Winterize Your Roof

Determining And Controlling Snow Loads
Deep snow can be deep trouble and rain on top of snow can significantly add to the weight. Even a partial roof collapse can cause extensive damage to the interior contents of a business. When all that snow comes in, it melts and can flood the building.

The age of the building can be a major factor in the snow load risk. Newer building codes provide much better guidance for estimating snow loads, particularly the increased loads near changes in roof elevations where snow drifts and snow falling from the upper roof can build up on the lower roof near the step. Older roofs can also suffer from corrosion of members and connections, which can reduce its ability to resist high snow loads. Buildings with lightweight roofs, such as metal buildings or built up roofs on bar joists, generally provide less protection from overload than heavy roofs. The safety margins used by engineers are based on a combination of the weight of the roof and the snow loads. Consequently, there is usually a larger margin of safety against excess snow loads for heavy roofs than for lightweight roofs.

For flat roofs, the step-down area between roof sections is particularly susceptible to roof overload because of the tendency for ice and snow collection, especially during periods of windy weather. Roof top equipment and roof projections, such as mechanical equipment that is over 2 feet tall, causes snow accumulation due to drift, creating the need for higher snow load consideration in these areas. An even more serious condition can be created when a taller building or a taller addition is built adjacent to an existing building. Unless the existing building is strengthened in the area next to the new taller building or addition, snow accumulation on the lower roof near the step could produce much higher loads than those considered by the original designer for the existing building.

The best source for determining how much snow load a building can handle is the design plan. These designs can range from having a snow load of 10 to 20 lbs per square foot in Mid-Atlantic States, to between 40 and 70 lbs per square foot in New England. IBHS offers these general guidelines to help estimate the weight of snow:

• Fresh snow: 10-12 inches of new snow is equal to one inch of water or about 5 lbs per square foot of roof space, so you could have up to 4 feet of new snow before you need to worry.
• Packed snow: 3-5 inches of old snow is equal to one inch of water or about 5 lbs per square foot of roof space, so anything more than 2 feet of old snow could be dangerous.
• The total accumulated weight of two feet of old snow and two feet of new snow could be as high as 60 lbs per square foot of roof space, which is getting toward the design limits of even the best designed roof.
• If there's ice it's much heavier, with one inch equaling about a foot of fresh snow.

For safe removal that won't endanger you or damage your roof, consult a roofing contractor.

Watch For Ice Dams
Avoid the costly collision of hot and cold and reduce the risk that ice dams will form and create a soggy mess. When heat from the interior of a building with a sloped roof escapes into the attic space, it warms the underside of the roof. Meanwhile, the roof eave outside the heated space remains a colder temperature. As snow accumulates on the rooftop, it melts over the warmer portion of the attic and runs down the roof. When it encounters the cold edge of the roof it refreezes.

The refrozen water along the roof edge creates an "ice damming" condition, and consequently, the melted snow running down the roof begins to back up underneath the roof covering. This water will soak the roof sheathing and leak into the attic unless there is a barrier above the sheathing. An appropriately installed secondary moisture barrier will help prevent the water from entering your business and damaging your structure and its contents.

Consider the following recommendations to help prevent your business from experiencing damage from freezing temperatures:

• If your roof covering is going to be replaced in the near future, ensure that a secondary moisture barrier is installed using at least two layers of underlayment cemented together or a self-adhering polymer modified bitumen sheet (similar to underlayment). The moisture barrier should extend from the edge of the eaves to at least 24" beyond the inside of the exterior wall.
• To help prevent ice damming, remove or relocate heat sources that are installed in open areas directly under the roof, such as an attic or mechanical room.
• Light fixtures in the ceiling below the open area that are directly under your roof, such as attic space or a mechanical room, should be insulated. Recessed light fixtures release heat if they are not insulated. Check to see if there is any visible light from these fixtures in the attic. If there is, they probably are not adequately sealed or insulated. You should seal or insulate those light fixtures immediately.
• If you have penetrations into the attic (e.g. partition walls, stack vents, electric chase, etc.), seal and insulate them so that daylight cannot be seen and airflow is minimal. Also, insulate, seal, weatherstrip or gasket all attic access doors.
• Attic penetrations and access doors that are not properly sealed and insulated allow for heated air to escape into the attic and can contribute to an ice damming condition.

Severe winter weather can't be avoided, but following these guidelines will help your business avoid the costly pitfalls that often accompany it. Maintaining your building is critical to continue to serve the customers and employees that have come to count on you.

Reprinted with permission from the Insurance Institute for Business & Home Safety (disastersafety.org)