Protect Your Building from Structural Damage Caused by Snow and Ice
By Michael Drinkwater
Every winter, buildings in the United States are structurally damaged by the weight of snow and ice. There are exceptional events that cause snow loading leading to damage that is unavoidable. However, most damage from the weight of snow and ice can be avoided with a combination of routine maintenance and taking certain actions once snow does fall.
When snow and ice accumulate on a roof, the force applied must be transferred to the ground. This means that roof framing should transfer weight to vertical components such as walls and posts. Structural damage from the weight of snow and ice is most commonly associated with the failure of roof framing to transfer loads to walls and/or posts. This means that the most effective way to avoid structural damage from the weight of snow and ice is to be sure that roof framing is capable of supporting loads.
In surveying structural damage from the weight of snow and ice on hundreds of structures, I have seen that, in most cases, damage could have been avoided if the structure had not been compromised prior to the storm. Often in a structural failure under the weight of snow and ice, the capacity of the roof-framing system had previously been diminished by a cause unrelated to the weight of ice and snow. Examples include wood decay from an ongoing leak, historical damage from a fire, or poor construction practices.
While this imperfection may serve satisfactorily in the absence of high loading, it can be the first to fail under the increased load from heavy snow and ice. The failure of one member can transfer loads to neighboring members that are otherwise competent but not designed to support additional loading, leading to a progressive failure of the roof-framing system.
Failure can also occur to an uncompromised roof if storm-related maintenance is not completed. It is common for competent structures to deflect or sag when a heavy snow and ice load is applied. This is an indication of stress but does not necessarily mean failure is imminent. Typically the structure will rebound when the snow and ice load is removed. That said, if deflection is observed and snow, ice, or rain on the snowpack is in the forecast, then it is prudent to remove the load from the roof.
Apart from the removal of snow and ice, care can be taken to be sure proper drainage is maintained. When snow and ice melt, the water flows downhill. If drainage paths are clogged by organic debris or ice, then ponded water can concentrate in relatively small areas. This is especially true on buildings with nominally flat roofs. Water ponding around a clogged drain can cause local ponded water that leads to structural damage even if prevailing snow loads do not approach the capacity of the building.
Protecting your building from structural damage due to the weight of snow and ice requires a combination of regular maintenance throughout the year and then immediate action once a significant event occurs. To get your building through the next big snowstorm undamaged, you can take the following steps.
Inspect roof framing and coverings. Framing that is compromised by ongoing or historical issues should be replaced and/or adequately reinforced. Roof coverings that are damaged or worn should be addressed to minimize the potential for leaks.
Inspect roof drainage. Roof water should have a clear pathway to the ground. Debris on the roof that that can obstruct drainage should be removed. This may also involve the installation of heating components to melt ice at key points in the drainage system.
When Snow and Ice Are On The Roof:
Be sure that snow, ice, and meltwater have a route off the building. Can snow slide off the roof? When snow melts, will drain pathways remain unclogged? Daily observations and maintenance may be needed until snow and ice on the roof diminish.
Consider whether the snow and ice can be removed while care is taken to not damage roof coverings. When loads approach design capacity and/or the structure deflects, this is a prudent option to avoid structural damage, especially if additional loading is forecasted.
Michael Drinkwater, P.E., is a forensic engineer for Donan, one of the largest forensic investigation firms in the country. Donan offers diverse services including forensic engineering, fire investigation, component testing, UAV services, and litigation support. For more information, visit www.donan.com.