Understanding Snow Loads in Construction

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November 6, 2015

How Snow and Ice Stress Structural Integrity

How much snow can a roof hold? Roof snow loads indicate the amount of additional force pressing down on a building when snow and ice pile up on the roof during winter storms.

That sounds simple enough, right?

Well there are a lot more variables than just the amount of snow on the roof.

Snow LoadDENSITY: The density of the snow makes a significant difference in the weight of the snow. Typically, snow in the western U.S. is drier and lighter than the moisture-laden snow experienced in the eastern part of the country.

The Federal Emergency Management Agency (FEMA) states one foot of freshly fallen powdery snow weighs about three pounds per square foot. Very wet and heavy snow can weigh seven times as much as dry snow.

ACCUMULATION: The snow from just one day does not tell the story, either. Today’s storm may only drop 4” of snow. However, if your roof already had 30” of snow on it left from several previous storms, that 4” of snow could be the straw that breaks the structure’s back.

DISTRIBUTION: Is the snow evenly balanced across the roof? If not, additional stress is placed on your structural system. Wind creates drifts, especially around dormers, roof valleys, and chimneys. Sometimes snow slides down the roof as it starts to melt, adding weight to the lower sections of the roofline.

VARIATIONS in TEMPERATURE: When snow begins to thaw, then refreezes as temperatures drop again, the weight shifts across the roof, creating additional stress on some areas.

MIXED MOISTURE: Light rains may help melt snow from the rooftops. However, sometimes in heavier rains the snow absorbs the rain between the frozen crystals, drastically increasing the weight— especially if the temperature drops quickly.

One inch of snow may weigh anywhere from one-quarter pound per square foot to one and three-quarter pounds per square foot. Only one inch of ice weighs in at just under five pounds per square foot— almost five times an average snow’s weight.

Snow Weight InforgraphicAll these variants create a challenge for determining safe snow loads. That’s why design and construction in snow country relies on professional engineers.

How the Snow Load is Determined

Structural snow loads across the U.S. are based on a number of factors, including:

• Recent ground snow information supplied by the National Weather Service

• The shape of the structure, including the roofline and roof obstructions

• The roof’s wind exposure

• The application of the structure and its number of occupants

• Thermal values of the building

Using this combination of data, snow loads for different areas are determined using engineering formulas. Local communities or states may amend the suggested snow loads— or write their own codes— if they choose to do so.

Roof snow loads for the following counties (in pounds-per-square-foot ground snow load) will give you some idea of the wide range of specifications across the U.S. (Keep in mind that specific locations within these counties may have snow loads modified by local authorities.)

• Alameda County, CA: 0 psf
• Dallas County, TX: 5 psf
• Ellis County, KS: 25 psf
• Moffat County, CO: 35 psf
• Itasca County, MN: 70 psf
• Aroostook, ME: 100 psf

METAL BUILDING ROOF SNOW LOADS

All RHINO metal buildings are guaranteed to meet or exceed all local codes for which it was designed— including roof snow loads— for the LIFETIME of the structure. As loads vary dramatically from one area— and one elevation— to another, it is the responsibility of the buyer to current building codes with their local building officials before ordering a RHINO steel building.

Every RHINO order includes three sets of plans stamped by an engineer licensed specifically by the state in which the steel building is to be constructed. RHINO’s buildings can be designed to withstand snows’ weight on roofs.

Discover all the benefits of RHINO Steel Building Systems before you commit to your next building project. Call RHINO today toll free at 888.320.7466 to learn more about roof snow loads.

Our experienced and friendly metal building specialists will answer all your questions and provide a free quote on the building of your choice. They are also available with helpful tips on designing your structure for maximum roof snow loads. 

- by Mat Brown,

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