The integrity of each element of a building is critical to its stability. However, no component carries more potential for property damage or personal injury than the roof. A poorly performing foundation or failing exterior cladding can be inconvenient, unsightly, and expensive to repair or replace, but a roof collapse can have devastating and long-term consequences.
Thankfully, it is exceedingly rare that a structural roof fails without warning. There are virtually always telltale signs that point to problems with the roof’s materials, design, functionality, or all three. Property owners can help protect themselves from costly repairs and legal liability by becoming familiar with these signs and calling on forensic engineering and building science experts as needed for guidance on repairs or total roof replacement.
Building by the Numbers: Minimum Design Loads for Buildings and Other Structures
Given the wide-ranging implications of roof collapses or any kind of structural failure, it is important that buildings be designed, constructed, and if necessary, repaired or reinforced following current industry standards. Those standards are codified in what is called the International Building Code (IBC), with further guidance on weights, material strengths, and other details of the design loads provided by the American Society of Civil Engineers (ASCE) in a document titled Standard 7 – Minimum Design Loads for Buildings and Other Structures.
The guidelines include detailed information on how roofs should be designed to accommodate primarily two different types of loads. The first is called dead load. It includes any static weight that will remain the same for the life of the roof, such as the structure itself, ceiling material, roofing material, insulation, mechanical/electrical/plumbing, and rooftop HVAC equipment. The second type are known as live loads. Live loads are variable and temporary forces associated with occurrences like snowfall, people walking on the roof, wind, etc.
Planning for snow loading in colder climates is an especially important aspect of roof design and construction. The ASCE standards address how the different building geometries and layouts affect snow accumulation and the resulting roof snow loads. For example, when parapet walls enclose a flat roof, they trap snow, leading to drifting snow along the parapet. In contrast, a curved roof or a flat roof with no parapets allows snow to slide or blow off, making it less prone to these issues. The presence of different roof heights can also be a challenge, as snow from the higher roof falls to the lower, producing large drifts. To ensure the structural integrity of the building, these drifts need to be accounted for in the design process.
The IBC/ASCE standards provide the information needed to build a roof that will support the loads it is exposed to, provided it is properly maintained. Problems arise when changes to the roof or the surrounding environment are not identified and addressed, and the structure is weakened.
Common Causes of Structural Roof Failures
There are many conditions that can contribute to problems with a roof. Below are seven of the most common causes of structural roof failures:
1. Deterioration and water intrusion
One of the leading causes of roof failures is the deterioration of structural components caused by water intrusion. Wood structures that are regularly or continually exposed to water can be attacked by mold and bacteria that ultimately cause them to decompose. Metal components, including metal truss plates, bar joists and roof decking, can be similarly affected and weakened by rust and corrosion. Water can gain access to roof framing in a number of ways, including failing shingles or roof membranes, as well as through interior condensation.
2. Changes in roof drainage
Positive drainage is critical to the health of a roof. An example of a detrimental change in roof drainage might be a situation where the drains are clogged with debris causing water to pond on the roof. At 62.4 pounds per cubic foot, the weight of standing water can very quickly add up and cause tremendous stress on the roof and supporting structures.
3. The addition of nearby or adjacent buildings
If some portion of an adjacent building’s roof surface sheds rainfall or snowmelt onto the roof of the older structure, that older roof may not have large enough drains to properly accommodate the runoff. Changes in the way snowfall drifts and accumulates on the older roof, or the weight of a drift falling from one roof to another, are other sources of destructive stress.
4. Changes in melting and runoff due to adding insulation
One of the steps property owners commonly take to make their buildings more energy efficient is adding insulation to prevent heat loss through the roof. Unfortunately, in some cases a certain amount of heat loss is helpful in that it facilitates the melting and sloughing of snowfall on the roof. When that process is curtailed, especially on older buildings, dangerous accumulations can develop, resulting in damage to the roof.
5. Improper structural modifications to roof trusses
When repairs or enhancements are made to the mechanical, electrical, or plumbing systems in a building, they are sometimes made at the expense of the roof structure. Structural members can have holes drilled in them, notches, or be altogether removed to accommodate conduit, plumbing, or ductwork. These modifications weaken the roof structure, causing deflection that can worsen over time and, in extreme cases, lead to roof failure.
6. Damage to roof trusses during installation
Another cause of weakened trusses and the resulting issues with a roof is damage during transportation or installation. Whether it is damage to the wood itself or to the metal plates used to connect members, this problem may remain hidden until excessive load is placed on the roof.
7. Inherent design errors
A roof not designed to the appropriate standards for its use, location, etc. is more prone to failure. For example, a roof in Minnesota must be able to accommodate the large live loads associated with the state’s heavy snowfalls. If the standards for Mississippi, where the maximum expected snow load is much lower, are used in error, problems ranging from deflection to full collapse can occur.
Vicious Circle: How Issues Combine to Decrease Roof Structural Performance
Any of the issues listed above can, on its own, cause a roof to weaken and ultimately fail. However, in most cases, serious problems arise not because of one factor, but due to the compounding of two or more of these issues. For example, a new addition is constructed next to an existing warehouse building. The addition is upwind of the original building, and has a higher roof, resulting in snow drifts forming on the roof of the old building. The weight of the snow drift causes the bar joists supporting the old roof to deflect, which causes ice to form in the roof drains. The roof over the original building collapses when the drift melts but the resulting water cannot drain off the roof due to deflection of the joists and blocked roof drains.
Consequently, performing regular maintenance is essential, as it can uncover small issues before they develop into major problems. And, when an issue is discovered, it is important to have a forensic engineer or building science expert perform a comprehensive inspection. Simply attributing a defect to the most visible or most likely cause can leave other contributing factors undiscovered, resulting in future problems.
From Assessment to Bracing Retrofits: What Pie Can Do for You
One of the most important investments building owners can make in preventing deterioration of the roof is having any changes in appearance or function that might be attributable to problems with the roof assessed promptly by a forensic engineer. If the structural analysis reveals that the integrity of the roof has been compromised, there are a number of methods that can be used to secure it and ensure that it meets the current building code and is safe for occupants.
Installing knee braces on the columns and roof beams is one way to address a deficient beam line. Another is to decrease the beam span by adding columns. You can also strengthen the beam by adding new members to make it wider or deeper. And in some cases, the problem with the existing beam is so significant that removal and replacement is the best option.
Fortunately, while every roof is unique, there is a way to remedy virtually any structural issue.
-Shannon Pierce and Clinton Standish