Under the Weather: Getting to the Root Cause of Wind and Snow Damage
June 24, 2022
Often, the cause of damage seems very obvious in a forensic structural investigation. Sagging in the roof or cracking in the ceiling after winter may be attributed to snow. In reality, these types of claims are a lot more complex. Therefore, it is essential to determine the root cause of the damage. We do this for three main reasons:
- To determine the insurance coverage.
- To determine the required repair.
- To address safety concerns of the structure.
Basic concepts in structural design
A structure is designed to carry the loads specified in the building codes. If the loading exceeds those design loads, a properly designed structure will show early warning signs before a sudden collapse. This is known as brittle behaviour. An adequately prepared structure will fail in ductile behaviour. Examples of early warning signs of failure include:
- Cracks in the walls, windows, or ceilings
- Inoperable doors and windows
- Determine the root cause of the damage
- Determine the proper structural repair
- Prevent major failure in future.
- We are licensed to fly across Canada
- We have drone operators in Eastern and Western Canada
- Our drones can fly in cold, hot, and windy conditions
- We can live broadcast video by drone
Structures are designed to satisfy the building code’s life safety and serviceability requirements. For instance, the ceiling falling on your head is a Life Safety concern. If doors or windows are inoperable after some time, that is a Serviceability issue. If early warning signs of failure are observed, the structure should be inspected.
Example 1: Sagging roof under snow load
Figure 1: Roof deflection after snow season.
A sag in the roof after winter can be attributed to snow. However, it could be due to structural or constructional deficiencies.
Figure 2: The complete load path in a roof structure.
The gravity loads shown in figure 2 are dead loads, the self-weight of the structure, and the snow load. These loads sit on the rafters and move through the rafters to the columns. Because of these loads, the rafters are under compression. As a result, some part of the inclined compression force at the joint goes to the columns through compression. The other part of it creates outward thrust force. This force is carried by the ceiling joists, where it generates tension force.
Figure 3: A demonstration of suction under wind.
During a wind event, there is suction on the roof. The rafters open up, as a result, meaning the ridge board is under tension. The collar ties and columns are also under tension, depending on the wind speed.
Figure 4: The rafters, collar ties, and ridge board in a roof, with lateral bracing of the collar ties and bracing of the gable roof.
We accessed the attic for a more detailed view of the structure. Pre-existing construction deficiencies were observed, including inadequate splice and undersized rafters.
Figure 5: Structural deficiencies in the roof.
Rafters should always be continuous and properly sized. They also needed to be aligned in some areas.
Figure 6: The misaligned and discontinuous rafters in the attic.
Our investigation revealed that pre-existing structural deficiencies caused the loss.
Example 2: Cracks in the ceiling under snow
Figure 7: The cracked part of the ceiling.
Here, the crack was formed under the weight of the snow. However, we needed to find the root cause.
Figure 8: An overall view of the attic space.
We found temporary construction struts in the attic. Ideally, the roof should have a complete load path comprising rafters, collar ties, and ceiling joists. Construction struts are typically placed during the installation of the ridge board or rafters. They are supposed to be removed after the construction. Otherwise, they cause cracking.
Figure 9: Illustration of a roof section with an unintended load path.
If there is a construction strut on the ridge board, then the loads on the roof go through the construction strut instead of the rafters. Then they go to the ceiling joist, which is intended for something other than that load. Therefore, we concluded that the root cause was an unintended load path.
Example 3: Cracks in the ceiling under snow
Figure 10: The cracked ceiling.
Cracks in the ceilings or sagging in the roof are usually early warning signs of damage. Therefore, it is important to do a structural inspection to determine the root cause and pre-existing conditions. In this example, there were cracks in the ceilings. Inspection of the attic revealed that the original construction had two trusses on either side of the wall.
Figure 11: An illustration of a roof section.
The owner added new rafters and collar ties. However, there was no continuous ceiling joist or rafter tie.
Figure 12 illustrates a roof section demonstrating a discontinuous ceiling joist.
Because the ceiling joist was not continuous, it could not carry the tension force. Hence, it formed cracking. Cracking must always be inspected to determine the root cause of the damage. This also helps determine the correct repair method.
Figure 13: The attic view of the house.
The root cause for this loss was an unintended load path. When adding rafters to the top of the trusses, the engineer should have appropriately connected the trusses to ensure continuous rafter tie and ceiling joist.
Structural design concepts
Snow and wind loads are prescribed in building codes for every city in Canada. Additionally, snow and wind loads are based on the maximum probable wind and snow loads in 50 years. In structural design, we use safety factors to ensure some safety margin in the building. This way, if the loading exceeds the design load by a little bit, the structure can still carry that load.
Safety factors in structural design
In the National Building Code of Canada, snow load is amplified by 50%, and wind load is amplified by 40%.
Table 1: An example of actual snow and wind loading values vs. the design values set in NBC 2015.
When designing the roof, 50% must be added to these provided values. If ground snow is below the design value, the roof should still carry that load without breaking. It might deflect more, but it should not break.
Example 4: Roof collapse under snow
Figure 14: The collapsed barn roof.
The roof of a barn collapsed after a snow season. Our inspection of the structure revealed trusses that were sitting on wall studs.
Figure 15: The components of the damaged roof
There was a wall stud under every truss and a plate at the top of the wall studs, with trusses bearing on the plate. If the truss is removed, the plate cannot take the load because it becomes a beam.
Figure 16: An illustration of the failure of the bearing plate.
The owner made big openings in the structure without actually putting a beam, as indicated in figure 16. As a result, the top plates failed, and the barn roof collapsed. Therefore, we determined the root cause of damage to be non-engineered alterations.
Example 5: Failure of the roof after snow
Figure 17: The collapsed barn roof.
Our drone inspection of this structure revealed the root cause of the damage to be the long-term deterioration of the barn.
Example 6: Failure of the roof after a snowstorm
Figure 18: The collapsed roof.
A roof partially collapsed after a snowstorm. We inspected the structure and obtained the weather data for the day of the loss. This data provided the wind direction, speed, and snow accumulation in the area circled in figure 19.
Figure 19: An aerial view of the building.
The root cause of this collapse was the combined effects of snow and wind. In this case, the loads were the root cause of the collapse.
Example 7: Damage to a barn after wind
The braces of a barn broke after a windstorm. The barn only had cross bracing at the ends.
Figure 20: A side view of the barn, with actual and design wind speed specifications.
The actual wind speed on the day of the incident was within the safety margin. If the barn was designed properly, the brace should have stayed intact under this wind speed. This was an indication of pre-existing issues. We discovered that there needed to be more bracing for the barn.
Example 8: Cladding blown off by the wind
Figure 21: The damaged structure.
In this case, the cladding was blown off during a rainstorm. The actual wind was 82 km/h, whereas the design wind was 84.7 km/h. So, the actual wind speed was lower than the design speed.
Figure 22: Insufficient fastening of cladding observed in the structure.
Our investigation revealed that the fasteners were just screws withdrawn from the woodcuts. This was insufficient fastening of cladding to framing. Cladding should be properly fastened to a proper backing, so it does not get withdrawn easily by wind.
Figure 23: Illustration of the discontinuous joist observed.
We identified other pre-existing construction deficiencies and a discontinuous joist on the roof. Therefore, the true cause of the loss was pre-existing construction deficiencies.
Example 9: Damage to a house after a tornado
The door trim of a house showed damage after a tornado in 2018.
Figure 24: The widening gap from bottom to top.
The claim was that the gap shown in figure 24 happened because of the tornado. However, we found signs of soil settlement under the garage.
Figure 25: Flashing pulling away from the wall.
One sign of soil settlement was the flashing pulling away from the exterior wall of the building. The gap was narrow at the bottom and wider at the top, which was a clear indication of a garage rotation due to soil sediment.
Extreme snow and wind load conditions may or may not be the root cause of damage, depending on their magnitude. The true root cause of damage might be the loads in extreme cases. But often, the root causes are pre-existing design and construction deficiencies, non-engineered alterations, unintended load paths, and long-term deterioration. Forensic structural engineering is, therefore, needed to:
We typically use drones to inspect unsafe or inaccessible spaces. In these cases, we fly a micro drone into the structure to inspect the areas that need investigation.
Figure 26: A screenshot from drone footage of an unsafe barn to enter.
Our drone operators usually collaborate with forensic structural engineers to identify and inspect specific areas of the scene. Our forensic investigators then make notes and record the required data.
Drones provide access to the rafters of the structure after a roof collapse. Our operators fly very slowly and illuminate the area for clearer visualization. This is a team effort between the drone operator and the engineer, who provides direction on where to fly for data collection.
When an unsafe or confined space prevents us from seeing the pattern to determine the true cause, we rely on drone footage.
Inspecting high altitudes
While many drone companies offer scene inspection services, they do not need to have the policing background and tactical units experience our operators do. The experience of our drone inspection team allows us to fly into confined areas and higher altitudes to gather data.
Figure 27: A high-altitude image captured using a drone.
Our expertise enables us to access even the most confined spaces to get clear images of the beams, rafters, and other structural components. In doing so, we can collect the empirical data required for the structural engineer to assess the scene.
Inspecting confined spaces
Instead of sending the engineer beneath a structure, we use a drone. We can then illuminate the areas of interest for the engineer to gather data.
Figure 28: Critical damage captured using a drone.
Drone inspection allows us to gather accurate data and keep safety at the forefront. By doing this, we can get the required data without putting the engineer in harm’s way. Another advantage is the ability to gather data to identify deficiencies promptly. Some of our drone inspections take approximately 10 minutes.
Figure 29: An image obtained through a live broadcast.
If an engineer cannot attend the scene, the drone operator can gather the data on their behalf or through a live remote broadcast to the engineer. If an adjuster needs to see the live broadcast, we can link them in so they can view the data in real time.
Our remote lights provide sufficient illumination in dark, confined spaces like basements or attics. We have also been experimenting with a panorama 360° view of the interior or exterior structure.
Figure 30: A panoramic view of a residential building that suffered a multimillion-dollar fire loss.
Aerial data can provide burn patterns and the overall scope of the loss, especially in large claims. Our drone service is not limited to structural investigations; it is for all disciplines. Our team can also conduct site mapping using a drone.
Figure 31: Drone mapping of the scene.
Site mapping allows us to promptly get the house’s site plan and exterior dimensions. This is the same technology used in collision reconstruction in law enforcement.
The DroneDeploy software also allows us to cut through any structure to see the slope of the roof and the difference in elevation between the roof and the ground. We can also draw another path to get the exact dimensions of the house. This technology can see the extent of damage in large losses.
In cases of explosions or fires, the burn patterns and debris fields are visible from the aerial view, which provides a different perspective of the scene. With this technology, five and a half acres of land can be mapped in only a 25-minute flight.
Thermal imaging is widely used for search-and-rescue missions in law enforcement. We have now brought that technology to Origin and Cause.
Figure 32: RGB imaging from a scene we inspected.
We can detect water penetration through heat loss. We use the thermal camera to map the scene in a thermal spectrum. You can use that for a building inspection as well.