Marquees & Snow Load: Roof Geometry Matters

  Posted by: Chief Editor, Selmore - Marquee Hire, on July 28, 2017 12:16 pm Leave your thoughts
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snow load roof geometry

When it comes to marquees & snow load, roof geometry really does matter - here's how!

Being a dynamic matter, snow fall on roofs is greatly affected by shape and overall design of the roofing structure. This article aims to show that with snow load, roof geometry makes a big difference.

The downfall of snow is one thing to have your eye on, but how it is affected by the shapes and slopes of the marquee structure also has great bearing on how the snow load risk will emerge.

With a variety of marquee structures, each with their own unique geometric arrangement, the marquee hire contractor has to consider the variable risks involved in the case of snow load.

What are the shapes of roof which temporary structures typically have?

Depending on whether they are officially classed as 'temporary'

snow load roof geometry

By Federal Emergency Management Agency (FEMA)

or 'semi-permanent', demountable event structures may have one of the following roof features (as examples):

  • Stepped
  • Monoslope
  • Flat
  • Gable

This is by no means a comprehensive list of roof shapes, but we hope through it you get the picture - being able to envision how snow fall might behave differently when laid down upon one of the many roof structures illustrated to the right.

One obvious omission here, would be the pointy top roofs, familiarly associated with stretch tents and star tents.

To go one further, the spherical dome shape roof of a geodesic event tent is again deserving on its own level of consideration.

Aside from general classification of the shape of roof, what other physical aspects affect snow load accumulation?

When it comes to the matter of snow accumulation on roofs, the physical properties of traction and gradient are critical. Increased physical resistance either by the way of traction, or just a flatter surface, will deter snow shedding, drift or slide, thus leaving the load as a more evenly distributed burden...less likely to slide as compared with an unassisted, steeper incline.

A few points to reference on this issue of roof traction & slope (pitch):

Roof pitch & Snow Load

  • Low slopes, or pitch = low snow shed
    • slopes as low as 10 degrees have been observed to experience snow shed
  • High slopes, or pitch = high snow shed
  • Roof angle > angle of repose = snow sliding
    • snow will not slide where angle is greater than 30 degrees

Roof traction & Snow shed

Where the marquee roof has been augmented with tactile additions like cleats, dimples or snow guards, this will also improve the roof's ability to avoid a snow slide.

What other environmental agents will affect the snow load and its behaviour?

The most obvious of the points to appreciate below will be wind, of course, because as a major external force you would expect that it can exert a persuasive amount of pressure upon snow and from varying directions, at different intensities to create a significant shift in snow load.

The other factors, may be a little less obvious, but we hope to do the job of explaining as best as we can:

  • Wind
    • far has the largest effect on snow load
    • open area results in much greater snow shedding
    • ...also where there are obstructions, drifting and variable load occurs
  • Non-insulation
    • is common with event marquees, the temperature contrast between internal & external is large...
      • melting of snow load and can lead to slide
        • where there is an overhand of roof over the top of a structure wall
          • ice dams, as melted ice (water) re-freezes without exposure to warmth, stops snow shedding
  • Neighbouring building
    • ...where there is a higher existing neighbouring building...
      • there is a potential for snow drift and accumulation upon your marquee roof

This overview should give greater insight into the effects of geometry and its related factors on snow load.

For further information on snow load and marquees, feel free to contact us.

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