That's a good question! The short answer is I had no accurate way of knowing anything for sure.
The longer answer is that there's little technical information on the
native NZ timber I used (Totara), especially for the younger
trees I was using, but it's known for being generally strong and durable, which was a good starting point. And I knew I needed each rafter to span 4m so, as an approximation, I set up a 4m span between two blocks of
wood on the ground and then placed different poles on there and loaded up the mid-points of each them with weight (people, or whatever else I needed, up to about 200kg). I could then see how much each pole was flexing and used this to grade the poles in order of strength. I noticed there was a significant increase in flex beyond a certain minimum diameter, so I only used the poles that had minimal to zero flex, keeping the smaller ones for smaller spans elsewhere in the building.
It's difficult to calculate the weight of the earth roof above, but I was pretty that even with sodden wet soil up there, I felt the weight on each rafter would be fine (It's a 50m2 roof with 20 rafter poles, so each pole only needs to support 1.5m2 of roof, plus a decent portion of that area will be towards each end of each pole where they're a lot stronger.) Fortunately I'm in a climate where I don't need to worry about snow loading.
The living earth roof has now been up for several years and weathered many a storm and downpour with no issues so in the end it's worked out really well, and the roof looks amazing (inside and out!). I definitely understand your concern, as I was a little bit hesitant using my rustic approach to structural engineering, but I think a bit of
common sense goes a long way. Not to mention I think it would have been near impossible (or at least incredibly expensive) getting proper engineering calculations done, as each pole was unique in size, shape, taper etc.
I hope that helps - I'm hoping to do a video or two on the living roof in the next few weeks if that's of interest. And (eventually) I'll have one on how I finished and weatherproofed the hole in the centre, which gave me a few sleepless nights trying to work out how best to do it!
Thanks
Jon
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