This is a long-delayed post about building my reciprocal roof frame. This entry documents the details of building my reciprocal roof frame, including the type of wood I used, the number of rafters, and the work process itself. I hope that this will be useful for individuals who want to attempt building a similar frame. With the general lack of information about reciprocal roof construction on the internet, I hope this will become a primary resource. Here we go…
What is a Reciprocal Roof?
A reciprocal roof is a beautiful and simple self-supporting structure that can be composed of as few as three rafters, and up to any imaginable quantity (within reason, of course). Reciprocal roofs require no center support, they are quick to construct, and they can be built using round poles or dimensional lumber (perhaps with some creative notching). They are extremely strong, perfect for round buildings, and very appropriate for living roofs, as well. The reciprocal roof design was developed by Graham Brown in 1987.
My Reciprocal Roof Frame
Here are some specs of my reciprocal roof frame:
- 14 primary rafters
- Black locust poles except for one pin oak (used green)
- All rafters have a diameter of roughly 5″ at the top end of the poles (heavy!)
- 14 secondary rafters
- Pin oak and a few black locust (used mostly green)
- Diameter of 3-4″ at top end of poles
In total, the roof frame is composed of 28 poles. This frame sits on the walls of my load-bearing cob house.
The Charlie Stick
You will need a sturdy center post, or “charlie stick” to temporarily prop up the rafters. You can either find a pole with a natural Y at the top, or find a really sturdy pole and nail two 2x4s to the top to keep the first rafter from wanting to slide off. Saw the charlie stick to the desired height, based on what the pitch of your roof will be and the height of your walls. There’s a little bit of math there. Also, aim to leave it perhaps a little long, because the roof could settle when you take out the stick.
Prop up your post with some braces. Don’t worry about burying it – it will just be that much more difficult to get it out later. You should have someone standing by to watch the stick while the first few rafters go up to make sure it is steady.
Slide your first rafter up over your wall (or wall frame) and up onto the charlie stick. Leave some extra length at the top and at the wall. (You can always determine your exact eave length later.) Decide if you will go clockwise or counter-clockwise, and grab the next pole, and prop it on top of the first, beyond the charlie stick point of contact. (You want the charlie stick to end up outside of the inner circle.)
Tie it with strong, heavy gauge wire to prevent it from sliding. You might want to wire it to your wall frame, or if you are doing a load-bearing cob house (like me), you should have buried strong dead men in your walls with wire to attach the rafter. (My dead men and wire were not perfectly positioned or perfectly strong, and many poles were loosely attached or not attached at all. No biggie.)
Continue around and around. The space between each rafter (at the wall) should be roughly even. Keeping things even will also result in a more circular inner ring (the hole at the top of the roof). Determine how large you would like your inner ring to be early in the process, and use a guide, such as a large dish or disc to help with setting the poles evenly.
The Last Rafter
Depending on how tight or loose your frame is, it may be a tight squeeze for your last rafter. Don’t worry if it is. (It was very tight for me both times.) The last rafter slides on top of the previous rafter, and under the very first pole. If it is a tight fit, you should still be able to leverage the rafter into place. (Extra hands always help with this!) Once it’s in place, lash it with wire. Now you’re ready to remove the charlie stick!
Removing the Charlie Stick
Undo your braces, grab a mini sledge hammer, and knock your charlie stick out of its support position. If all goes well, your roof will now bear its own weight! (It seems very unlikely that anyone could really mess this up, resulting in collapse… have no fear… it’s not that scary!)
You now have a self-supporting reciprocal roof frame! This process can take as little as a morning to complete, with enough extra hands. I had the help of five others and we finished the frame in perhaps five hours.
The Secondary Rafters
You can choose to have many secondary rafters, a la Tony Wrench, or just a single pole between each main pair. (Or none!) Now, I cannot speak with absolute authority on this (or any of this, really…), but my own secondary rafter design was fairly simple. Following the spiral, I raised each secondary rafter individually, pre-drilled a hole through the pole and into the primary rafter, and hammered a giant (maybe 5″ long?) spike nail into each. (I did a second spike nail into each pole later, after doing the decking.) That was it. That was the only attachment at the top to the frame.
Now, there is even more detail I could dive into with this whole process, but I’ll leave it at this for now. If you have any questions about building a reciprocal roof, please contact me! Also, I highly recommend the following resources for additional construction information. Good luck!
Other Reciprocal Roof Resources
- For general information on reciprocal roofs, see my Natural Building 101: The Reciprocal Roof: Beauty, Strength, and Simplicity in a Roof Frame at Green Building Elements
- Simon Dale’s low impact woodland home has a gorgeous reciprocal roof with gnarly rafters and beautiful slab decking (construction information here)
- Tony Wrench’s website has some photos and information about his living reciprocal roof
- The LessPress Snail Cabin has a reciprocal roof made with dimensional lumber. Also, be sure to check out their Excel spreadsheet for calculating beam lengths and positions for both circular buildings and otherwise
- For math nerds, visit The Pavilion for a very technical description of how reciprocal roofs function
- Zone5 has a brief description with some images of a Tony Wrench-style reciprocal roof construction for a roundhouse here and here
- Check out a scale reciprocal roof model and design using dimensional lumber at Casa de Baro
- Cae Mabon in Snowdonia of North Wales features several buildings with reciprocal roofs. Images here.
- Design Forward has a very brief snippet about the history of the reciprocal roof design