Roofs - Collapse and Performance Failures
By: Michael Payne, PSU B.A.E./M.A.E., Fall 2012

Introduction

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Figure 1: Examlpe of a flat roof. Source: EPDM Rubber Flat Roofing via Flickr


To introduce this topic, it is first important to have the reader understand the terminology of a roof system so as not to confuse it with what is generally referred to as a roof. According to the National Roofing Contractors Association, or NRCA, a roof assembly is “a system of interacting roof components including the roof deck, vapor retarder (if present), insulation and membrane.” However, a roof system is “a system interacting roof components generally consisting of a membrane … and roof insulation (not including the roof deck) designed to weatherproof and sometimes improve the building’s thermal resistance.” (NRCA 2005, p467). Therefore, this article will refer to the structural portion of the assembly as "roof" and the performance, or weatherproofing, portion of the assembly as "roof system" or "roofing".

A roof that has a low pitch (less than or equal to 25% slope) is referred to as a flat roof or low sloped roof, and is the most common roof type in commercial construction (Smith 2011, WEB). An example of a flat roof can be seen in Figure 1. There are many different roof system types available for use with the flat roof. The most common among these roofing types are the built-up membrane, modified bitumen membrane, single-ply membrane, low-sloped metal panel system, and Spray Polyurethane Foam system (NRCA, WEB). Similar to other building systems, roof systems are very susceptible to failures. The most common are performance grade failures such as water and air intrusion. However, more serious failures such as up-lift and collapse occur all too often. This report will focus on examining these common roofing failures and analyzing the cause behind the failures. It will also use several industry case examples of various failure types to help the reader understand the failures and to provide a solution as to how these failures can be prevented in the future.


Keywords:

Roof Assembly, Roof System, Roofing, Performance Failure, Wind Uplift, Roof Collapse


Types of Commercial Roofs


To understand typical roofing failures, an introduction to common commercial roofing types is required. Commercial roofs are usually flat roofs with minimal slope. A typical roof assembly consists of a structural base element, such as concrete planks, metal deck, or fibrous deck. This base usually spans the steel/wood frame or concrete/masonry load bearing walls. Above the base is the roof system. This usually consists of an insulation layer and a substrate layer that are either mechanically attached, fully adhered, or ballasted to the deck. Finally the protective roofing membrane covers the system and prevents damage and water intrusion down into the rest of the system.. There are many variations of roof systems used in today's construction, but the most commonly used are the built-up membrane, modified bitumen membrane, single-ply membrane, low-sloped metal panel, and spray polyurethane foam systems (NRCA, WEB).

Built-Up
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Figure 2: Example of a Built-up roof. Source: Facility Engineering Associates,P.C (FEA)

The built-up roofing system, (BUR), has been used for over a century, and is the oldest roofing system still commonly used today (although not as commonly as the other systems are). BUR systems is often referred to as "tar and gravel" or "tar pitch" roofs (NRCA, WEB). As these names suggest, the BUR system is either an asphalt or coal tar based product in which alternating layers of tar, or bitumen, and reinforcing fabrics, or ply sheets, are combined to create a waterproof membrane (Smith 2011, WEB). It is recommended that a system be at least 4 plies thick to allow for proper strength. This system is usually hot applied, and is usually either directly applied to a substrate, or mechanically fastened by a base ply. Because BUR systems are relatively susceptible to weathering such as Ultraviolet radiation (UV) damage, a surface cover consisting of aggregate, cap sheets, hot asphalt, or elastomeric coatings is used for final protection. An example of a built-up roof can be seen in Figure 2.

Modified Bitumen

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Figure 3: Example of a mod bit roof. Source: FEA

Modified Bitumen membrane systems (often shortened to "mod bit") have been used since the 1960's and are perhaps one of the most common choices for roof systems used today. They consist of rolled sheets of reinforcing fabrics with hot polymer-modified bitumen. An example of a mod bit roof can be seen in Figure 3. For redundancy this system consists of 2-ply construction, with a separate base and cap sheet. These plies are rolled down perpendicularly in a lapped fashion and are fully adhered to the substrate. Polymers are added to the bitumen to change the characteristics to improve the system. Two common types of polymer enhanced bitumen are the Styrene-butadiene-styrene (SBS) and Atactic polypropylene (APP) bitumen. SBS mod bit systems have a rubberized quality that allows for good cold temperature flexibility, and they are either hot applied using mopped asphalt or cold applied using adhesives. APP mod bit systems have a plasticized quality and are torch applied to the substrate (NRCA, WEB). Surface coatings such as small aggregate granules, metal foil, or liquid-applied surfacing are used to protect against weathering.

Single-Ply

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Figure 4: Example of an EPDM roof system. Source: FEA

As the name suggests, single-ply membrane systems are a single ply system in which a certain composite is manufactured into 7.5 ftx50ft rolled sheets and rolled out in a lapped fashion onto the substrate. Because only one ply is needed, construction costs go down. However, the trade off is that little redundancy protects against impact or intrusion. There are two types of single-ply membranes. Thermoplastic membranes can be repeatedly reheated and softened. Thermoset membranes, on the other hand, are set permanently once manufactured. Different single-ply systems are often referred to by the acronym of the chemicals that make up the membrane sheet. The most common single-ply membrane types include the Ethylene Propylene Diene Terpolymer (EPDM), Polyvinyl Chloride (PVC), and Thermoplastic Olefin (TPO) membranes (NRCA, WEB). All three of these single-ply systems are roughly 30-100 mil thick (1mil=0.001 inch). Although these systems can all be mechanically attached or loose laid and ballasted, the most common construction has the substrate mechanically attached and the membrane fully adhered to the substrate. Loose laid with a ballast is also common.

EPDM roof systems, like the one shown in Figure 4, are often referred to as "rubber roofs" due to their look and black color (though other colors are available) and are one of the most common roof types used today. They are very weather resistant and have great low temperature flexibility (Smith 2011, WEB). The chemical make up is of propylene (natural gas) and ethylene (oil). EPDM are thermoset single-ply roof systems which require all seams to be sealed with liquid adhesives or seam tape. PVC membrane is thermoplastic and is produced by calendaring, extrusion, or spread coating. To help prevent flexibility, plasticizers or other stabilizers are chemically mixed into the sheets. Because PVC roofing is thermoplastic, lap seams can be heat welded rather than adhered. PVC roofs are commonly grey or white in color. TPO membranes are also thermoplastic and are produced by calendaring with lamination, extrusion with lamination, or extrusion coating. TPO systems are produced with a mix of propylene, ethylene polymers, and polyester reinforcing. TPO is usually a white colored membrane, which helps keep roof surface temperatures cooler and prevent heat absorption into the building.

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Figure 5: Example of a metal panel roof. Source: FEA
Metal Panel
Low sloping metal panel roof systems, like the one seen in Figure 5, use structural metal panels set to at least a 4 degree slope to create a hydrostatic and impact resistant roof system (Smith 2011, WEB). An advantage to this system is the structural quality allows it to span far distances. Because the panel system is meant to act as a weatherproof membrane, it must be ensured that detailing and installation are done properly to create a fully continuous system. All laps or inconsistencies in the metal panel construction should be sealed using a waterproof and UV resistant sealant. The NRCA recommends that an asphalt saturated felt underlayment also be included as a redundancy factor to help shed any intruding water that gets past the metal panel system. In cold weather areas, an ice dam protection sheet made of mod bit membrane should be considered as well (NRCA, WEB).

Spray Polyurethane Foam

Spray polyurethane foam-based roof systems are commonly referred to as SPF systems. This system consists of a 2 component liquid (Isocyanate and polyol) that ,when sprayed onto the roof deck in a 1:1 ratio, chemically reacts to form a rigid, closed cell polyurethane foam (NRCA, WEB). This foam acts as the substrate and outer barrier for the system (although some systems are designed to have the foam be fully supported on a backup felt or roof deck). To enhance the insulating quality and R-value of the system, the spray is simply applied thicker. To help protect the outer face of the foam, a spray-applied elastomeric coating is often applied. Coating types include acrylics, butyl rubber, silicone, and other elastomers;and these coatings provide weatherproofing, UV protection, impact resistance, and fire resistance for the SPS system. The choice of chemicals for the coating depend on the environment the system must endure, such as sunlight, moisture, humidity, and temperature (SMith 2011, WEB).


Performance Failures


Like all other systems in a building, roofs can fail. In fact, roofs account for the majority of necessary building repairs and building litigation cases in the United States (Jacobus 2011,WEB). However, most of the failures that occur in a roof system are not major collapses, but simply performance failures. Although each roof system has its own specific failure modes, there are a variety of common performance failures that can result in necessary repairs. Some of these failures that require repair include the following:

  • Blistering
  • Splitting
  • Punctures/ Penetrations
  • Wrinkles
  • Flashing Installation
  • Surfacing
  • Fasteners
  • Shrinkage
  • Ponding
  • Leaks or moisture intrusion
  • Abuse, neglect, and lack of maintenance

Blistering

Blistering occurs when a gas or vapor source such as liquid water gets trapped within the plies of the roof membrane or between the membrane and substrate. As temperatures increase throughout the day, the gas or vapor expands and pressure pushes out on the membrane. This creates bubbles or raised strips called blisters. The gas or vapor usually gets into the system during construction. Most often it is due to applying the membrane on a wet substrate, using wet insulation from improper storage, continuing construction during rain or snow, or using insufficiently hot asphalt mops or torches with mod bit membranes.Blistering is the most common performance failure in roof systems (Warseck 2003, p32), but also one of the most minor issues. Because the system is not penetrated in any way, the integrity of the system remains intact. However, blistering can lead to weakened membranes and cause greater failures and leaks if neglected.

Splitting

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Figure 6: Example of a splitting failure. Source: FEA
Splitting refers to the stress cracks that form in a membrane or joint due to movement, such as that seen in Figure 6. This failure can occur for many different reasons. It is common in roof systems that are aged. As the system ages, it stiffens and shrinkage occurs. This creates a situation where events such as temperature change, impact load, or differential movement from the substrate can overstress the membrane causing splits. Research done by the U.S. Department of Commerce in the 1960's helped prove that thermal expansion,in general, must be taken into account over the whole life of the building to ensure overstressing does not occur (Cullen 1965,p1-6) .


Splitting is especially common in membrane flashing set over metal . As metal has a higher thermal expansion coefficient than membrane materials, differential movement occurs, stretching the membrane. At expansion joints, roofing details or application are often insufficient for the movement that can occur and the membrane is overstressed. This overstress and stretching creates a splitting failure as well. Splitting can be full depth, however usually it is more likely surface cracking. Water intrusion and weakened durability are the main concerns with this failure type.


Punctures/Penetrations

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Figure 7: Example of a puncture failure. Source: FEA
Punctures and penetrations refer to intrusions in the form of holes or tears in the roof system. Unlike splitting, these failures are usually full depth failures, meaning the membrane layer is fully compromised. This allows for water and air intrusion under the roof system which can be a serious issue. Penetrations and punctures are one of the most easily avoidable failures that occur on a roof top, as they are almost always caused by people. Even though debris or even hail storms can potentially cause this failure, it is most often due to sloppy construction over finished roofs, careless installation and maintenance crews that access roofs, or foot traffic over the roofing membrane (Warseck 2003, p34).


For example, Figure 7 shows a puncture failure caused by careless construction wrinkles in an EPDM membrane. This failure is particularly a problem with single-ply roofs such as EPDM because the thin nature of the membrane allows for easier punctures due to wear or impact, and the lack of redundancy . It is usually recommended that walk pads, added membrane layers, or other protective devices be used at high maintenance paths/areas to help prevent wear and impact punctures (Madsen 2004,p46). Finding the location of punctures can be difficult on some roofs, such as ballasted roofs, but they must not be ignored.Failures of this type can be detrimental to the interior of the building and the life of the roof assembly, and should be corrected quickly by patching or sealing.

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Figure 8: Example of a wrinkle failure. Source: FEA
Wrinkles
Wrinkles are raised ridges in membranes and flashing caused by poor placement or differential movement. An example of a wrinkle can be seen in Figure 8. This failure type is common when fully adhering rolled membrane types such as single-ply or mod bit systems and their flashings. As the membrane is unrolled and placed, poor care or haste by the roofer or difficult roof geometry can cause wrinkles to form. Although it is possible to fix this quickly by reapplying or rolling out with tools, it often needs completely redone or unfortunately is ignored. Often differential movement between the perimeter and roof structure will create 45 degree wrinkles along the perimeter flashing in a roof system (Warseck 2003, p34).

Although wrinkle failures often are not very problematic, they can cause serious issues if the wrinkles extend to the edge of the material. An edge wrinkle creates a hole in the system, referred to as a fishmouth due to the common shape of wrinkle, that allows water to penetrate through the waterproofing part of the system (Warseck 2003,p34). Even minor wrinkles can eventually lead to problems, however. Wrinkles are raised up from the rest of the system, creating a tripping hazard and allowing for easier wear and tear or punctures to occur. Therefore, it is recommended to cut out and replace wrinkled roofing or cover with patches to help prevent further problems.

Flashing

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Figure 9: Example of a flashing failure. Source: FEA
Roof flashing is an integral part of a roof system in that it helps weatherproof the different joints and connections of a roof system. This includes locations such as where the roof meets the perimeter or building wall, where designed roof penetrations for vents and mechanical equipment come through the roof, and at built up curbs. Flashing can refer to membrane flashing or metal flashing, and it is important to understand where each is used. When flashing failures occur, it can create serious issues, as flashing is usually used at locations where the roofing system terminates. This creates a serious water intrusion problems as water not only penetrates the surface,but gets down to the structure and interior of the building. Flashing failure is almost always caused by a construction flaw or lack of long term maintenance.


A common flashing failure is lap failure. This occurs when manufacturer guidelines aren't followed and flashing laps are either too short or not properly adhered by adhesives, cold bitumen, or fasteners. Figure 9 shows a membrane flashing failure where an incorrect adhesive was used and the flashing peeled. Even though flashing may appear adequate, the life of the flashing can be seriously compromised. Other common flashing failure occurs where proper detailing is not followed at wall counterflashing, sealant locations, and termination points (Warseck 2003, p32-34).

Surfacing

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Figure 10: Example of a surfacing failuref. Source: FEA
Surfacing refers to the protective covering placed over many roofing system. This can be for protection against debris or impact, protection against UV radiation and other weathering, or ballasting purposes, and includes such things as roof granules, gravel, and elastomeric coatings. When the surfacing is worn off due to age, impact, weathering, etc., the roofing system no longer has the outer coating and the remainder of the system starts to have integrity issues. This failure type normally takes a long time to develop and usually requires repeat maintenance or replacement to correct. Figure 10 shows an example of an aged surface failure.




Fasteners

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Figure 11: Example of a fastener failure Source: FEA
Fastener failure occurs in mechanically fastened roof membrane systems and metal roof systems. Improper fastener details, incorrect fastener placement, improper fastener length, poor fastener installation, and weathering can create problems with the fasteners of the system. Roof uplift forces can create fatigue on fasteners that cause enlarged holes around the fastener which can allow water to penetrate the system. This is especially an issue with metal roofs and coping where a lack of redundancy creates serious intrusion when fasteners pull out allowing water to get past the panels. This can also happen due to shrinkage of a membrane pulling on the fasteners stretching the hole. Figure 11 shows a rusting metal panel with failing fasteners that have rusted. Water can be seen ponding at the fastener locations, allowing leaks to occur.

Shrinkage

Shrinkage is a failure mode that usually occurs at the roof membrane (most often found in single-ply membranes). In this failure, the membrane shrinks due to weathering or improper manufacturing/installation. The heat and UV radiation from the sun can eventually break down the membrane in a process called heat aging, causing hardness and shrinkage to occur. Also, if the chemical make up of the membrane is manufactured incorrectly or coatings are used that are not compatible with the membrane, reactions can create shrinkage or weaken the membrane which augments the heat aging process. This shrinkage can cause stress cracks or wrinkles within the membrane due to differential shrinkage.

Shrinkage can also cause membranes to pull out or debond from flashings, laps, perimeters, and at other locations.This can cause bending/ pullout of fasteners, tearing of the membrane, or creation of gaps allowing for water intrusion (Nelson 1985, p87). Because differential shrinkage becomes more of an issue as membrane coverage increases, it is recommended that expansion joints are used along large roof sections to help create smaller sections (Cullen 1965, p5). Repairs and replacement are necessary in shrinkage failures to ensure the integrity of the remainder of the system is kept.

Ponding

According to the NRCA, ponding occurs when
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Figure 12: Example of a ponding failure. Source: FEA.
puddled water remains on a roof for over 48 hours (although some manufacturers set a 24 hour limit) (James1997, p56). This water can be caused by rain, melted snow/ice, large amounts of condensation from mechanical equipment, or from failed rooftop water storage. Ponding can occur from poor drainage due to slope, clogged drains/scuppers, and debris or ice causing pooling, The root cause is usually poor roof design, construction, or maintenance.


Ponding failure can be very problematic, and should be monitored and repaired as soon as possible. Ponding water can cause staining, debris accumulation, mold or plant growth, and other issues that may damage or deteriorate roof membranes. If left untreated, leaking and damage to wood or fasteners can often occur at ponding locations. In some circumstances, ponding water can cause deck and structural framing deflection , and may even cause overbearing failure/collapse (which will be covered further in a later section). To help prevent ponding from occurring, roof slope should be greater than or equal to 1/4 inch per foot. In addition, it is imperative to ensure gradual slope decrease from ridge points to drainage points with no slope change. Also a good maintenance plan should be implemented to clean debris and monitor any problematic areas, such as that seen in Figure 12.

Leaks or Moisture Intrusion

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Figure 13: Example of a moisture intrusion failure. Source: FEA
Moisture intrusion is the most critical performance failure that occurs in roof systems. If a roof system is leaking, it has completely failed in doing the job it was designed to do (see definition of roof system in Introduction). As already discussed in this report, all roof types (BUR, mod bit, metal,etc) are susceptible to leaks, and almost all other failures that have been mentioned can eventually lead to this failure. In addition to poor membrane and flashing installation/performance, other causes to moisture intrusion include poor drain details/ construction, rain during construction or uncovered repairs, poorly installed expansion joints, wind driven rain, and a multitude of other various issues. Leaky roofs are a huge problem in the building industry, causing billions of dollars each year in damage and repairs.


Water intrusion can ruin the integrity of the entire roof assembly, including the roof deck, insulation, and structure (it can even cause eventual collapse in certain circumstances). Water intrusion into the interior of the building is also an issue. Water can ruin interior finishes, short electrical lines, cause staining, ruin storage or equipment, rot wood or rust metal, and cause microbial growth such as mold and become a health issue. Figure 13 shows an example of interior water intruding and staining fireproofing under a metal panel roof system. Therefore, it is critical that roof systems are detailed/constructed to ensure water tightness, and all eventual water intrusion failures are quickly and properly repaired to prevent further problems. Approved test methods to ensure water tightness include common tests that have been used for a long time such as flood testing, as well as infrared, nuclear, impedance, high-voltage, and other testing that are newer to the industry (Various 2010, p.18-25).

Abuse, Neglect, and Lack of Maintenance

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Figure 14: Example of an abuse/neglect failure. Source: FEA
This section goes into detail on one of the biggest underlying culprits to many of the performance failures listed above. Although improper construction and natural weathering are common reasons for failure, the abuse, neglect, or lack of maintenance that a roof sees during its lifetime can ultimately cause many problems to arise. Abuse can range from heavy foot traffic in areas with little extra protection to careless maintenance crews working from the roof on other equipment/systems. For example, the most common abuse situations include mechanical equipment technicians and window washers (Warseck 2003). These maintenance crews have little care for the roofing system as they work on their designated system. Often, scraps, fasteners, and other debris from these crews are left behind. Worse yet, these crews can often unknowingly cause accidental damage to membranes, flashing, metals, etc. as they perform their work.


Another major issue in many roofs is owner neglect. All too often owners believe that the roof should last on its own and never check up on it. This allows failures to go unnoticed and get worse with time. Figure 14 shows a roof location where ponding and vegetation growth are a problem and should be fixed and maintained. Finally, even when owners do check on their roofs, a lack of maintenance or inadequate maintenance plans can cause further failure. Finances, business continuity, and lack of manpower or knowledge can cause owners to provide inadequate maintenance to their buildings (Madsen 2004,p45). Too much deferred maintenance can lead small maintenance problems to become larger, more costly failures. To help combat this, it is often recommended that multiple roof inspections occur each year (as well as after any major storm event), and any problems are fixed in a timely manner. Also improper repairs can simply "cover up problems" and allow them to resurface or get worse. Often a lack of understanding will have maintenance crews repair with caulk, roofing cement, and other inadequate materials when leaks are found on roofs. It is a necessity to always follow maintenance instructions given by roofing manufacturers to ensure the best life out of the roof system and to maintain warranties.


Structural Failure and Roof Collapse


Although most roof-related failures that occur are simply performance issues to the roofing system, there are several failure types that can be more serious. These types of failures are strength-related failures and can create large openings in the roofing system, damage interior systems and spaces, or even lead to partial or full collapse of the building. The ultimate cause of these failure types is an overloading of the roof. Whether it be positive or negative pressure, if roofs are pushed past their limit, failure will occur. The most common types of strength related failures are wind uplift and overloading caused by excessive snow, sitting water, or live load.

Wind Uplift Failure

As wind interacts with a typical flat-roofed structure, positive pressure occurs at the wind-facing wall (windward wall), while negative pressures occur on the roof and opposite (or leeward) and side walls (As seen in Figure 15). In a more realistic, non-typical roof (e.g. different roof elevations, the existence of large parapet walls or overhangs, internal pressures etc), uplift forces can be behave differently. Therefore, it is important to analyze these roof loads on a case to case basis.
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Figure 15: Wind pressures on typical flat roof building.
In most existing buildings today, wind load design was based off of the "Wind Load" section of the American Society of Civil Engineers standard ASCE-7. As Figure 16 shows, uplift forces are critical along the edges of the roof (the end zones), with the greatest design uplift pressures occurring at the building corners (corner zones). The dimension marked "a" in the figure represents "10 percent of least horizontal dimension or 0.4h, whichever is smaller, but not less than either 4% of least horizontal dimension or 3 ft (0.9 m)" (ASCE 2005).
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Figure 16: Wind load zones per ASCE-7 05.


Wind uplift failure occurs when the negative pressure of passing wind pulls up on the roof assembly of a structure. This uplift pressure can cause stress-related damage to various roofing system parts, pull off the weatherproofing (or the whole roofing system) and expose the interior, or even create failures in the structural assembly of the roof causing partial or full roof collapse.

In roof systems in which the weatherproof membrane is adhered to a substrate that is then fastened/adhered to the deck, the chance of uplift failure of the roofing system is greatly reduced. The reason for this is that in an adhered membrane situation, the main force resistance is taken by the insulation and the attachment method to the deck. In situations where the membrane is loose between contact points, such as mechanically fastened single-ply membranes, the membrane can be pulled by wind forces getting under the membrane. This causes the membrane to inflate, or tent, and causes stresses on the membrane and at attachment points (Malpezzi and Gillenwater 1993,p123). Even after adhering the membrane to a substrate, it is necessary to ensure fastener spacing along the perimeter of the system and on edge and corner substrate sections is adequate to meet manufacturer guidelines to prevent uplift movement. Often a testing standard such as Factory Mutual (FM) or Underwriters Laboratory (UL) is used to rate the system being constructed. It is also important to realize that wind pressures are dynamic, and not static. This cyclic loading pattern can cause back out failure of fasteners and cause fatigue failure within the different components of the roofing system.

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Figure 17: Metal roof panel blow off failure. Source: MBK Marjie via Flickr
A more serious issue arises when uplift forces create a pull-off scenario on a roof. This can refer to pull-off of components, such as inadequately fastened perimeter flashings, or total roof pull-off. This happens most often on metal panel roofs and mechanically attached single-ply roofs, but can occur on all roofing systems.. In metal panel roofs, seam connection is vital to the integrity of the roof. If uplift forces compromise this connection, the roof panels can blow off the building, exposing the interior space, as can be seen in Figure 17. Seam failure can be caused by poor detailing for edge uplift forces, weakening and prying of the seam due to cyclic loading and plastic deformation of the thin metal, or an increase in uplift force due to large interior pressurization (Dixon and Prevatt 2010). Pull-off failure is problematic because it removes the weatherproof layer of a building. Once it occurs, cold air and water can enter and heat can escape. Water intrusion is by far the worst problem because it can cause serious interior leaks which can ruin structural components, electronics, storage, and finishes. Worse yet, water intrusion can create mold issues. Due to the seriousness of this failure, it needs to be repaired very quickly.

Finally, in certain circumstances, wind uplift forces can create building collapse failure. This can be caused by additional forces on the structure due to roof pull-off or from excessive forces on the structural system of the roof assembly. When roof pull-off occurs on buildings such as metal panel structures additional wind forces acting on the walls can overturn the light structure. Also, if the roof structure itself is not properly designed for such large uplift forces or it sees repeated loading, the connections can weaken or fail. Often during heavy wind events such as hurricanes or blizzards, heavy rainfall mixes with the uplift forces creating additional live load. This additional loading can exacerbate the weakened condition and progressive collapse of the roof structure can occur.

There are three main circumstances that create uplift failure in roofs. The first is high wind pressure failure with wind speeds that exceed design forces for a particular roof (often typhoon strength winds). Due to expense, it is uneconomical to design all roofs to withstand excessively large wind speeds from storms like major hurricanes or tornadoes. Because these type of high-wind storms are not all that common, this failure mode accounts for very little of the uplift failure cases in the U.S. The second is uplift failure due to faulty or inadequate design. Although it can be a big issue on unique buildings, problematic roof design also does not account for many uplift failures. Many building codes and manufacturers include testing procedures (such as ASTM pull tests) and guidelines (such as FM rating) to ensure the design is adequate. For example, FM-90 is a common minimum uplift rating for roof components that require the component to withstand up to 90psf uplift force. The third failure mode is due to poor construction within the roof assembly. Surprisingly, over 80% of uplift failures are caused by poor construction methods or non-compliance to existing wind standards or manufacturer guidelines (Schneider 2008, p19). Often FM standards or other manufacturer guidelines are not met in perimeter connection details or fastener spacing at the perimeter is inadequate.

Overloading Roof Failure

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Figure 18: Example of an overloading roof failure. Source: Zieak via Flickr
Arguably the most serious roof failure is overloading/overbearing failure. Overloading is a structural failure in which excessive static load is applied to the roof assembly. Overloading can be caused by several different scenarios, including excessive live load, rain/water load, or snow/ice load. This failure type seems to get the most media and legal attention as well, as it often leads to roof and building collapse (and, unfortunately, serious injury and death often result). An example of overloading failure collapse can be seen in Figure 18. However, it is important to note that this failure type does not always cause collapse. This is due to the safety factors and redundancy that are included into many roof structural designs. Additionally, overloading can also refer to overbearing the membrane or other roofing components and may not harm the structure at all.

Overloading failure can be minimal, such that the membrane, substrate, or metal work on the roof are the only affected roof components. Excessive live load is usually the culprit in this case. Too much foot traffic or moving/storing equipment and materials on a roof can cause the membrane and insulation to deform. Also, putting weight on metal work at perimeters and other locations can cause them to deform and have failure at lap joints allowing water intrusion. In addition, deformed membrane and substrates can cause ponding to occur, which can lead to more serious issues. Therefore, this overloading can lessen the life of the roofing components and cause weatherproofing issues. To prevent this type of overloading, protection against foot traffic (such as pads or thicker membrane) should be used in construction, and maintenance crews should understand the limitations of the system when accessing the roof for work and storage.

In circumstances where large amounts of live/dead load are applied to a roof, deflection and structure failure become problematic. Roof events where a large amount of people congregate can cause vibration and deflection issues, especially if the roof design was not rated for such loads (according to ASCE-7 05 Table 4.1, uniform roof live load design for a typical flat roof is 20psf, while a roof used for assembly is designed for 100psf). Excessive live load can also become an issue when rooftop equipment is replaced due to maintenance or added for new occupancy use. If weight, rooftop location, and other details are not carefully planned for items such as chiller units, this load can cause deflection, punching shear failure through the deck, and structural damage. Of course, if the load becomes too great or vibration is intense, the structure can eventually fail and may lead to collapse.

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Figure 19: Large roof ponding can cause overloading failure, Source: FEA
As discussed in an earlier section, ponding water (like that seen in Figure 19) can be very problematic on roof assemblies, and excessive amounts can cause overloading failure to occur. Poor drainage and heavy rainfall is often to blame for dangerous ponding situations. At 62.4 pounds per cubic foot, water loads can increase quickly. For example, if a roof structure has a bay size of 30 feet in each direction, and an inch of rain is ponded on that bay due to a clogged drain, the water weight for that bay would be almost 5000 pounds alone. When heavy rainfall puts multiple inches of rain onto a roof, this weight can increase dramatically (remember that flat roofs are often designed to a minimum of 20 pounds per square foot (if snow load does not control), and just 4 inches of water exceeds this). As water build up occurs, structural framing can start to fail under the load or due to unbalanced loading conditions with adjacent bays. Additionally, as water accumulates at low points, the deck can deform further allowing for further drainage problems and ponding to occur, and parapet walls can create a "pool" effect trapping large amounts of water. The final result of this is often progressive or sudden roof collapse.

Because of the seriousness of this failure type, it is imperative to ensure that ponding water does not occur on flat roofs. As mentioned earlier, roof slope and drainage need to be maintained. Roof drains need to have covers to prevent debris from clogging them, and maintenance plans should include a regular inspection of drains and scuppers to ensure performance. In situations where large volumes of water can accumulate (such as on walls with high parapet walls) overflow secondary drains or scuppers need to be included to allow water shedding in heavy rainfall or clogged primary drainage situations. If slope issues are present, additional tapered insulation can be placed at problematic areas to help fix slope issues. Note that much of this is also part of code requirements to ensure that proper performance is met.

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Figure 20: Roof overloading failure due to snow, Source: Pete Wann via Flickr.
Finally, large amounts of snow and ice can be problematic for roof assemblies, often causing overloading failure to occur. Because flat roofs cannot shed snow or ice like high sloped roofs, designing for snow conditions can be difficult. Large snow storms can often exceed design snow load conditions, creating problems such as collapse like the metal panel roof in ​Figure 20​. However, a popular misconception is that roof collapse under snow load only happens during blizzard conditions. In reality, many roof failures occur due to snow accumulation over several storms. If snow does not have the opportunity to melt before the next storm, the weight becomes additive (Zurich 2003). In many cases, older snow compacts and turns to ice as it melts, or rain after a snow saturates a snow, creating even heavier loads.

Roof failure under excessive snow load can be due to several factors including the following:
  • Incorrect snow load design. Often code reductions for roof snow load versus ground load are misinterpreted, or do not account for things such as snow drift.
  • Poor design/construction of the structure. Bay size, connection details, or poor workmanship can lead to weakened structures that cannot support design snow loads.
  • Poor drainage due to ice. Often snow and ice can freeze over drains and scuppers preventing melted snow from draining.
  • Unbalanced snow load. Like in other load failures, an unbalanced loading situation between adjacent bays can cause failure or collapse to occur (Zurich 2003)

Unlike other building failures, snow load failure can be very difficult to prevent. Although proper roof design and maintenance of drainage systems are key to preventing the failure, it is almost impossible to design for every snow storm. Often, the majority of roof collapses occur during large snow storms with quick accumulation. In these cases, building owners should realize when too much snow is accumulating, and an emergency shedding plan should be put into effect. That is, there should be a plan in place to shovel off or manually melt snow on flat roofs to help shed weight quickly. For buildings in problematic snow regions, it is smart to have a snow shedding plan in place for each snow, even if it is not a major storm. Good warning signs to snow overloading failure include sagging decks, deforming or bowing steel frames, sprinkler heads pushing through ceilings, and large cracking sounds (Zurich 2003). It is important to start shedding plans early on, as waiting for further failure signs may create a dangerous situation. In all cases, the building should be evacuated and all precautions should be taken if large amounts of snow accumulate or failure signs occur.

Case Studies:


As mentioned previously, although the annual amount of roof failures in the United States is very high, most failures go unnoticed by the media or engineering community. However, there have been many instances that,either by the significance of the building or the significance of the failure, do bring national attention to the project. Some are caused by natural disasters, such as the roof uplift failure of the Louisiana Superdome during Hurricane Katrina. Others are caused by overloading due to poor design/lack of maintenance, such as the Knickerbocker Theater collapse in Washington D.C.. When looking back at these specific case studies it can be seen how important roof design is in the integrity of a building.

Uplift Roof Failure Case Study: Louisiana Superdome (2005)

Although it is not a flat roof, the Louisiana Superdome Roofing Failure in Hurricane Katrina in 2005 was a very serious and media-covered event which proves to be a prime example of a roof uplift failure due to strong wind. As over 10,000 people took shelter from what would become the U.S's most costly weather event in history, winds around 125 miles per hour from Katrina hit the stadium's large domed roof. This roof was a steel structured roof with steel decking. The weatherproofing consisted of a renovated EPDM roof membrane which was fully adhered to a rigid polyisocyanurate board (iso-board) substrate that was mechanically attached to the steel deck. With uplift pressures exceeding 100 pounds per square foot, the roof assembly eventually failed twice. First, the EPDM single-ply was ripped off the insulation. Then under further loading, the uplift pressures pulled up between the insulation and steel deck, eventually causing large sections of deck to fail and collapse. Amazingly, none of the people taking shelter in the stadium were injured, but large amounts of damage was done. This included roof assembly damage, structure damage, interior damage in the dome, and serious water intrusion.

Fluttering Membrane.JPG
Figure 21: Superdome roof fluttering before failure

It was determined that failure of the EPDM membrane was due to several factors. In addition to large suction pressures acting on the roof, design flaws of the EPDM roof restoration accounted for the failure. The lack of an air barrier in the 2002 replacement roof allowed wind forces to get under the roof system increasing the upward pressure. As high wind speeds continued, debris pummeled the thin membrane and mechanical equipment ripped off the roof causing holes and increasing pressures further. Eventually, sustained pressures started to peel the adhered EPDM membrane off the substrate and create a "fluttering" sensation (as seen in Figure 21). Insufficiently fastened lap strips then gave way and the single-ply membrane peeled completely off much of domed roof. After this first failure mode occurred, sustained winds continued pushing up on the rigid insulation. Eventually large portions of the steel deck became overstressed from the forces and failed, collapsing. Although no deaths occurred in this event, the Superdome roof uplift failure was very serious and cost millions to repair (Progar 2012).

To read more about this specific case study and the lessons learned from the failure, please visit the Louisiana Superdome Roofing Failure in Hurricane Katrina page on the Failures Wiki webpage.


Overloading Roof Collapse Case Study: Knickerbocker Theater (1922)


The Knickerbocker Theater collapse became a legendary case study focused on structural roof collapse and preventing design negligence. The theater building was a silent film theater constructed in Washington D.C.in 1917, becoming the largest theater in the area with a maximum capacity of 1700 patrons. The structure was designed with either 40 foot tall hollow tile with face brick or structural masonry bearing walls at the exterior with 4 interior columns for intermediate strength. One main girder (referred to as T11) ran from the northwest wall to an interior column as 3 secondary girders and a beam hung off this girder. Remaining beams ran parallel to the main girder at 9.5 feet on center to support the roof, which consisted of a 3-inch concrete slab. Final steel designs were changed by the fabricator to include heavy plate girders to replace some of the trusses ( with no engineering checks), and lateral stability issues during construction prompted the addition of a concrete belt course on the northwest wall and struts between the columns. During a blizzard that produced approximately 28 inches of snow in the winter of 1922, the strength of the roof structure was overstressed after the connection at T11 and the northwest wall failed and the secondary beams ripped off their seats on the main joist. This caused the large roof to collapse inward, as shown in Figure 22, killing 95 patrons and injuring 133 others during a silent film presentation.

Knickerbocker courtesy of the liberty of congress.jpg
Figure 22: Knickerbocker roof collapse. Source: The Library of Congress
Investigators at the time found evidence that the fatal roof collapse was due to both negligent designs and excessive snow loading on the roof at the time of failure. Although the roof was said to meet current code and inspections, it was found that there was a large amount of defects in the design which created a system that was inadequate to carry the required load, let alone a heavy snow load. In addition, seven total defects in the structure were found to directly result in the failure. First of all, T11 was overstressed and deflected due to unchecked fabrication changes that created a shallower truss and non-spliced moment plates. At the northwest wall connection point, proper anchorage was not present, the seat was only extended 6 inches onto the bearing wall, and bearing stresses created an eccentricity that was not accounted for in the design. Also, the bottom chord of the truss was not reinforced properly and each channel was independently subject to weight, torsion, tension and eccentric bearing forces not accounted for in the design. The top chord connection was missing a cover plate which put the system strength on a 1/2 inch gusset plate. Finally, out of plane stiffness was problematic at the northwest wall due to excessive windows and slenderness, and lateral stiffness at the columns was inadequate.

Through the investigations of the collapse it is thought that the failure mode of the roof occurred in the following steps:

1. Excessive snow load added to inadequately designed roof system.
2. Truss deflection under load created outward thrust on slender northwest wall, which caused it to move 5 inches outward.
3. Thrust worsened by eccentric bearing load, and twisting occurred at bottom chord of T11 due to non-perpendicular connection to the wall.
4. Top chord of truss became overloaded due to shallow design, and buckling and gusset plate failure occurred.
5. Channels bearing down on the seat were crushed due to excessive loading
6. T11 failed, followed by the secondary structural members, and the roof collapsed under the load.

Further investigations done by other forensics teams concluded that failure could have also been due to multiple beam failure, slab failure due to inadequately sized concrete deck, and column failure. Investigation also found that the main cause of failure and death was due to negligence by the architect, steel fabricator, hollow tile sub-contractor, concrete inspector, and project foreman. The seriousness of their neglect caused them to be indicted with counts of manslaughter, and their careers were ruined. The lessons learned from this serious case study included implementation of better inspections, standardizing the building code, and creating the requirement of licensing for all parties involved with design to ensure competency (Kijak 2012).


To learn more about this specific case study, please visit The Knickerbocker Theater Collapse page on the Failures Wiki webpage.

Also, additional roof failure case studies can be found by searching the general Failures Wiki webpage.

Conclusion


After reviewing some of the most common failure scenarios for the performance of a roofing system and the integrity of a roof system, several conclusions can be made. First, and foremost, it is a necessity that both designers and contractors have an understanding of the systems involved, and work together closely to obtain full attention to detail. Items such as location, building type, occupancy, loading, scenarios, etc. should be thoroughly analyzed as well. Just as important, owners or facility managers must also understand the roofing system they have and follow all recommended maintenance or replacement procedures given by the system manufacturer. Second, although minor in many cases, performance failures to roofing systems can eventually lead to greater (and more expensive) problems, and should, therefore, not be ignored. Preventing deferred maintenance by repairing the problem early can save a building owner and other parties involved a lot of money and possibly even court time. Finally, it is important that we, as an industry, learn from our mistakes. By understanding past roof failures, we should be able to improve both the design and integrity of the systems we create, and prevent catastrophic failure from occurring again.




Annotated Bibliography



ASCE. (2005). "Wind Loads". "Minimum Design Loads for Buildings and Other Structures, ASCE/SEI. p(41). <http://www.ce.udel.edu/courses/CIEG407/CIEG_407_Protected/Chapter%206%20Wind%20Loads.pdf>.
(accessed October 24, 2012).
  • ASCE-7 2005 is used to show help explain uplift force locations on a low-sloped roof.

Cullen, William C. (1965). "Effects of Thermal Shrinkage on Built-up Roofing." U.S. Department of Commerce, National Bureau of Standards. p(1-6). <http://digicoll.manoa.hawaii.edu/techreports/PDF/NBS89.pdf>. (accessed September 26, 2012).
  • This journal article goes into further detail about a specif failure type prevalent among buildings. This aids the wiki page as a detailed example to a failure.

Dixon, Craig, and Prevatt, David. (2010). "What Do We Learn from the Wind Uplift Tests of Roof Systems." ASCE Structural Congress 2010.<http://www.davidoprevatt.com/wp-content/uploads/2010/02/dixon-prevatt-struct-cong-wind-uplift-testing-01-25-2010.pdf>. (accessed September 26, 2012).
  • This journal article goes into further detail about a specif failure type prevalent among buildings. This aids the wiki page as a detailed example to a failure.

Jacobus, Casey(June 2011). "Why Buildings Fail." Greater Charlotte Biz, Novemebr 2011. <http://www.greatercharlottebiz.com/article.asp?id=1267>.11/35 (accessed October 15, 2012).
  • This article is used to help explain the significance of roof repair and litigation within the building industry.

Kijak, Steven. (2011) "Knickerbocker Theatre, Washington D.C. (January 28, 1922)." Failures Wiki. < http://failures.wikispaces.com/ Knickerbocker+Theatre+Collapse >. (accessed October 24, 2012).
  • This Wiki page goes into detail about the historic roof collapse of the Knickerbocker Theatre in Washington D.C.

Kirby, James. (May 1997). "Avoiding Ponding Water by Positive Drainage." Professional Roofing. NRCA p(56). <http://docserver.nrca.net/technical/5649.pdf>. (accessed November 23, 2012).
  • This article is used to introduce ponding water and the significance of the failure

Madsen, Jana. (February 2004). "The Top 10 Most Common Roof Problems." Buildings Magazine. p(44-47). <http://www.buildings.com/tabid/3334/ArticleID/1735/Default.asp>. (accessed September 26, 2012).
  • This article is used to introduce and explain several different common failure types for roofs and go into detail on the causes behind them.

Malpezzi, Joseph and Gillenwater, Richard. (April 1993). "Static vs. Dynamic: A Wind Uplift Testing Study". 10th Conference on Roofing Technology. NRCA. p(123-130). <http://docserver.nrca.net/technical/1827.pdf>. (Accessed November 10, 2012).
  • This article is used to go into detail about dynamic uplift loading and the consequences

Nelson, Peter.E. (1985). "Failure Investigation and Testing of Single-Ply Roofing Membranes." National Roofing Contractors Association. p(86-88). <http://docserver.nrca.net/technical/285.pdf>.(accessed September 26, 2012).
  • This journal article goes into further detail about the failure and testing procedures of a specific type of commercial roof system. This aids the wiki page as a detailed example to a failure.

NRCA. (2012) "Roof System Types." National Roofing Contractors Association. <http://www.nrca.net/consumer/roofsystems.aspx>. (accessed September 26, 2012).
  • Several different types of roof systems found in buildings will be identified and explained. This technical page on the NRCA website goes into detail on many roof types and aids in the introduction to roof types.

NRCA. (2005) "The NRCA Roofing and Waterproofing Manual—Fifth Edition (2006 Update)". p(467). <
http://www.nrca.net/rp/technical/manual/06pdfs/467_578.pdf>. (accessed September 26, 2012).
  • The NRCA Roofing Manual will be used to help define roof system and differentiate it from the generic term of roof.

Progar, Josh. (2011) "2005 Louisiana Superdome Roof Failure During Hurricane Katrina." Failures Wiki. < http://failures.wikispaces.com/ Louisiana+Superdome+Roofing+Failure+in+Hurricane+Katrina >. (accessed October 24, 2012).
  • This Wiki page goes into detail about the Hurricane Katrina wind uplift failure at the Superdome in Louisiana.

Schneider, Reinhard. (June 2008) "Wind Uplift Solutions - Increasing the Durability of Roofing Systems." RCI Interface, Roof Consultants Institute. p(19-26). <http://www.rci-online.org/interface/2008-06-schneider.pdf>. (accessed September 26, 2012).
  • This journal article goes into further detail about a specif failure type prevalent among buildings. This aids the wiki page as a detailed example to a failure.

Smith, Tom. (October 21 2011) "Building Envelope Design Guide-Roofing Systems." Whole Building Design Guide. <
http://www.wbdg.org/design/env_roofing.php#desc>. (accessed October 1 2012).
  • This design guide is used to come up with roof system information for each roof type. ALso, detailing and other design criteria useful for the description of the roof systems and their failures are used.

Various. (Winter 2010) "Roof Leaks: Pinpointing and Repairing." Waterproof!@ magazine. p(18-25). <
http://www.waterproofmag.com/back_issues/201001/roof-leaks.php>. (accessed September 26, 2012).
  • This journal article goes into further detail about a specif failure type prevalent among buildings. This aids the wiki page as a detailed example to a failure.

Warseck, Karen. (April 2003) "Roof Failure: Effect and Cause." Building Operating Management. p(32-36). <
http://www.facilitiesnet.com/roofing/article/Roof-Failure-Effect-and-cause--1575#>. (accessed September 26, 2012).
  • This journal is used to introduce and explain several different types of roof failure and their underlying causes.

Zurich. (November 2003). "Snow Loading Roof Collapse." Zurich Services Corporation. <http://mysite.verizon.net/res133uz5/sitebuildercontent/sitebuilderfiles/snowloading.pdf>.(Accessed October 12 2012).
  • This article explains warning signs and steps to take to prevent roof collapse due to snow load.



onding water
· Flashing
· Surfacing
· Fasteners
· Shrinkage
· Ponding water
· Abuse, neglect, and lack of maintenance
even after adhering the membrane to a substrate, it is necessary to ensure fastener spacing along the perimeter of the system and on edge and corner substrates is adequate to meet manufacturer guidelines to prevent uplift movement. Often a testing standard such as Factory Mutual (FM) or Underwriters Laboratory (UL) is used to rate the system being constructed.