Helping you understand the risk to health and safety posed by snow load on roof of marquees
Roof snow loads on marquees pose real structural risk to the stability of the tent and where this risk arises, marquee hire contractors must have a management procedure in place for avoiding personal damage or injury.
However, this is not a guarantee, by any means, that as contractors we ought to assume that this negates the need for risk management planning, or else excuses us from knowing that a temporary structure has a much lower threshold (incomparably so) for bearing up under the weather conditions than a permanent building, like a house.
So what are the principles behind managing risk of roof snow load on marquees?
As with any subject matter, to gain a firm grasp of the topic enough to really establish any mastery over its execution, it profits to get the first things first.
With snow load, there is nothing different.
Making light of a subject, introduces the assumption and in the area of project management, this of course increases risk and therefore potential for loss by the stakeholders involved.
Where these stakeholders are event guests, snow load poses a very real material risk to health and safety. Therefore, you have a duty of care.
Snow load is diverse: no two loads of snow are exactly the same!
Snow fall from snow storms are a natural occurence and with all of the variables involved in this matter of precipitation, it is impossible to say one snow storm is identical to another.
Because of this fact, we must also treat the product of the snow storm also as being entirely different one from another and therefore in need of its own unique assessment (monitoring) and response.
Now, snow load on the ground we all might be familiar with, but how a snow load behaves on a roof is again entirely different to the ground variety - and furthermore, differs from one roof type to another.
Due to this unwieldy issue of snow load and getting a handle on defining it for the sake of tackling the problem competently, the International Building Code was proposed which comprehensively satisfies the definition of snow load on roofs.
Within this code there are 5 contributory factors identified as affecting the diversity of snow loads on roofs:
- Wind exposure of roof
- Roof slope
- Roof shape
- Roof obstructions
- Thermal condition of the building
The variability of the above points is by no means all that impacts on roof snow load, but they have been identified as major factors involved in the matter of snow load.
Snow in itself is also varied & each type of snow is to be treated differently
As a type of geological precipitation, snow, as a definitive happening is not really alike one occurence from another. Therefore snow must be treated uniquely every time, however though about within a framework understanding of how it typically behaves.
Some factors affecting snow fall and therefore snow load include:
- Atmosphere (weather conditions)
All these points above add their own persuasive measure to determining the overall snow density. Snow density is of course intrinsically linked to snow load (weight).
Being aware of how these points contribute to snow density will help you go about monitoring and discerning the risk profile of the current snow storm/fall in the best manner.
Are there guideline ranges of snow density which would help load estimation?
When estimating snow load, there is a standard benchmark which the U.S. Federal Emergency Management Agency - FEMA advocates. This measure of density relates to a standard condition of 1 foot depth of snow:
- Light (dry ) snow = 3lbs/sq. ft.
- Heavy (wet) snow = 21lbs/sq. ft.
- Ice = 57lbs/sq. ft.
Whilst these estimates above are experimental and therefore produced on the basis of other controls being constant, such as: flat, wide/open roof, free from obstruction (a rarity in practice!), they still offer some yardstick for estimating snow load on marquee roofs in various environments.
In practice then if most roofs aren't 'flat, wide, open and free' - doesn't that make for unbalanced snow load
Unbalanced snow load is how it really is in practice, which makes for a far more uneasy assessment of risk associated with snow load.
In practice, because of design variation and actual physical variances with every marquee roof structure, there will exist various depths of snow along a marquee roof in the event of a storm and therefore a variable snow load.
The variability of load, likewise presents a variable risk profile to be managed by a marquee hire contractor.
Are there any particular snow conditions which affect unbalanced snow loads more?
Snowfall differs from one shower to another and even once fallen it's ability to relocate from its original place of settling to another is very different depending on environmental factors.
Two main determinants of unbalanced snow load are:
Drifting snow is described as snow that has settled upon a roof and transported by wind between two locations on the same roof.
Where the arrival location of the drift is an obstruction on the roof, like a duct for ventilation for example, the snow depth at the point of obstruction (collection point) will be greater than the flat roof, creating a greater snow load.
Sliding snow is described as being snow settled at higher parts of a sloping roof which slides down the gradient of the slope and accumulates at lower portions of the sloping roof, increasing the snow depth there and likewise creating an unbalanced snow load across the roof of the structure.
As well as this variable snow load exerting uneven pressure across the roof, the dynamic force of the sliding load from high to lower parts creates a force of impact at the lower level, which in itself can create an overload and breach the threshold of the structures stability.
When monitoring snow load risk, what other factors should a marquee contractor bear in mind?
Monitoring snow load risk begins with having local knowledge of the meteorological patterns of the site climate. Having an idea of seasonal patterns will be a good initial indicator of risk. Beyond that, temperature monitoring and keeping up-to-date with the reports of local weather will enable you to stay abreast of changes.
Next is snow fall rate...
Snowfall rate itself dictates how early a marquee contractor or health and safety manager would begin monitoring.
If the rate of snow fall is fast, then the issue of snow load will emerge more quickly than during light snow fall, thus monitoring should begin earlier as not to be caught out.
Ambient temperature needs monitoring also...
Where environmental temperatures fluctuate rapidly above and below freezing point, this can present great hazard to the snow load of a marquee roof.
The hazard emerges from snow melting to water which trickles down from higher parts of the roof to lower parts, forming ice dams, especially at the guttering causing a great snow load imbalance (remembering the density of ice is some almost 20 times heavier than light snow). Events like this can cause structural tipping point and subsequent collapse.
What about sleet? That stuff, when it's neither continuous rain nor snow, but a bit of both?
If snow is laid down first on a marquee roof and it rains subsequently, depending on the duration of rain you might get different outcomes.
But essentially understand that adding rain to snow will increase the load.
The rain intensity, roof geometry and drainage each contribute to the additional risk of snow load.
For example, where the rain intensity is high and over a longer duration, this might lead to the snow on the roof being washed away, dissapating snow load with it...
...but where the intensity is less and rain showers occur within a short duration, this might lead to snow saturation and therefore a rapid increase in snow load.
Are there any risk management template which we could recommend to document snow load handling?
Selmore Event Hire have designed a suite of risk management health and safety templates for marquee hire contractors, which we would encourage you to purchase and download.
Categorised in: A Guide To Event Health and Safety
This post was written by selmore