Most commercially used marquees in Britain use a 20degree angled roof (3m high apex for 6m marquees with 2m leg height) as that gives the greatest wind-resistance by design, obviously it still needs well anchoring down.
When designing a marquee we consider the likely weather the marquee has to withstand in Britain: rain, snow and wind.
Rain is reasonably straight forward, any waterproof cover with enough angle on the roof for run off would do the job.
Snow and wind are perhaps surprisingly treated together, this is because it is very difficult to design for both in the same marquee. Consider alpine chalets that have very steep roofs to prevent snow settling – if you used this design in a marquee it would make it far too susceptible to wind as described below. So in general marquees in Britain are designed to be strongest against wind and are not rated for snow-loading (hire companies get round this by leaving heaters on to prevent snow settling).
The taller a marquee is the greater the wind force it has to resist. The strength of this force is increased by the square of the height increase so if the height of a marquee is doubled the force it has to resist actually goes up by 4 times as much. The lower the apex height the more wind resistant a structure will be.
As the roof angle increases the apex gets higher and the surface area of the roof increases. By the law used in hydraulics the pressure per square metre is constant, if the area increases then there is more upward thrust when the wind gets inside the marquee. To reduce the likelihood of damage again the apex should be as low as possible.
It’s no coincidence the industry standard for a pitched roof is 20degrees (making it a 3m apex for 6m marquees on a 2m leg), it’s been found to be the best angle for helping the wind to roll over the roof of the marquee.
By way of example here’s a photograph of the 2012 Showmans Show (2013 not available as yet). The marquees highlighted in red use a 20 degree pitched roof, the ones highlighted in blue don’t use a pitched roof (they are curved or peaked) and the ones highlighted green are marquees that we can’t recognise as 20 degree pitch roofs or not. Note that 20 degree pitch roofs are by far the most popular and the marquees that may use other angled roofs are the smaller ones – In 3m and 4m widths the overall roof areas are smaller where you can use a higher apex/steeper roof with fewer problems.
When buying a larger marquee look for those that feature a 20 degree roof pitch, on a 6m wide marquee on 2m legs that means a 3m high apex. They may cost a little more to manufacture but it is worth it. There’s a reason it is the industry standard.
Thanks for reading
The above was written by Colin King, former principal lecturer in Engineering.
Translated in to ‘normal’ English by Spencer!