Ziggy has the best info I’ve found on reciprocal roofs at his Year of Mud website. See: How to Build a Reciprocal Roof Frame
A reciprocal roof is a simple self-supporting structure made of poles or dimensional lumber that are arranged in a spiral pattern. No center support is needed. They are perfect for round buildings. Developed by Graham Brown in 1987, this type of roof is extremely strong and can incorporate a living roof of plants. They’re fast to build. A reciprocal roof on a small house can be erected in a few hours. It is also affordable, since it can be made with peeled poles from a local forest. (Poles can be gathered with a low-cost firewood permit, typically costing around $25 or so.)
The basic process involves bracing the initial pole called a Charlie stick and then working either clock-wise or counter clock-wise, placing and tying poles in position. The last pole goes over the previous pole and under the Charlie stick. Then, just knock out the bracing under the Charlie stick and the frame is self-supporting. Secondary rafters can be added between primary rafters.
Check out Ziggy’s site for pics and complete details. He even provides a good list of resources for further reading.
The forces acting on a reciprocal roof result in two things; 1) the beams twist under load in non-dimensional timber (ie logs), and 2) bending or sagging under load, or with time. It is the latter which can effect walls, but under all but major overload conditions this is trivial, and can usually be safely ignored. One or two inches of deflection is trivial in an eight foot wall, and this would represent a very large deflection in a reciprocal roof. Most of the forces involved cause a twisting effect which doesn’t appear to have any serious spreading forces on the walls.
i knew the yurt relied on tension bands to stop the roof from pushing the yurt walls out. but i thought reciprical roofs were very different and only excert downwood pressure. therefore a tension band or equivelent was not required. am i mislead?
Detailed information about reciprocal roofs on the web is difficult to come by, but from what I can gather, yes, there are horizontal forces that must be contained with anchors in the wall. Tony Wrench has a book with more details on reciprocal roofs: http://thatroundhouse.info/
I might mention that these reciprocal roof systems are very similar to a typical yurt roof design, with the rafter poles themselves forming the circular interior compression ring. As with yurt roofs, they need to be tied securely where they connect with the wall so that they stay put, and there needs to be a strong tension band or bond beam around the perimeter at the top of the wall to keep the roof from depressing and tending to expand the wall at that point.
The link is not working or his site might be down.
Try it again. It works for me.