with rational, legally justifiable methods, processes, and policies.

With increasing snow loads due to climate change, roof collapse is becoming a frequent occurrence. Increasing snow loads are a key area of concern not only for designers of new buildings but for owners of existing buildings that are suffering roof failures and other structural problems.

Failures are typically caused by a combination of events, including snow loads that exceed design and building conditions. Critical snow overload risks can result from increased depths, increased densities, snow drifts, rain-soaked snow, and unbalanced snow. Building condition contributing to higher risks includes improper maintenance, inadequate design, construction errors, and improper snow removal.

The most recent climate change science states that changes in temperatures, rainfall, snow accumulations, wind loads, erosion, permafrost thaw, accelerated weathering or deterioration of materials, and other severe weather events will all affect the safety and reliability of buildings and housing in Canada’s North. Specific to changing snow loads for buildings, the many changes in the Northern climate can affect how much snow falls in storms, how much snow accumulates on the ground, the density of the snowfall and snowpack, the amount of rainfall that falls on the snow, and the timing of the winter snow season.

Identifying Actions

Although snow load increases cannot be mitigated, proper planning, maintenance, assessment, and snow removal procedures can significantly reduce the risk of roof collapse.

Before the start of the snow season, a building snow removal plan shall be developed by the building owner, manager, and competent individuals. This plan shall describe the method for snow removal from the roof of the building during heavy snow winters. A copy of the plan shall be stored in an off-site location and updated a minimum of every 5 years or after significant additions/changes have been made to the roof. The snow removal plan should establish:

  • Whether snow can be safely removed from the roof in a timely manner by competent individuals
  • The competent individual who will carry out roof snow removal
  • The sequence for snow removal, including associated risks
  • That the snow removal plan does not create additional unbalanced snow loads
  • That occupational health and safety and fall protection requirements have been implemented
  • Any structural evaluations that should be considered to develop a plan for long-term remedial
  • Action when a roof has required frequent snow/ice removal
  • Barrier placement to protect the public, if required

The Standard also identifies actions to divert snow melt away from the roof and building and the use of snow guards or snow fences to retain snow and ice to minimize the potential for large volumes of snow to slide from higher-level roofs and slippery roofing material.

Implementation

The Standard outlines several protocols to be followed when completing snow removal. Snow removal from roofs should be undertaken if there are no adverse visual and audible warning signs of potential roof collapse and heavy community snow conditions are observed, and should not be undertaken during a blizzard, snowstorm, or other adverse weather conditions are present. Areas onto which snow will be dumped from a roof shall be secured and marked to protect pedestrians and prevent access to the building. Snow removed from the roof shall not be placed near the foundation of the building and shall be stored at a community-approved storage location well away from the building to minimize permafrost degradation and potential drainage issues.

Personnel removing roof snow shall note whether there are any hazards on the roof that might be hidden by the snow and need to be marked as trip hazards, including roof drains, skylights, vents, and other equipment. Snow removal personnel should always use caution by remaining alert to unexpected sounds or movement around surfaces that have been weighed down by snow (or water from melted snow) because these surfaces could collapse.

To reduce the risk of creating an unbalanced load on the roof, procedures for removing snow from a component of the roof should ensure that snow is removed infrequently in small amounts at a time, snow is removed uniformly across the roof, snow removal avoids the creation of additional snow piles on the roof or adjacent roofs, and snow removals leaves a 5 cm layer of snow on the roof to reduce the risk of damage to the roof.

Outcomes and Monitoring Progress

The Standard outlines building maintenance practices to reduce the risk of collapse. Practices that increase snow overloading risks that should be avoided include:

  • Re-roofing over three or more layers of shingles;
  • Modifications to the structure that cause weakening of the roof’s load tolerances;
  • The addition of mechanical units, antennae, solar panels, or other equipment to the roof without proper assessment or reinforcement;
  • The installation of additional insulation on the roof;
  • The conversion of interiors to open floor plans through the removal of structural and/or shear walls;
  • Any changes or additions made to the exterior or interior of a building that can affect its structural integrity

Building maintenance should be practiced to reduce risks of roof degradation and collapse and include, at a minimum, annual inspection and condition reporting of:

  • The roof and make necessary repairs as soon as possible;
  • Flashing and roof parapets for damage from previous snow loads and repair as soon as possible;
  • Metal members and connectors on roofs (especially older roofs) for corrosion and reinforcement as necessary;
  • Areas of previous ice build-up to identify solutions to prevent future issues;
  • Any visually apparent deflection of roof framing members or unexplained cracks in the building and seek expertise from a competent individual where this is present;
  • Documentation of cracks and other visual indicators of stress

Roof overload conditions are related to ground snow loads. Measurements of snow conditions should be conducted to determine the level of snow overloading risks posed to many of the buildings in a community, particularly during heavy snow winters. Community snow measurements provide valuable guidance needed to determine the relative severity of the heavy snow conditions, the need for roof snow removal, and the potential risks for roof collapses. Communities should monitor snow accumulations through:

  • Community measurements of snow depths or snow water equivalents taken by the community at a consistent location each winter;
  • Local knowledge of snow conditions that indicates unusually high snow depths or heavy snow weights, typically spanning 30 years;
  • Measurements taken near the community by agencies, provided that the snow conditions at the agency measurement site are representative of snow conditions in the community

Resources

Link to Full Case Study