Roof Damage Discoveries from March 2003 Blizzard
The weight of ice and snow from the March 2003 DenverMetro area snowstorm caused significant damage to many structural components of roof systems, including catastrophic collapse. However, the snowstorm also caused damage to other roofing components. Weight of ice and snow, along with sliding and/or drifting snow, caused damage to the roof coverings and waterproofing characteristics of many roof systems. Damage to the roof coverings and waterproofing typically resulted in water infiltration and water damage claims.
During our various observations of damaged roof systems, there were two characteristics that were noticeably common. First, when damage was observed to the roof coverings, the roof was typically nearing the end of its useful life or had previous long-term damage. The loadings created by the snowstorm would then exacerbate an already poor condition. Second, the roof systems that experienced deflection or structural damage were typically older roofs, which were not designed and/or constructed to current design standards. Depending on the age of the roof system, a roof structure that was properly designed when it was originally built may be able to safely withstand the types of loads created by drifting snow accumulation on the roof during a large storm event. In general, current minimum design requirements for roofs are more stringent than the design criteria used during the construction of older homes.
PIE also observed many low-sloped built-up roofs that experienced damage from the snowstorm. In general, the roof support members deflected during the snowstorm, resulting in a separation between the felts and the perimeter metal edge flashing of the system. The roofs were typically at the end of their useful life, which results in a very brittle and aged system. Thus, with deflection of the roof system, an already brittle and aged system separates from the perimeter edge flashing and experiences cracking. The flashing separations and cracking results in water intrusion into the buildings. In addition to low-sloped roof systems, many sloped roofs experienced similar damage, as the deflection caused separations at the flashings. The separations of the flashings from their original position resulted in water infiltration.
Deflection of roof sheathing between support members was also observed. The sheathing deflection typically resulted in a “wavy” look of the roof, as observed from the exterior. In most cases, the roof sheathing was already weakened by improper roof ventilation, which allows moisture from the home to build up in the attic. The moisture forms condensation on the underside of the sheathing, typically causing expansion, buckling and delaminating of the sheathing. The degrading of the sheathing results in reduced nail holding strength, deflection between the supports and a reduction in strength. In other cases, the roof sheathing was already bowing prior to the snowstorm due to multiple layers of roofing materials or long-term deflection. Thus, the loadings created during the snowstorm worsened an already weakened or deflected condition causing additional deflection of the sheathing between the supports.
The snowstorm also caused damage to standing seam metal roofs. Deflections of the standing seam roof panels occurred with large, multiple-span lengths. At the ends of the metal roof panels, there were deflections between supports that exceeded design expectations. The deflections caused buckles in the panels, which in some cases were severe enough to fracture the panel and result in leakage into the building. The sliding of snow on metal roofs can reduce the snow load, but can also damage components of the roof. The downslope movement of the snow can shear off plumbing vents and other roof penetrations. In addition, moving snow in valleys can cause problems with the waterproofing seals along the valley.
Plumbing vents and other roof penetrations were commonly damaged by the weight of ice and snow. In some cases, the sliding snow caused separation of the pipes from the items they were connected to on the interior of the home. The pipe jacks were occasionally broken just below the roofline and a repair of the jack was necessary. The jack being pushed over from the impact of the snow creates an opening between the jack and the flashing. These openings allowed water to infiltrate the interior of the space and, in some cases, to cause interior water damage.
The buildup of ice and snow at the roof edges was also a problem. Ice damming, which allows water to back up below the roof covering and damage the roof sheathing, was a common occurrence. The backup of water resulted in interior finish damage, and in some cases, the water was able to back up below the roof covering to an extent that the water infiltrated the wall framing and manifested as biological growth in the walls. The buildup of ice and snow caused gutters to be ripped off of the roof, as the gutters were unable to support the increased weight of the snow and ice at the roof edges.
Thus, it was our experience that the March 2003 snowstorm caused a large amount of roof damage that was not considered catastrophic, but resulted in water damage and potential for further damage. The ice and snow loadings worsened already poor or degraded conditions, which increased the extent of damage. To help prevent long-term damage and worsened conditions from future snow loading events, owners should perform regular investigations and maintenance. Regular roof investigations and maintenance can help reduce the damage and resultant costs associated with future weather-related events.
As a result of the March 2003 Blizzard, Pie Consulting & Engineering has investigated in excess of 200 structures involving claims of damage due to weight of ice and snow. These claims run the spectrum from large commercial roofs experiencing catastrophic collapse to residential roofs with minor damage to components.