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Snow and Wind Damage: Unravelling and Understanding the Root Cause

December 9, 2021

Forensic Investigative Analysis is crucial to understanding how damage is caused. During snow and wind storms, most times the cause of damage seems obvious such as the sagging of the roof after heavy snow, cracking in the ceiling due to snow, and damage to walls after hurricanes. In most cases, the cause of loss seems obvious but it is not. It is important to determine the root cause of the damage for the following reasons:

  • To determine insurance coverage
  • To address safety concerns of the structure
  • To correctly determine the extent of repairs

Structural Design 101

There are some basic structural concepts specified in structural design. A structure is designed to carry a certain amount of load or weight that is specified in the building design codes. The guidelines are mentioned below:

  • If the loading exceeds the specified design load, early warning signs will be visible on properly designed structures before a sudden collapse (Brittle Behaviour). Properly designed structures fail in Ductile Behaviour.
  • Early warning signs include cracks in the ceilings and walls, deflections, and cracks in doors and windows.
  • Structures are designed to fulfill the Life Safety (the ceiling should not collapse on you) and Serviceability (ceiling gypsum boards should not crack, doors should be operable) requirements of the building code.
  • If early warning signs of failure are noticed, the building should be inspected for Life Safety and Serviceability issues.

Here are some examples:

Case Study 1: Sagging Roof after Snow Season

If you find your roof sagging after the snow season, it might be due to the weight of the snow. While the roof definitely sagged due to the weight of the snow, there may also be underlying structural deficiencies.

On the roof, there is not just the snow load but the gravity load and the self-load of the structure as well. The gravity load is sitting on the rafters causing the rafter to compress due to these loads. The inclined compression force goes to some part of the columns and the other part creates a horizontal force known as outward thrust force. This is carried by the ceiling joist. The outward thrust force causes tension in the ceiling joist. The entire system is a complete load path.

During wind events, you have a suction on the roof. The rafters are going to open up causing the joint to be under tension along with the collar tie and columns depending on the wind speed.

The sagging in the roof had pre-existing construction deficiencies including an inadequate splice, inadequate rafters, misaligned rafters, and discontinuous rafters. These were the root cause of the damage.

Case Study 2: Crack in the Ceiling after Snow Season

In the attic, there were temporary construction struts. The roof structure consists of rafters, collar ties, and ceiling joists making it a complete load path. When a strut is placed, it alters the intended load path and they are supposed to be removed post-construction.

With the placement of the construction strut, the load goes through the construction strut to the ceiling joist rather than the rafters. The ceiling was not supposed to bear this load, so the root cause was the unintended load path and not the snow.

These are early warning signs which call for a structural investigation to understand the root cause of the damage.

Structural Design Concepts

  • Snow and wind loads are prescribed in building loads for every city in Canada
  • Snow and wind loads are based on maximum probable wind and snow loads in 50 years
  • Safety factors are used in structural design to ensure the presence of some safety margin in the building. If the load exceeds by a little bit, the structure can still bear the load
  • In the National Building Code of Canada, the snow load is amplified by 50% and the wind load is amplified by 40%

Case Study 3: Roof Collapsing after Snow

Our experts were asked to inspect the structure and it was found that the root cause of the damage was the long-term deterioration of the barn.

Case Study 4: Roof Collapse Under Snow

The barn roof collapsed after the snow season. The trusses are sitting on the wall studs. There is a plate at the top of the wall studs and the trusses are bearing on the plate. If the truss is removed, the plate cannot take the load as the plate becomes a beam. The barn collapsed due to non-engineered alterations as the owner put the plates without the beams.

Case Study 5: Partial Collapse of the Roof after Snowstorm 

The team at Origin and Cause was asked to inspect why the roof collapsed – if it was due to the snow, wind or if there was any other reasoning behind the damage.

By analyzing the weather data of the day and knowing the wind direction and wind speed, the team was able to calculate the snow accumulation. It was found that the root cause of the failure was the combined effect of the wind and snow proving in this instance the loads were the actual root cause of the collapse.

Case Study 6: Damage to the Barn

The barn was a 30 ft x 180 ft barn that faced damage to the braces due to the wind speed. The design wind speed was 91.8 km/hr and the actual wind speed was 98 km/hr. Although the actual wind speed was more, the root cause was not the wind as it fell within the safety margin (+40% for wind). The root cause of the damage was the pre-existing structural fault that to inadequate lateral bracing and support.

Case Study 7: Damage to the Door during Tornado

In 2018, during a tornado, there was damage to the door trim. The claim was that the gap was caused due to the tornado. Post investigation, signs of soil settlement under the garage were found. The flashing was pulling away from the exterior wall. The gap was narrow at the bottom and wider at the top. This was a clear indication of some sort of rotation of the garage due to soil sediment.

Case Study 8: Cladding Blown off By the Wind

The cladding was blown off during a wind storm. The actual wind speed was lower than the design wind speed. The fasteners were withdrawn from the wood girths and there was insufficient fastening and cladding to the framing. There was a lack of proper backing to prevent being blown off due to the wind.

The investigation also revealed that there was a discontinuous joist under the roof of the shelter. The true cause of the damage was pre-existing construction deficiencies.

Summary of Wind and Snow Damage

Snow and wind damage can be caused due to:
  • Extreme snow/wind that exceeds design code values
  • Pre-existing design/construction deficiency
  • Non-engineered alterations
  • Un-intended load paths
  • Long-term deterioration
Forensic Investigation is required to determine:
  • The true root cause of the damage
  • To determine the extent of damage repair
  • To prevent failure in the future

Origin and Cause: Your Choice of Forensic Investigative Firms

As stated above, Origin and Cause has inspected various scenarios of wind and storm damages and has been able to determine the true cause of damage. It is important to not take the case at face value but to investigate every piece of evidence thoroughly to understand the true cause of the damage. If your structural design is showing any kinds of early warning signs, contact Origin and Cause to help figure out the actual cause of damage and take corrective action before any major damage occurs. If you’ve faced any form of damage due to wind or snow, we are here to help determine what caused the structure to collapse. Contact us at 1-888-624-3473 to conduct a thorough, evidence-backed forensic investigation.