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Structural Engineering NY | MA

Snow Accumulation on Roofs: When to Worry

As Winter progresses and a series of snow storms deposit an ever increasing depth of snow on the roof of a building, we are often called by concerned owners wondering how they should determine if their roof is overloaded with snow and should be shoveled. One of the most overlooked ways that snow will damage your home is the actual process of removing it! Scraping snow off your roof can damage the shingles and take years off of your roof’s life.

Snow accumulation on the roof of a building is influenced by several factors.

A sloped roof with a pitch of 8:12 or greater tends to hold less snow than roofs with a lesser pitch. Flatter roofs such as those with a roof pitch of 4:12 tend to hold more snow.

Wind tends to blow snow off of flat roofs and can have the effect of piling snow on a roof with a parapet. Wind can also remove snow from the windward side of a pitched roof and deposit the snow on the leeward side of a sloped roof leading to an unbalanced and potentially hazardous loading.

The cubic weight of snow has a wide variation depending on density. Light fluffy snow can have a weight of as little as 7 pounds per cubic foot while wet dense snow can weigh as much as 25 pounds per cubic foot. Deeper snow tends to have a greater weight per cubic foot as it is compressed by the snow above.

Roof snow accumulation can be influenced by various factors such as adjacent buildings, roof projections, mechanical units, changes in roof elevation, friction of the roofing material and various other factors.

Newer homes that are well insulated tend to have cooler roof surfaces than older homes. 

These cooler roof surfaces tend to melt less snow and as a result have greater snow accumulation that older homes when all other factors are the same.

We often are called by building owners who ask a question such as ; “I have 30” of snow of the roof of my building. Is that too much? Should I consider hiring a crew to shovel it off?”.  The answer usually requires additional information. The answer the engineer is trying to determine is if the weight of snow and ice on the roof exceeds what it was designed for.

Building codes (and more broadly by ASCE 7-10) provide a structural engineer with a ground snow load that varies depending on historical snow fall in a geographic region. Ground snow loads in the NorthEast range from a low of 25 pounds per square foot in some coastal areas to a high of 100 pounds per square foot in Northern Maine.

An engineer uses that code prescribed ground snow load to calculate a design roof snow load by applying various factors that take into account several variables including roof slope, building use, thermal factor, roof friction and wind exposure.

If it is late in the Winter and the roof has already accumulated significant ice and dense snow in the lower layers this can add substantial weight. One of the greatest loading conditions can occur in late Winter/early Spring when there is a heavy rain falling on an existing snow pack on a roof.

To determine the actual weight of snow on a roof a test sample of the roof snow can be removed in a 12”x12” area down to the roof deck and captured in a container, melted and a volume of water calculated. Based on this the weight of the snow per square foot on the roof can be determined. An engineer can evaluate the roof structure and determine its design capacity. Using this information an educated decision can be made about the necessity of removing the snow from the roof.

Although every roof is different and an engineer should be called in if there is a concern, for many roofs in the NorthEast a rule of thumb is that if a roof has in excess of 18” of snow either an evaluation should be done by a professional engineer or the snow should be removed.

An inspection can also reveal potential weak spots or places where your roof structure, deck, or framing might fail.  Additions or modifications can affect your home’s ability to support snow.
Residential wood frame structures are generally very resilient. But, eventually, if it is not built or designed properly, it will fail.  These type of structures often give warning signs when they overloaded such as cracks in your walls and ceilings. Look for doors or windows that seem warped or present difficulty closing.

Shoveling snow off of a roof can be a hazardous and expensive proposition and should be carefully considered before proceeding. All safety regulations should be taken into account.

Additionally it is important to remove the snow in a manner that doesn’t make the roof unstable such as removing all the snow from one side of a gable roof but not the other.

(Snow Accumulation, snow loads, snow storms / Mon, 30 Dec 2013 15:02:00 +0000)




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