SNOW LOADS AND METAL BUILDINGS: WHAT YOU SHOULD KNOW

When you were a kid, the only thing massive amounts of snow meant was a snow day out of school. It's too bad that as you grow up, you have to start thinking negatively about a weather phenomenon that can be so beautiful and fun.

The truth is that, while snow can be pretty to look at, it can also be pretty difficult for your building to support. How can you prepare for the potential of a heavy weight settling on your roof and staying there for days?

Easy. You erect a metal building. Metal buildings are strong and durable, much better suited to hold up all that white stuff than wood frame construction.

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TYPES OF SNOW
You may have heard of the Eskimo’s hundred words for snow. We don't have that many in English, but we do separate snow into different elements.

Snowflakes - single ice crystals that fall from the clouds
Hoarfrost - a grayish-white crystalline deposit of frozen water vapor formed in clear still weather on vegetation, fences, and other structures
Graupel - also called soft hail or snow pellets, graupel is precipitation that forms when supercooled droplets of water are collected and freeze on falling snowflakes 0.08 to 0.20 inch balls of rime frost
Polycrystals - snowflakes made up of multiple individual ice crystals
Besides taking on different forms, snow can also come in different consistencies depending on the temperature and amount of liquid water it contains. Snow can be a light, easily-blown powder or it can be an extremely heavy, water-laden packing snow.

Sometimes snow becomes covered in ice, adding to its weight and creating other problems with building roofs. To design any building to withstand the variety of snow and ice it may experience, you have to calculate the snow load.

CALCULATING SNOW LOAD FOR METAL BUILDINGS
Snow loads are determined by a combination of factors, including:

Roof shape, elevations and slope
Roof obstructions
Wind exposure
Thermal condition of the building
Recent ground snow information from the National Weather Service
What the building is used for
In addition to these factors, engineers must take into account the differences in snow according to the season, humidity, and altitude. Taken altogether, these factors create different densities of snow and how long it is likely to remain.

According to FEMA's Snow Load Safety Guide, "The weight of one foot of fresh snow ranges from three pounds per square foot for light, dry snow to 21 pounds per square foot for wet, heavy snow."

Add in drifting and debris from surrounding trees, and the balance of roof-top equipment for HVAC and firewalls, and your roof must support much more than its own weight.

States and local communities often amend suggested snow loads and building code to match the prevailing weather. Alameda County in California typically has a snow load of zero psf while Aroostook, Maine (the most northern part of the continental United States) can expect 100 psf snow loads. Dallas, TX typically has a snow load of five psf.

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DESIGN CONSIDERATIONS
There are several aspects to roof design that require some thought when taking potential snow load into accounts.

Roof pitch - the higher the roof pitch, the less the snow accumulates. Areas of high snow fall benefit from almost any level of pitch to encourage snow to drop off the roof. Flat roofs may require snow removal service.
Wind load - geographical locations prone to heavy snows probably experience high winds as well.
Geometric features - some geometric features tend to accumulate heavier snow loads than others. Saw tooth and stepped roofs tend to accumulate more snow than gable or multi-span roofs or mono-sloped roof designs.
Accessories - Snow guards, roof cleats, and other roofing accessories can keep snow distribution more even.
The best way to ensure your metal building is designed to withstand typical snow loads is to work directly with the metal building manufacturer. There may be a design in place that accommodates your need. If not, the manufacturer can custom engineer the frame and roof system for your climate.


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