### The roof beams.

I put in three sets of cross-braces between the uprights for the yurt, apart from the doorway which only has them top and bottom.

The strength of the structure of a yurt comes from enclosing the top of the upright beams so that the roof doesn’t collapse. This is called a ‘tension cable’ and I added two of these, on opposite sides, to the outside of the top of the walls where I hoped they would nestle safely under the ‘eaves’.

Most Mongolian yurts are made with a 30 degree pitch in the roof so that snow will slide off and not collapse the roof. Deep snow is a very rare event in Cornwall UK so I decided to make my roof with a 24 degree pitch, enough for the endless rain and if necessary I would push the snow off from the inside.

Although I failed my maths ‘O’ level 3 times, in adult years I have got better at calculating stuff when it is applied to practical situations. I even run the Parish Council audits where I live. Those sums in school maths where you are given the diameter of a bath plug and the amount of water in a bath to calculate how long it takes to empty still leave me with a sense of the futility of existence. I mean, who cares?

This calculation was more like the trigonometry question (which I also couldn’t understand) where you had to calculate the height of a tree by knowing what time it is and measuring the shadow of the tree. But what do I know? I found an equation online about calculating the length of the third side of a triangle. It is a squared + b squared = c squared .

The first side of the triangle [a] is the height of the apex of the roof over that of the external vertical supports (160cm above floor) which was 85cm. This gave me a total height at the centre of the yurt of 245 cm above the floor.

The second side [b] is basically the radius of the yurt which I had at 264.5cm.

The calculation I made was [a] = 7225 + [b] = 69696 = [c] 76921. Un-squaring this gave me 277.3cm, which I increased to 280cm to give a bit of overhang to the roofbeams. I still don’t know if this was the right calculation but anyway, it worked OK. I added roofbeam guides to the top of each pole to hold them in place while I figured out how to join them at the centre.