Snow-Induced+Roof+Failures+and+Prevention+Methods

Snow-Induced Roof Failures and Prevention Methods
 * An Overview **

//Alyssa Stangl | BAE/MAE | Pennsylvania State University 2013 //

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Introduction
Every year, winter brings everything from a light dusting of snow to intense snow storms such as that in // Figure 1 // – these storms are known to cause roof collapses and failures. As a result of this, snow is an important design factor for structures throughout most of the United States. Extreme snow loads have always been a cause for concern for designers and owners alike, and they are often responsible for roof failures and other structural problems, causing millions of dollars in damages and operation interruptions (Zurich 2003). It is the engineer’s responsibility to design a structure to withstand the extreme and sometimes unpredictable loads.

A few types of snow loads known to cause failures include snow drifts, unbalanced snow loads, rain-on-snow loads, and sliding snow. However, structural failures and problems are often the result of improper maintenance, inadequate design, construction errors, and improper snow removal. Many designers and investigators may also argue that inadequate code requirements for snow loading are to blame for many snow-induced roof failures. As a result of these allegations and several bad collapses, some code changes have been implemented. Snow loads can be intense and can cause devastating collapses, but these complications may be avoided by implementing a few preventative measures and by performing prompt and proper snow removal. The following article will provide an overview of the different types of snow loads typically causing failure, a few case studies, prevention methods, proper snow removal methods, and code changes.

Key Words
snow, accumulation, snow drifts, sliding snow, roof failure, preparation, prevention, ASCE 7, snow removal, warning signs, prefabricated metal building

Types of Snow Loads
There a several different types of snow loads that are notorious for causing snow-induced roof failures. These include snow drifts, unbalanced snow loads, rain-on-snow loads, and sliding snow. Usually, engineers design buildings to withstand these loads; however, roof geometry and heavy snowfalls can exacerbate the effects of the loads, causing failures. Many snow-induced roof failures are due to loads that are higher than the anticipated average on a small area of the roof, as opposed to a uniform load over the entire roof (O’Rourke 2008). This unbalanced loading condition can be difficult to anticipate and design for; however, e ngineers are required by the International Building Code/ ASCE 7 to calculate and design for these different types of snow loads. See Figure 2 below to view different problems that can result from snow drifts.

Snow Drifts
Snow drift is the first load condition to be discussed. Snow drifts need three elements to form: a source of drifting snow, wind capable of lifting snow, and a geometric irregularity (O’Rourke 2008). Mr. O’Rourke in //Lessons Learned: Structural Collapse from Snow Loads// claims that the most common geometric irregularities are roof steps or changes in roof elevation, parapet walls, and gable roofs. Roof steps are very common and can experience both leeward and windward drifts. For leeward drifts, the snow source is the roof above, and, for windward drifts, the snow source comes from the lower roof. Both are a result of winds carrying and depositing snow up against the wall connecting the roof step. Larger drifts, however, are often the result of windward drifting on a flat lower roof (Buska 2001). Roof st ep snow drifts are responsible for about 20% of snow induced roof failure studies according to a study performed by Mr. Michael O'Rourke (O'Rourke 2008). //Figure 2// shows the effect of Roof Step Drifting (provided Courtesy of James Buska from // Minimizing the Adverse Effects of Snow and Ice on Roofs). // //Figure 3// shows the differences between windward and leeward drift conditions.





Parapet walls, as mentioned above, are another common cause of snow drifting. In this case, the drifts are caused by windward drifts pulling snow from the roof level up to the parapet wall. As the snow drift increases in size, the wind will streamline over the parapet causing the snow drift to form the traditional right triangle associate with snow drifts. However, initially, a vortex can be created due to the shape of the snow. Parapet walls become more of a problem as the wall increases in height and the roof gets larger. The taller wall allows more snow to accumulate, and the large roof provides more snow for drift formation. According to Michael O'Rourke, parapet wall drifts are responsible for about 11% of snow induced roof failures (O'Rourke 2008).

Gable roofs also provide a unique opportunity for drift formation. Drifts associated with gable roofs result from leeward winds. B ased on the behavior of the wind creating the drift condition, gable roofs with slopes of 0.5/12 can experience drift formations. Otherwise, the wind streamlines do not connect properly to allow snow drifts to form. This is currently the slope indicated by //ASCE 7 - Minimum Design Loads for Buildings// (O'Rourke 2008). Finally, several drift conditions can occur at the same location at the same time creating a “combined drift” condition. An example of this would be a parapet wall on the edge of a roof that has a slope of 0.5/12 or greater. Here, the resulting drift is the result of two combined drift effects: the parapet effect and the gable roof effect. These types of loads are very difficult to anticipate and design for, but care must be taken to recognize when the possibility for combined drift exists.

Sliding Snow


Sliding snow is another source of roof performance problems; this occurs when snow slides from a high roof onto a roof below. This is of particular importance on roofs that are slippery and unobstructed (do not have snow guards). When sliding occurs, additional snow loads can be imposed on a lower roof. ASCE 7 currently has provisions for the calculation of sliding snow loads; however, if the snow falls from a substantial height, the issue of dynamic loads may be imposed on the lower roof, which is not addressed by ASCE 7 (Buska 2001). Sliding snow also has the possibility of dragging roof elements with it such as mechanical equipment, parapet cladding, roofing materials, and much more (Buska 2001). //Figure 4// shows a plumbing vent that has been displaced due to snow sliding on an unobstructed metal roof //. //

Others
Some other sources of roof failures include ice dams, rain-on-snow, and misguided snow removal attempts. Ice damns usually form on the eaves; snow is able to melt on a heated roof and runs down the roof to the building eaves. At the eaves, where the temperature falls below freezing, the water will turn to ice. The thickness of the ice dam increases as more snow melts and refreezes at the eaves. Michael O'Rourke discusses a case where the ice dame reached a thickness of 6 inches for a 100 foot long eave (O'Rourke 2008). The load of the ice can become great enough to exceed the load bearing capacity of the structure or cause damage to other building elements. See //Figure 5// for a schematic of ice dam formation and potential associated problems.

Rain-on-snow is something that ASCE 7 now addresses in its provisions. Rain-on-snow is often not alone the cause of roof failure. It is usually just the "straw that broke the camel's back." According to Mr. O'Rourke, the rain-on-snow surcharge load at failure for most case studies is usually only about 4 to 5 psf (O'Rourke 2008). When compared to snow loads of 50 psf, the 4 to 5 psf seem insignificant but can be just enough to cause overloading of the structure.

There are also some cases of human negligence that can lead to snow-induced roof failures. For example, at one facility, officials attempted to remove snow from a garage using a fire hose. Due to the extra weight of the water, the roof finally collapsed (O'Rourke 2008). See the section below for proper snow removal techniques and methods of preventing roof failures.

Factors Contributing to Snow Induced Roof Failures
Large amounts of snow can accumulate and cause increased loads that a building designer may not have anticipated. However, there are other factors that can increase the likelihood of a snow-induced failure. These contributing factors include inadequate design, unoccupied status of the building, roof contour, roof obstructions, extra roof insulation, lack of emergency procedures, and poor workmanship or job-site control (Hoover 1996). Each of these factors has the power to cause a roof failure; however, if combined, failure becomes even more likely. //Figure 6// shows a metal building that collapsed due to higher than anticipated snow loads. ====

Inadequate design has become a less common cause of snow-induced roof failures since ASCE 7 included provisions for snow drifts and sliding snow. However, in the past, building codes did not include provisions for these things. Therefore, designs would not be strong enough to handle imminent drift loads. Building codes were only concerned with the maximum anticipated uniform snow load. However, even today, unanticipated combinations of snow, wind, live, and dead loads can cause roof failures, even when the designer considered all loading conditions as required by the building code (Hoover 1996).

Unoccupied buildings are often more susceptible to roof failures due to snow loads. If no one is returning to the building, no one will know if a problem is developing. Often roof failures happen slowly, and warning signs appear; but if no one is there to see these signs, the unoccupied building will fail. When a building is unoccupied, it is important to have a maintenance plan in place and emergency procedures established (Hoover 1996).

Roof contours and obstructions, as mentioned before, can create unique drifting conditions, especially when combined drift occurs. Roofs with irregularities should be closely monitored during heavy snow falls. Drift conditions can form rapidly, giving little time for emergency action if necessary (Hoover 1996).

Recently, the building industry has become very focused on energy efficient designs, meaning extra insulation to limit heat escape through the roof. This seems perfectly fine from an energy standpoint; however, from a snow loading standpoint, this could be a problem. The increased insulation decreases the rate of snow melting, allowing additional snow accumulation. This increases the snow load the building must carry. Furthermore, when renovating or retrofitting a building, it is important to keep things such as snow loading in mind; the building was designed for a specific snow load based on the building's thermal properties. If insulation is added or the mechanical system is updated, the designer should consider the impact on the building's structural loads (Hoover 1996).

Lack of emergency procedures is another common cause of roof failures. Many facilities will fail to create proper procedures for damage detection or preventative measures to avoid snow-induced collapses. Every facility should have a plan for action when heavy snowfall occurs and when, if ever, signs of failure begin to appear. Another common cause of snow-induced roof failures is poor workmanship or lack of quality control. A building could be design properly for all snow load conditions. However, if it is not constructed properly, it will not perform as intended, increasing the risk of catastrophic failure (Hoover 1996).

Snow Loads and the Building Code
In this section, the ASCE 7 code requirements for snow load calculations and design are discussed. To begin, Chapter 7 of ASCE 7-10 has been adopted by the International Building Code 2012. This chapter covers the proper procedure and specific requirements for the calculation of snow loads, which include uniform roof snow loads, sloped roof snow loads, flat roof snow loads, partial loading, unbalanced loads, drifts, sliding snow, and rain-on-snow surcharge loading. Designers are required to design buildings to meet the loads required by the current ASCE 7-10 standard. However, building codes have not always provided provisions for calculating snow load conditions beyond the flat roof snow load. This is discussed in more detail below.

ASCE 7-10 Chapter 7 contains the provisions for the calculation of the minimum design snow loads for a building. Every snow load calculation begins with a ground snow load, Pg. Section 7.2 directs the designer to Figure 7-1 which is the ground snow load map. Here the designer either selects the ground snow load for the appropriate location or must perform a case study to determine the proper ground snow load to be used. Next, Section 7.3 combines the effects of roof exposure, thermal conditions, and building occupancy to calculate the flat roof snow load to be used for design (ASCE 7-10). Section 7.4 outlines the procedure of the calculation of sloped roof snow loads. To determine the sloped roof load, the flat roof snow load is modified by a roof slope factor, Cs, which is determined based on the roof slope and thermal conditions (ASCE 7-10). Continuing through the snow load provisions, partial loading conditions are addressed in Section 7.5 for continuous beam systems and other continuous systems that could be affected by unbalanced loads. Section 7.6 discusses Unbalanced Roof Snow Loads in more detail, requiring that "Balanced and unbalanced loads shall be analyzed separately. Winds from all directions shall be accounted for when establishing unbalanced loads." The idea here is to determine the worst case snow loading condition for the specific structural system in question; hip and gable roofs, curved roofs, folded plate, barrel vault and saw-tooth roofs, and dome roofs are all addressed in ASCE 7-10. Next, requirements for the determination of drift loads are provided. Section 7.7 discusses the determination of Drifts on Lower Roofs, including lower roofs and adjacent structures. Adjacent structures is a new code provision introduced in ASCE 7-10. Section 7.8 goes on to outline the determination of snow loads created by roof projections and parapets. Finally, Sections 7.9-7.12 list the provisions for sliding snow, rain-on-snow surcharge loads, ponding instability, and existing roofs. Reference ASCE 7-10 Chapter 7 for more details about the calculation of snow loads.

Older versions of the building codes and ASCE 7 did not include provisions for things such as snow drifts which are often responsible for roof failures. Most older codes mention that snow drifts need be considered in design; however, no method of determination is outlined. Also, ground snow loads, in older code versions, were less than are currently found in ASCE 7-10. For example, the North-East region of the United States was remapped after a snowstorm between January 6-8, 1996. A second winter storm hit the North-East on January 12, 1996. Many buildings collapsed from the excessive snow accumulation. The snow accumulations from the storms in 1996 were found to exceed the 50-year recurrence interval; some were even found to exceed the 100-year recurrence interval. As a result, it was determined that a remapping of the ground snow loads in the North-East was necessary (DeGaetano 1997). As a result, in the current version of ASCE 7, the ground snow load map lists higher values for some areas and lists many others as site specific case study regions.

Snow-Induced Roof Failure Cases
Snow loads cause many roof failures every year, some years and locations resulting in more failures than others. The collapses range from serviceability failures to full structural collapse. Every collapse has different effects on the community depending on the building use, location, and the severity of collapse. This section will go into more detail about a specific case and some general effects of a two intense winters.

**Winter of 2011 - New England**
In the winter of 2011, New England was devastated by a series of snow storms. Along with heavy snows, a series of freeze-thaw cycles plagued the region. Many collapses occurred and the exact number is unknown. However, many people had narrow escapes from the collapses. According to the New York Times, two men were working on the second floor of a 120-year-old building in Middletown, Connecticut. They saw the roof beams begin to buckle and ran out of the building as quickly as possible. Moments later, the building collapsed (Seelye 2011). The constant back-to-back storms has left many people shut in their homes. As stated by Katherine Seelye in her New York Times article, "The effects have rippled through every aspect of life, cooping up people with cabin fever...". Many businesses suffered time and product losses. One example was Mike's Barber Shop; the owner was able to salvage a few items from the shop but most items were lost. Furthermore, schools were closed for fear of that the roofs would collapse at any moment. It is apparent that the lives of anyone in Middletown, Connecticut were put on hold as a result of this series of snow storms (Seelye 2011). This series of storms was also tracked closely by AccuWeather. AccuWeather describes in incident in Lynn, Massachusetts, were a senior services building roof collapsed under tons of snow. Fifteen people were inside the building when the roof collapsed; most escaped unharmed except two men that were later freed from the rubble by rescue workers (AccuWeather 2011).

**Winter of 2013 - New England**
In February of 2013, more than 16 structures collapsed under snow loads in East Haven. Efforts were made to remove snow from local roofs; most, however, were too little too late. Many of the roofs collapsed despite the best efforts of the community. Officials of the town recommended that residents hire experienced contractors, with a home improvement registration, to remove snow from the roofs of their homes (NBC Connecticut 2013).

**Snowdrift Roof Collapse - Waterville, Maine**
One particular case that will be discussed in detail occurred in February of 1994 in Waterville, Maine. On February 9th, a new junior high school was opened. A few days later, the roof collapsed (Zallen 2001). The school is divided into 6 sections and is irregularly shaped. The roof elevations change depending on the use of the building segment. The average height difference between adjacent segments is 7 feet. The building roof system is composed of a wood fiber deck tongue and groove planks. These are nailed to the open web joists. The joist chords are made of laminated wood and the webs are made of hollow steel tubes. Joists are supported by concrete and masonry bearing walls. Some of the joists cantilever over the exterior masonry bearing walls (Zallen 2001).

The roof was designed to handle 75.4 psf. The dead load at the time of collapse was 19.7 psf, leaving about 56 psf for the uniform snow load (Zallen2001). The designers did not include additional loading condition for drifting snow. The 1967 National Building Code only required a 40 psf uniform snow load for design. However, ANSI, which supersedes the National Building Code, requires 56 psf for design and recommends methods of accounting for drifting snow (Zallen 2001).

The day of the collapse was very windy; most snow was removed from the roof except for those caught at the roof steps. The average density of the snow drift, including snow and ice, was about 24 pounds per cubic foot. Several of the joists failed under the load. The deck was supposed to distribute the load to surrounding joists; however, the load on the nearby joists was already at maximum capacity, causing these joists to fail as well (Zallen 2001). The load on the open web joists were determined to be 1.7 to 2.0 times the allowable load. According to Zallen Engineering, "... the most probable cause of failure is that the ultimate strength of a joist was exceeded by the loading, that joist failed, and the adjoining joists could not sustain the additional load transferred to them, causing the failure to propagate (Zallen 2001)."

It was determined after some code analysis that if the design engineer would have followed the ANSI code recommendations for snowdrift loading, the roof most likely would not have collapsed; it would, however, have been significantly overloaded (Zallen 2001). The snow drift, however, was higher and more dense than that anticipated by ANSI.

Other Cases
Below is a list of several different snow loading case studies. The names below are linked to another Failures Wiki page that discusses the case in detail. Please review these cases for more information.


 * Hubert H. Humphrey Metrodome Roof Snow Collapse of 2010
 * CW Post College Auditorium Collapse
 * Hartford Civic Center (Johnson)
 * Knickerbocker Theatre Washington DC
 * Katowice Poland

Snow-Induced Roof Failure Prevention
There are several things an owner can to do to prevent a roof collapse due to a snow event. There are also things that one can do if he or she suspects that there is a problem with a roof, which includes knowing the warning signs for failures. These things are divided into several categories: Things to Do Before Snow Accumulation, Things to Do After Snow Accumulation, and Warning Signs of Potential Collapse/Overstressing. Snow removal methods could be included in this section; however, this detailed process has been given its own section. See below.

**Before Snow Accumulation**
Before a winter storm hits, there are certain measures a building or home owner can take to help limit potential building damage and safety issues. First, always have an emergency response plan in place before a snow event occurs, and make sure that everyone has easy access to the plan. Next, the maximum safe snow depth for the building's roof structure should be determined. This can be determined by referencing the roof live load capacity on building drawings or specifications (CNA 2010). The roof should also be regularly inspected for structural integrity and should be repaired as required. The roof should also be inspected to ensure that proper draining will occur; this includes clearing all drains, downspouts, gutters, etc (CNA 2010). Furthermore, if the owner wants to be proactive, he or she may solicit a local roofing contractor with the proper credentials for snow removal. This ensures that in the event of a storm a proper contractor will be available (Zurich 2003).

**After Snow Accumulation**
When snow begins to accumulate on a roof, it is very important to closely monitor the snow depth, especially in areas where snow drifts are possible (See section above regarding Snow Drifts for more information). According to the CNA Risk Control Bulletin, snow should be removed from the roof before snow depth reaches 50% of the "safe" maximum depth (CNA 2010). If the snow accumulation becomes too close to the roof live load capacity, DO NOT send workers to the roof to remove snow. Furthermore, snow should only be removed during the snow storm if additional accumulations are expected to exceed the roof maximum depth, and snow removal is absolutely necessary to maintain structural integrity. Next, snow should always be removed in layers to limit the occurrence of unbalanced live loading. Avoid the formation of snow piles on the roof for the same reason (CNA 2010). Finally, snow and ice should be cleared from drains and gutters to allow proper drainage of melting snow (CNA 2010). It is important to remember that snow removal can be a dangerous process if done incorrectly; it is recommended that owners hire a properly qualified snow removal, roof contractor to clear the roof.

**Warning Signs of Potential Collapse and Overstressing**
Before a collapse occurs, there are often warning signs of overstressing. If any of these things occur, evacuate the building immediately - failure is imminent! The following are common signs of overstressing as provided by Zurich Corporation in //Risk Topics// and by the Professional Loss Adjusters in //Finding Nemo - Avoiding Snow Roof Failure//.

Failure signs include but are not limited to:
 * Sagging steel roof members
 * cracked or split wood members
 * sprinkler heads pushing down ceiling tiles
 * doors that pop open
 * doors or windows that are difficult to open
 * bowed pipes or conduit
 * creaking, cracking or popping sounds

If any of these things occur, evacuate the building, and contact a qualified structural engineer.

Proper Snow Removal Methods
After a snow event, the method of snow removal is very important. The key is to avoid creating an unbalanced loading condition by removing snow in an incorrect pattern. Because the process of removing snow can cause a roof failure if done improperly, there a many guides for proper snow removal (Professional Loss Adjusters 2013). As an example, //RiskTopics: Snow Loading Roof Collapse// prepared by Zurich Corporation provides a thorough step by step guide for the snow removal process for one type of steel framed building.

// Note: // //It is important to note that the correct snow removal process and sequence depends on the type of roof system and framing components in addition to any unbalanced loading conditions that may exist. As such, it is recommend that a structural engineer be consulted for any major snow removal situation//.

**Preparation for Snow Removal**
Before removing any snow, the following items must be addressed. First, can the snow safely be removed by the owner or a contractor? If not, a properly qualified contractor or structural engineer should be contacted. Second, determine if there is time to remove the snow before failure occurs. If the building is showing warning signs of failure, a structural engineer should be contacted to determine if snow can be safely removed and, if possible, snow removal should occur promptly. Also, it is a good idea to select a contractor in advance of a snow event and retain them on contract as mentioned previously (Zurich 2003). Remember, before any snow is removed, it is recommended that a professional be contacted.

**Snow Removal Process**
The following snow removal process is outlined by Zurich Corporation and is specifically for prefab-metal buildings (Zurich 2003). A design professional should be contacted before any major snow removal begins.

1. Remove drifted snow first. 2. Remove snow from the middle bays of the roof structure. 3. Remove snow evenly from both sides of the roof so that the live load remains balanced as possible. 4. Do not remove snow from one roof and place it on another. 5. Be careful when removing snow or ice to not cause damage to the roof membrane. 6. Avoid walking on areas of the roof and compacting snow on adjacent roof segments.

A diagram of this snow removal procedure for a metal building, along with some additional tips, can be seen in //Figure 7// below.



Conclusion
Snow storms and winter weather conditions create unique loads on buildings that designers are required by code to consider in building design. However, sometimes the forces of nature are too strong and overloading of structures occurs. Many roof failures occur every year in the United States, particularly in the Northern regions of the country. These failures can cause millions of dollars in damages and operations interruptions. It is important for a building or facility owner to understand the building and know its limitations prior to a snow event. A plan for action and building monitoring during and after a snow event should be in place. Also, a professional should always be consulted before snow removal begins. Remember, if the building shows any signs of overloading or fatigue, evacuate immediately. Snow is beautiful but we must be wary as designers and owners of its effects on buildings and the associated potential for building failure.

Annotated Bibliography

 * AccuWeather (2011). “Roofs Collapsing Under Tons of Snow in New England.”  (Sept. 16, 2013).**
 * This news article briefly describes the failure of several roofs in the New England area in January 2011. Snow accumulation amounts and frequency of failure were reported.


 * ASCE Standard 7 (2010). Minimum Design Loads for Buildings and Other Structures, American Society of Civil Engineers. Chapter 7, 29-41.**
 * This will be used to explain the calculation of snow loads and how engineers are expected to account for snow. I will also be using figures from this standard in order to illustrate different loading types.


 * Buska, J. Tobiasson, W. (2001). “Minimizing the Adverse Effects of Snow and Ice on Roofs.” International Conference on Building Envelope Systems and Tehcnologies. 339-346.**
 * This document uses ASCE 7 to step the reader through the effects of geometry on snow loading. It also explains how problems can be avoided if more functional architectural designs are developed.


 * CNA (2010). “Snow Loading and Roof Collapse.” Risk Control Bulletin, CNA Financial Corporation. 1-3.**
 * This article discusses different ways to prevent roof collapse due to snow loading. It also discusses the loss scenarios and loss analysis associated with filing a claim with an insurance company. It also lists lessons learned from past failures.


 * DeGaetano, A., Schmidlin, T., and Wilks, D. (May 1997). "Evaluation of East Coast Snow Loads Following January 1996 Storms." Journal of Performance of Constructed Facilities, ASCE, Vol. 11 - 2, 90-94.**
 * This article discusses the impact of the winter of 1996 on the ASCE 7 ground snow loads. As a result of many failures, the ground snow loads were increased and many areas became site specific case-study regions.


 * Geis, J., Strobel, K, and Liel, A. (2012). “Snow-Induced Building Failures.” Journal of Performance of Constructed Facilities, ASCE, July/August 2012 Edition, 377-388.**
 * This journal discusses snow-induced failures all over the U.S. and between 1989 and 2009. It determines the failure rates of different building types and causes for snow-induced failure. It also discusses building failure trends.


 * Holicky, M., and Sykora, M. (2009). “Failures of Roofs under Snow Load: Causes and Reliability Analysis.” Forensic Engineering Congress 2009, ASCE 2009, 444-453.**
 * This journal article discusses roof failures between 2005-2006 due to snow loadings. The article discusses the modes of failure due. It also explores the possible necessary changes to code values in order to build more reliable structures.


 * Hoover, S. R. (1996). “Preventing Snow Load Roof Failures.” Plant Engineering September 1996, 115-116.**
 * This article discusses measures one can take to prevent snow load failure in a structure unable to handle the loads. It describes the factors that will lead to building damage and preventative measures.


 * Larson, J. B., DeLeon M. A., and Ahuja D. (2012). “Case Study: Failure at an Abutting Lower Roof due to Snowdrift.” Forensic Engineering Congress 2012, ASCE 2013, 1278-1287.**
 * This journal article discusses a lower roof failure due to snow drift. The case study finds construction deficiencies; however, the snow drift causing failure was twice what the code calls for design. The article calls for code changes.


 * NBC Connecticut (2013). “Several Buildings Collapse Under Snow.”  (Sept. 16, 2013).**
 * This news article discusses the collapse of 16 buildings across Connecticut due to snow loads in February 2013. It gives tips to homeowners for snow removal.


 * O’Rourke, Michael (2008). “Lessons Learned: Structural Collapse from Snow Loads.” STRUCTURE Magazine, Lessons Learned, November 2008, 42-44.**
 * This magazine article categorizes roof collapses based on histories from forensic consulting. This article discusses different causes of snow failures. It also describes how snow loads will behave on gabled roofs, parapet walls, and multilevel roofs. It also discusses combined drifts, rain on snow loads, ice dams, sliding snow, and other odd conditions.


 * O’Rourke, Michael (1990). “Roof Snowdrifts Due to Blizzards.” Journal of Structural Engineering, ASCE, 116(3), 641-658.**
 * This journal discusses the two types of drifts that occur on multilevel roofs. It also talks about some cases associated with failures due to snow drifts. It also analyzes the increase in snow drifts as a result of snow, increased wind, and other factors associated with a blizzard.


 * O’Rourke, Michael., Wikoff, Jennifer (2013). “Snow Related Roof Collapse and Implications for Building Codes.” STRUCTURE Magazine, Structural Forensics, January 2013, 18-21.**
 * This magazine article discusses the winter of 2010-2011. This winter was very snowy and a lot of snow-induced failures occurred. This article explains the background of grown vs. drift snow loads. It then compares actual snow loads to simulated snow loads and discusses what went wrong that caused failures.


 * Professional Loss Adjusters, Inc. (2013). “Finding Nemo: Avoiding Snow Load Collapse.”  (Sept. 16, 2013).**
 * This web based article gives practical advice for determining if failure is imminent and how to prevent failures. It provides warning signs, safety issues associated with unbalanced snow loads and sliding snow, and how to remove the snow safely.


 * Seelye, Katherine Q. (2011). “Winter’s Punch Crumbles Roofs in New England.” The New York Times,  (September 6, 2013).**
 * This new article explains the roof collapses associated with an intense series of snowstorms in the New England area in February of 2011. It also goes into detail about how the multiple failures have affected the towns functionality as well as the emotional impact.


 * Zallen, Rubin. (2001). “Snowdrift Causes Roof Collapse.” Zallen Engineering: Forensic Engineering in Construction. No. 4, 1-7.**
 * This article discusses a specific roof failure on a junior high school in Waterville, Maine on February 9, 1994. It explains the building architecture before the collapse, and the roof after the collapse occurred. The article also explains causes of collapse as well as shortcomings in the design. Also, an investigation of the code at the time of collapse is discussed showing inadequacies in the existing code at the time.


 * Zurich Services Corporation. (2003) “Risk Topics: Snow Loading Roof Collapse.” Risk Engineering, Zurich Services Corporation 2003, 2-7010, 2-7.**
 * This is a very comprehensive journal article discussing snow load roof collapses. It explains why collapses occur with examples and images, how to prepare for a storm, warning signs for collapse, action to prevent collapse or damage, and snow removal and preparation with diagrams.