Jan Corriveau wrote:Hello Jared,
I was wondering the same question... I've also seen people doing it like in this youtube video:
It's basically a velcro plate embedded in earth bags a few layers below with 2 vertical pieces of plywood (Those are strengthened by the next 2 layers of earthbags pushing against it's sides). Once the rafters are installed, you continue the last 2 layers of earthbags to tighten everything together.
See this video and forward at 8:06
I hope this helps
I read this whole thread and have also watched the video that is linked above to how the family at https://www.youtube.com/watch?v=Bh6N_WWGwkU&feature=share
attached their rafters for building a reciprocal roof on a round earthbag structure. I think their method of installing the rafters on top of velcro plates with attached sidewalls for the rafters to drop in-between is a brilliant idea. The plates are anchored by the next row of earthbags. The extra height of the attached sidewalls allows the rafters to be aligned level with each other even when the earthbag wall is not. Brilliant!
A reciprocal roof does not need a bond beam or a tension ring like a radiating yurt roof. A reciprocal roof twists under its own weight causing it to tighten upon itself rather than spread like a radiating roof.
If you are building a rectangular wall system the rafters can be supported by a ridge beam, or a truss in the shape of a gable or as a shed style roof.
Any of these roof designs exert very little pressure out to the side.
For building a small rectangular shed interlocking the bags at the corners and anchoring the rafters to the walls in the fashion shown in the video above should be plenty adequate for structural integrity. Ideally, earthbag walls have reinforced tensile strength in every row from the two-strands of 4-point barbed wire. Embedding steel bolts in the walls has shown to rust over time. Better to marry gravity and mass as an anchor as the Anasazi Natives have done for over 1200 years!