Posted on July 30, 2012
Recently, CIE has performed a number of roofing claim investigations where the cause was stated as weight of ice and snow. A defective condition that has been making the rounds claiming to be caused by the weight of ice and snow are vertical cracks. While we have found one instance of horizontal cracking attributed to the weight of ice, direct damage to the roof shingles is unlikely. Some of the claims that have been investigated:
CIE has found a range of snow and ice accumulation loads ranging from 20 psf (pounds per square foot) to 30.5 psf. Normally, this loading would cause bending but typically does not break anything. Most design codes in the region require at least a 30 psf ground snow load. The design code tolerance allows for a specified amount of bending at the design load. Typically, a 1" in 20' is the tolerance for most roof structures. A 1" deflection in even 10' is quite small and will not cause cracking of competent asphalt shingles. The deflection of the plywood between rafters is also pretty shallow, an eighth of an inch is the maximum expected deflection of the plywood sheathing between rafters. Even if you the amount was double this amount, the shingles wouldn't crack unless they were well past their design life.
Several Roofing Contractors and Public Adjusters have claimed that their clients roofs have failed with vertical cracks on many of the slopes. Typcially, the roofs are 10-20 years old and there are a number of vertical cracks located on opposing slopes. The vertical cracks are more extenisve on the west and south slopes. After the first few claims came in we checked with some of our peers in Minnesota, Wisconsin and Calgary, because we didn't see any damage. The frigid area consultants confirmed that roof direct damage caused by ice or the weight of ice and snow was rare.
Vertical cracks occur to shingles when the tensile strength of the shingle is exceeded. Frequently, the vertical cracks are found clustered in vertical patterns and conform to a 3' repeating pattern horizontally. It is easy to understand why this problem is attributed to excessive deflection caused by snow and ice loading. However, CIE examined a number of these and schematically diagrammed the locations and compared these locations to the rafter and truss supports locations. It's fairly easy for an engineer to calculate the expected deflection from a load, typically, the amount of deflection, short of a broken rafter or truss, is less than 1" in a span of 15'. Ice lifting a shingle and causing a crack, kind of like watching an iceberg melt, isn't happening fast enough to cause the stress needed. These dogs won't hunt. When we had the first claim of this nature we decided to thoroughly understand the mechanisms causing the problem.
What can cause the tensile loading. Roofing contractors and other benefiting parties would have you believe the vertical cracks are from the snow and ice load. Industry professionals often cite thermal cracking. So WHAT is THERMAL CRACKING? Most materials will expand and contract with hot and cold temperatures.
Many of the rafter problems observed this winter were due to the lack of lateral restraint. That is, the horizontal load from the rafter pushed on the top of the wall and the wall leaned out. The visual evidence of this is to look at the ridge line and see a sag in structure. Cathedral ceilings were a common source of these failures; there are special construction details needed to properly support a cathedral ceiling which are often overlooked. Even conventional ceilings were found with defective lateral restraint; typically, the rafters weren't connected to the ceiling joists, or ceiling joists ran the wrong way. We found a lot of "wrong ways" while investigating structural defects and collapes.
A number of the total collapses we observed had components of decay in the rafters. If you take a rafter that is undersized to start with and decay begins to chew on the fibers, the member gets overloaded pretty quickly. The snow events are a convenient way to demolish a delapitated structure. It is my opinion that the proximate cause of many of these collapses is not the result of ice and snow weight, but the strength reduction afforded by the neglect and decay. Decay is an inherent vice that progressively weakens the wood member.
Our team has been surprised by the number of trusses that failed. As previously mentioned, the snow loads approached the design load but rarely exceeded it. Even with the loads exceeded, the margins of safety would have prevented a collapse or member failure. Ask a fireman what they think of trusses, while they are an efficient use of materials, they rely on the integrity of each piece.
CIE investigations found manufacturing defects, design defects and installation defects. The most common failure involved metal plates with wooden trusses. We commonly found plates pulled off the members. Once you lose one member, the truss starts to tear apart leading to failure. Once one truss fails, there is a domino affect and more start to fail until there is a catestrophic collapse. The manufacturing defects found were typically poor quality control on the placement and embedment of the plates. Design defects included undersizing members and not accommodating extra loading needs. Installation failures included the lack of bracing per manufacturers specifications. The problem with trusses is that they can partially fail and remain hidden for years. CIE was called onto a claim where two trusses had punched through a ceiling. After climbing the roof, we did not find any unusual loading scenario; it was a long, wide slightly pitched roof. The attic revealed dozens of trusses of the same construction and after pulling back insulation we found an additional six (6) trusses that had failed connection plates. Disaster averted! What surprised me was that there were engineered repairs made previously to a few trusses on the other slope. CIE recommended the trusses be measured and independently analysed since the design loads were not met and the failure occurred due connection failures and a lack of bracing.
We have seen some proposed contractor solutions that often exceed the demands and needs of the loss. As an example, countless times contractors have stated that the whole roof needs to come off and be replaced. Fortunately, for some of our clients, we have the resources from a couple of structural contractors who are pretty clever in fixing these problems at small fraction of the replacement costs. Here are a couple of the simpler solutions that can be effected:
Unfortunately, many of the problems have nott been identified or recognized. This past winter collapsed a lot of the already weakened structures, it may take another dozen years before the structures weakened by this past snow and ice buildup cause a claim. We have also visited a number of structures where there was great concern on the part of the property owner, most of the time we found just excessive deflection cracks in the drywall and found a simple solution to prevent a reoccurrence. It is just as rewarding to provide peace of mind!