Insights
Collapses of wood-framed roofs are frequently reported in North America, ranging from simple agricultural roofs to complex roofs in industrial buildings. Controllable factors such as chemical attacks, moisture attacks, lack of maintenance, inadequate materials, design, or construction are often found to be contributing or primary causes. Furthermore, negligence in handling controllable factors severely impacts the scale and scope of damages.
From the perspective of J.S. Held forensic engineers in Canada, this paper will examine common causes and contributors to roof collapses and identify which parties are typically liable. This discussion will be of value to lawyers, adjusters, and underwriters.
As extreme weather events are becoming the new global norm, many wood-framed roof collapses are occurring during significant weather-related events such as windstorms and heavy snowfalls. While these collapses appear simple to analyze from a causation perspective, a complete determination of causation is often challenging as design, construction, and manufacturing deficiencies frequently contribute to the scope of the collapse and scale of the resulting damages.
Pitched wood-framed roofs are some of North America's most common roof structures for residential and agricultural structures whereas commercial structures frequently use flat wood-framed roofs. In both cases, prefabricated wood roof trusses are often utilized. Roof trusses are strong in resisting vertical loads; however, they are rather poorly suited to resisting lateral loads. Therefore, roof trusses must rely on a lateral load distribution and bracing system so lateral loads are transferred through the structure to the foundations.
Once applied loads approach the ultimate capacity of the trusses and/or bracing system, the roof structure will begin to deform. This deformation may lead to failure/collapse with very little additional load, or the deformation could reduce the ultimate capacity of the trusses by causing slip and deformation of the truss plates.
For low to moderate severity snow or wind events, the design and installation of the bracing system will frequently dictate the scale and scope of the roof failure/collapse. Well-designed and braced structures are significantly more resistant to progressive collapse, with localized collapse zones due to snow overloading, for example, as opposed to complete roof and/or structure collapses in poorly braced structures.
While many truss failures occur after significant weather-related events, causation investigations frequently identify preventable contributing factors. In some cases, the weather events are simply too severe for a roof structure to withstand, such as a direct hit from a tornado or major hurricane. In other cases, it is common for human erro r in design and/or construction to impact the scale and scope of resulting damage. In the following sections, we will explore some of these contributing factors and their impact on the occurrence and magnitude of loss.
1. Communication Gaps and the Risk of Collapse: Inadequate Design and Installation of Temporary or Permanent Bracing
Since the design, manufacture, and installation of metal-plated roof trusses often involve multiple parties, including a truss design engineer, truss manufacturer, contractor, the engineer of record (EOR), and the authority having jurisdiction (AHJ), one of the most common and easily preventable causes of loss is a lack of communication between parties.
For instance, lack of communication may lead to the omission of temporary roof bracing during construction. Collapse debris of inadequately braced roofs during construction typically show trusses neatly folded over on the ground as shown in Figure 1.
Figure 1 – Roof collapse aftermath with neatly stacked trusses typically indicates inadequate or missing temporary roof/building bracing during erection.
The omission of temporary bracing during truss installation is a common and dangerous practice in the construction industry. The economic and life safety risks resulting from insufficient temporary bracing are typically far greater than what an erection contractor may perceive, such as complete collapse of the structure, damage to adjacent structures and equipment, and loss of life. Inadequate bracing during construction is often a primary cause of collapse in the construction industry. It is paramount that the walls and roof are adequately braced during construction or the whole structure can collapse without warning.
Issued for construction (IFC) truss shop drawings typically delegate the permanent bracing design for the global stability of the roof structure to the EOR; however, the EOR may overlook the structure bracing design requirements. The contractor may overlook the footnotes in the truss design drawings and simply install the premanufactured trusses according to the design layout without knowing the temporary and/or permanent lateral bracing requirements, or by assuming that the roof structure was already adequately designed and reviewed by the truss design engineer and EOR. Such a series of miscommunications and assumptions could lead to the collapse of the roof trusses during or after installation.
Examples of temporary top chord bracing, permanent truss bracing, and temporary gable truss bracing are shown in Figures 2, 3, and 4, respectively.
Figure 2 – Typical temporary top chord bracing for erection (Source: BCSI Canada [1]).
Figure 3 – Typical permanent truss system bracing (Source: BCSI Canada).
Figure 4 – Example of gable truss bracing during truss installation (Source: BCSI Canada).
2. Design or Installation Errors
Common design errors involve failure to consider snow drift loads or dead loads from heavy rooftop units (RTUs) permanently installed on the roof. Either of these scenarios can lead to a roof collapse.
Frequently, RTUs are moved from their design location by mechanical contractors in response to site conditions without informing the truss engineer or EOR. In these cases, large dead loads [2] are placed at the areas on the roof that are not reinforced for the RTU loads, and localized deflections, due to creep [3] under the RTUs, may eventually lead to collapse.
3. Mishandling of Large Trusses
On many occasions, the mishandling of the trusses during the installation process by inexperienced contractors could have a negative impact on the integrity of the trusses. For instance, some inexperienced contractors may choose to use any equipment that is conveniently available on-site, such as a telehandler or forklift, to lift trusses without using spreader bars as shown in Figure 5. Lifting large trusses without spreader bars could damage the trusses causing slip or deformation of the plates, sometimes in a manner not easily visible, which can significantly reduce their design capacity. In many cases, the trusses may not show signs of distress until they are fully loaded. The damage in the trusses could incubate for years before resulting in a collapse due to a heavy snowstorm which exceeds the threshold of the reduced actual capacity of the roof.
Figure 5 – Example of how spreader bars are used for truss hoisting (Source: BCSI Canada).
4. Errors in Truss Manufacturing
In other cases, errors during the truss manufacturing process can play a major role in roof collapses. For instance, changing the truss bottom chords to Machine Stress-Rated (MSR) 1650Fb lumber while the truss drawings required MSR 2100Fb would generally cause the roof to be critically deficient in capacity and place it at severe risk of collapse under design service loads.
Economic realities are such that wood trusses are typically designed close to the capacity limits of the lumber grades and truss plates sizes chosen. As such, it is crucial that the manufacturers respect the minimum lumber grades and plate sizes/grades indicated in the truss drawings and request a review from the truss design engineer if the replacement of any truss members with a different grade of lumber is required due to unforeseen situations.
Figure 6 – Typical Lumber Stamps identify the lumber grade (Source: Canadian Lumber Standards Accreditation Board Quality Management in Canada’s Lumber Industry).
5. Improper Roof Maintenance
Roof maintenance is often overlooked by the general public simply because it is not easily accessible and/or the trusses are hidden in the attic. For instance, although the valley on the roof is often designed to account for snow drift, it is still recommended to use a snow rake to remove large accumulations of snow. Caution should be taken when using a snow rake on the roof so that no damage is caused to the roof surface material as this inadvertent damage could also lead to premature deterioration of the trusses should the damage be severe enough to allow moisture intrusion.
As the snow continues to accumulate in the roof valley over time, and after a few freeze-thaw cycles increasing the density of the snow, the roof may eventually collapse from overloading.
Figure 7 – Example of snow accumulation in a roof valley.
6. Inadequate Roof Waterproofing and Attic Ventilation
Installation errors in the building/roofing envelope is another common root cause of roof collapses. Deficiencies in the water management of the roofing, such as inadequate installation of the underlayment, having exposed nails on the shingles, or inadequate installation of roof flashings, etc., could promote water ingress into the attic and result in decomposition of the sheathing and trusses. As the decay continues to develop, it could negatively impact the integrity of the trusses by reducing their net capacity.
While the decomposition of a few truss members alone may not necessarily result in a roof collapse, it is a much larger problem when accompanied by poor attic ventilation. Decay of trusses and/or bracing could be exacerbated by the generally humid environment resulting from poor ventilation. This could lead to widespread decomposition of the roof trusses, resulting in a global reduction of truss capacity, thereby increasing the likelihood of a collapse.
Figure 8 – Decayed structural components in the attic due to poor roof waterproofing.
7. Poor Attic Insulation
Inadequate attic insulation can often be identified by the presence of ice damming along the eaves of the roof. An ice dam is formed when there is a breach of the warm air from the living space into the attic, mostly due to inadequate placement of insulation and/or vapour barrier at the bottom of the attic. The warm air that escapes into the attic would heat up the roof sheathing, melting the bottom layers of the snow above the roof. As the melted water travels near the edge, it then re-freezes near the roof edge due to the absence of heat along the roof overhang, forming an ice dam. The combined weight of ice and snow along the roof overhang that overstresses the truss heels, along with the decay of the trusses due to water damage from ongoing ice damming, could result in a localized collapse of the roof overhang.
Figure 9 – Schematic of ice damming (Source: https://extension.umn.edu/protecting-home-rain-and-ice/dealing-and-preventing-ice-dams).
8. Corroded Truss Plates
Corrosion of truss plates and/or nails is another of the prominent causal factors in roof collapse, and it is particularly common in agricultural and industrial buildings. While corroded truss plates could also occur in residential structures due to excessive moisture for prolonged periods in the attic, it is often not as widespread as those found in farms, agricultural, and industrial structures simply because the environments in these structures are often more corrosive and/or humid.
Figure 10 – Example of roof collapse for a cattle barn contributed by corroded connector plates.
Figure 11 – Example of a corroded connector plate in a collapsed roof.
In Canada, farm and agricultural buildings, such as livestock barns and riding arenas, are not required to be designed by engineers as commercial structures are. Farm buildings are currently governed by the 1995 National Farm Building Code, and it relaxes many requirements of the National Building Code (NBC) such as design loads and regulatory requirements in order to allow farmers to quickly construct and use structures on farms.
Many agricultural owners purchase ready-to-install standard truss packages for their applications without engaging an engineer to properly design the trusses or conducting adequate research on the required design specifications for their applications. While the normal or standard residential/commercial grade connector plates are fabricated from sheets that are hot dip galvanized to Coating Designation G90 in accordance with ASTM A924, these plates are not adequate for service in moist, corrosive, and dusty agricultural environments where they can develop severe corrosion in fewer than 10 years.
Commercial structures have the most clearly defined responsibilities in roof design and construction, and these typically outline the clearest path to establishing liability for a collapse from the professional and contractual responsibilities of the engineers and architects who owe a duty of care to the owners and the public.
1. Owner
In general, the responsibilities of the owner begin before a structure is constructed and extend beyond the completion of the construction to the full ownership period of the structure.
Before the beginning of construction, the owner is responsible for:
After the structure is in use, the owner is responsible for:
2. Contractor
A responsible contractor should use the best construction practices to ensure the structure is built as per the design drawings and National Building Code (NBC). In general, the contractor is responsible for:
3. Authority Having Jurisdiction (AHJ)
AHJ may include the municipality, city, or provincial government where the structure is built, or the department of municipal affairs, township or county building code offices, department of health and safety, etc. They are responsible for:
4. Coordinating Registered Professional of Record
The coordinating registered professional is typically an architect but can also be an engineer. In a single-disciplinary project, it is not uncommon that the engineer of record is also the coordinating engineer. The coordinating registered professionals are responsible for:
5. Structural Engineer of Record (SEOR)
The SEOR holds a crucial role in the integrity of the entire building structure. They are responsible for:
6. Truss Design Engineer
The truss design engineer is responsible for ensuring the roof is designed correctly by the truss designer. The main difference between a truss design engineer and the SEOR is that the scope of the truss design engineer is limited to the structural integrity of the individual trusses, whereas the SEOR is responsible for specifying loads and the structural integrity of the entire structure including the roof structure bracing. A truss design engineer typically works directly for a truss supplier/manufacturer or manufacturer of metal truss plates, and he/she is responsible for:
7. Roof Designer
The responsibility of a roof designer differs, yet overlaps to certain extents, with that of the truss design engineer. A roof designer is typically a technician who works directly for a builder or truss supplier/manufacturer and may be under the direct supervision of the truss design engineer for a specified project. The roof designer is responsible for:
With so many parties involved, liabilities for collapse are frequently shared. Furthermore, the responsibility of each party differs between occupancy types with residential and agricultural roofs requiring much less code-mandated professional oversight.
In the case of agricultural or residential roof design, the truss design engineer typically has more liability exposure than on commercial projects:
Commercial Roof Collapse Liabilities
As noted above, commercial structures require the involvement of architects, engineers, and coordinating professionals of record.
Agricultural and Farm Roof Collapse Liabilities
Residential Roof Collapse Liabilities
Bracing and other matters are complex design issues that are frequently misunderstood and neglected during design and/or construction of wood framed roofs. There are multiple parties involved which can make apportioning liability for a collapse challenging. Furthermore, preventable errors in design and construction can frequently exacerbate the magnitude of loss and resulting damages, but experts can help accurately assess collapse causation and identify liable parties by conducting a forensic investigation of the collapsed structural members.
We would like to thank our colleagues Yannick Roy and Wei Chiet Thong for providing insight and expertise that greatly assisted this research.
Yannick Roy is a Regional Lead in J.S. Held’s Forensic Architecture & Engineering practice. Mr. Roy specializes in structural forensic engineering and leading multi-disciplinary teams of engineers on large and complex losses. With over 20 years of experience, he is a registered professional engineer in Alberta, British Columbia, Yukon, Northwest Territories, Manitoba, Saskatchewan, and Quebec who has provided engineering services for the analysis of failures and restoration of a wide variety of damaged structures. As a forensic expert, he provides consultation to lawyers, insurance experts, contractors, and owners in relation to civil engineering litigations and claims related to structural failures, property damage, design/build issues, material failures, subrogation matters, professional liability and claims for engineering error and omission. His forensic experience includes failure analysis of various buildings and structural components and foundation systems as a result of fire, flood, vibration, impact, or natural disasters. Mr. Roy also provides expertise in causation determination and remediation/retrofit of damaged structures and foundation systems.
Yannick can be reached at [email protected] or +1 587 416 2314.
Wei Chiet Thong is a Senior Engineer in J.S. Held’s Forensic Architecture & Engineering practice. Mr. Thong is a detail-oriented forensic civil and structural engineer with various management experiences and complex technical skill sets in designing wood, steel, concrete, and masonry structures. He has more than 16 years of practical experience with projects focused on the energy, industrial, infrastructure and transportation, commercial, agricultural, and residential sectors. Mr. Thong is experienced in the design of new structures, as well as the repair and retrofitting of existing structures. He has also presented various written technical reports, including rebuttal reports, cause and origin reports, and research and analysis reports for lawyers. Mr. Thong has also presented engineering analyses and reports for the Minister of Municipal Affairs in making decisions on municipal or private claims related to declared disasters during his tenure with the Government of Alberta.
Wei can be reached at [email protected] or +1 587 404 6183.
[1] Prepared by the Structural Building Components Association (SBCA)
[2] Dead loads are permanent loads, such as mechanical HVAC equipment installed on a roof.
[3] Creep is a structural phenomenon whereby progressively larger deflections and/or deformations occur over a long period of time, typically underneath large dead loads.
[4] This bracing is only to ensure stability of individual truss members such as web members in compression and not for the roof system as a whole. The SEOR is responsible for overall roof and structure bracing.
[5] Truss Industry Standards include the BCSI Canada guide prepared by the Structural Building Component Association, typically included with every truss package purchase, that provides a wealth of information for safely erecting , installing, and bracing trusses.
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