Playing with Reciprocal Roof Designs

I am becoming more and more enamored with the idea of a reciprocal roof for my own round cob/straw bale home. But I am also hungry to learn more about the physics, strengths, and weaknesses of these beautiful, elegant, yet simple roofs.

Here are some pictures of a beautiful double reciprocal roof I found. These are of a double reciprocal roof built somewhere in Europe I believe in partnership with the Lincoln School of Architecture. The pictures are taken from the Lincoln School of architecture blog and  the Hill Holt Wood community blog.

The frame for the double reciprocal roof. So cool!

A view from the inside, when finished.

A detail shot of the interior. Notice the little supports added.

Image of the hall from the outside.

After seeing these I had to try designing my own roof. Tracing my own, to scale drawing of my round house I drew up a potential roof structure. It started as a simple reciprocal roof but then I started playing with a double reciprocal roof to make the spans shorter. The secondary rafters in my drawing bisect the major rafters and the squares with an X in them are where I was thinking of having posts.

My roof design, drawn to scale. It actually works with my footprint! The posts are all 10 feet from each other on the perimeter of the circle and somehow, magically, they don't interfere with any of the doors or windows! I wouldn't have to change anything!

There doesn’t seem to be much literature out there though on these roofs and I have many unanswered questions. I would love to talk to someone who could tell me if making it a double reciprocal roof makes it stronger or not and how much more complex it would be to build a double reciprocal roof. Another unanswered question is how much it matters if all angles are exactly uniform. Would the integrity of the structure be greatly compromised if slightly different diameter, rough sawn beams were used? Or different kinds of wood? Also, it seems that some of the genius of these structures are that they are totally self supporting, so I wonder if adding a few supporting posts somewhat randomly would actually compromise the strength of these structures by making the load less uniform. Or would adding a few supporting posts give more strength to these roofs? What is an ideal pitch for these roofs and how big can your spans be? I would also love to do a living roof but those are known to be quite heavy and in a Northeast climate you have to worry about snow load as well. How much weight can these reciprocal roofs actually take? If anyone knows someone who could answer some of these questions or help me figure out the answers please get me in contact!

My own research has turned up a few promising leads, but not much. The fact that the Lincoln School of Architecture built one of these is hopeful. Perhaps if I contact them I can get in contact with someone who better understands these roofs.

I also found a book called Reciprocal Frame Architecture by Olga Popovic Larsen that could be helpful but looks more like a coffee table book of pretty pictures than a How To book.

Then there is Brian Liloia from the Year of Mud Blog who definitely wins with the most google hits for reciprocal roofs. He built a living roof on a small cob structure, similar to what I hope to build, but I think his may be even smaller.

There is also a book called Building a Low Impact Roundhouse by Tony Wrench that seems to be recommended by some who have built their own reciprocal roofs and have a few good reviews on Amazon. Based on pictures their structure looks like it could be a similar size to what I want to build and is holding a lot of weight: a straw bale insulation layer, then a vapor barrier, and then a living roof. So that is also hopeful!

Thats a lot of straw and a lot of people standing on that roof! I think that plus a living roof would be sufficient insulation for me through a New England winter.

Now they have put the membrane over the straw and are beginning to put sod to build their living roof.

Lastly, I found a bit of info on the Lama Foundation website that looks promising. Here it looks like they were just playing around with a few small scale models, something I definitely want to do, but one picture in particular caught my eye:

Original caption taken from Lama Foundation website: "Is it strong? Yikes. You tell me. This is presenter Steve Swidler standing on the model."

Well, reciprocal roofs are definitely something I will continue to play with and explore. I am captivated by their simple elegance and would love to have one of my own… Like I said, if you find any more resources on these, whether people or print resources, please pass them on to me! Thanks.