Snow Loads and Metal Buildings: What You Should Know

Published October 13, 2017 by Whirlwind Team

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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.

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.


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.


Warning signs of snow load stress

If the actual snow load exceeds the design snow load, if the snow drifts or the roof installation was incorrect, you could suffer a building collapse or other emergency. If you’re wondering whether your building is over-stressed by heavy snow, look for these warning signs:

  • Horizontal deflection of the walls
  • Popping and creaking sounds
  • Sagging roof, ceiling, or metal members
  • Severe roof leaks
  • Bowing trusses
  • Doors or windows that are difficult or impossible to open or close

All of these are signs that the metal support members have bent and your building has the potential for catastrophic roof failure.

Tools for preventing or alleviating snow loads

The best way to deal with snow loads is to avoid them. If you design a roof with any amount of pitch, snow will be more likely to slide off. The higher the pitch, the less snow is likely to accumulate. However, if you must have a flat roof, you can obtain some useful tools to help minimize the snow load.

  • Roof rake - a tool that looks like a hoe on a 20-foot handle that is used to pull the snow down off the roof. It's also meant to alleviate the formation of ice dams.
  • De-icer - one of several heat emitting tools that melt the snow and ice off the roof. You can get a heat cable, heat tape, heated mesh, heated pans, and a cable device called Helmet Heat(R).
  • Snow guards - small plastic or metal pieces that attach to roof panels in a broken pattern that suspends the snow in a folding field to help it melt more easily and quickly.

De-icers and snow guards are permanently mounted on the roof. A roof rake is used by a person standing at ground level to pull the snow down off the roof rather like raking leaves.

Metal buildings are sturdy, durable structures designed to withstand wind loads and other extreme weather events but you must pay special attention to potential snow loads that could accumulate. Depending on the region of the US, you can expect snow loads of zero to 100 pounds per square foot.

Drifting snow can strain one section of roof while a heavy wet snow can create pressure on the entire roof. If the roof is unable to withstand the snow load, you will see the wall members deflecting horizontally, or the roof will begin to sag and leak. To keep snow loads from damaging the roof, you can use a roof rake to pull the snow down, or you can install a de-icer or snow guards to prevent accumulation from occurring.

If your metal building will be located in an area of the country that commonly experiences snow and ice, make sure the design engineer is aware of the local snow load building codes when you buy your metal building.

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