How to Build a Reciprocal Roof Frame

by ziggy on November 26, 2008 -- 23 comments -- Follow

reciprocal roof frame

A reciprocal roof frame

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…

Assembling Reciprocal Roof

Raising the reciprocal roof

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.

Reciprocal Roof Construction

Note “charlie stick” holding roof up during building

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.

Reciprocal Roof Rafters

Heavy poles!

Setting Rafters

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.)

Reciprocal Roof Construction

Mark notches the rafter slightly before tying it with heavy gauge wire

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.

reciprocal roof frame (wide)

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!

Learn More About How to Build a Reciprocal Roof Frame

Want to learn more about how to build a reciprocal roof frame and cob house building? Check out The Year of Mud: Building a Cob House book, which includes a chapter on how to build a reciprocal roof. (Use promo code NEWBLURB to save 20% off!)

Building a Cob House
By Brian “Ziggy&…

 

Other reciprocal roof resources

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Robert November 27, 2008 at 4:55 pm

Hi Ziggy

Thanks for this resource.
Just wanted to give a direct link to the Excel spreadsheet for calculating the dimensions of reciprocal frame roofs.

http://www.lesspress.com/twiki/pub/Media/SnailCabin/reciprocal-roof-2.xls
for round buildings (where the beams will all be the same length) and

http://www.lesspress.com/twiki/pub/Media/SnailCabin/reciprocal-roof.xls
for non-round buildings (where not all the beams are the same length) like our spiral-shaped cabin.

By the way, the roof of Snail Cabin has actually got a _double_ reciprocal frame – the main roof has twelve beams, with a six-beam reciprocal cupola on top. Anybody know of another building that has this type of roof? As far as I know it’s the first in the world.

Robert

Chaos Motor November 28, 2008 at 6:33 pm

All well and good, but I seriously doubt this was developed by Graham Brown in 1987, seeing as how the Native Americans were using the same basic design for teepees hundreds of years ago, and soldiers have been standing their rifles in the same manner for at least a couple hundred years.

ziggy November 28, 2008 at 7:07 pm

I’d be interested in reading more about that… I’m actually not convinced any Native Americans were doing something exactly like this design. Tipi poles are not really arranged in the same way, or self-supporting, from what I have read.

katie January 12, 2009 at 10:00 am

thanks for the photo-rich post. I also have looked online for reciprocal roof info, and this is a great addition to the info available!

ina leonte May 10, 2009 at 10:11 am

Hi! Just wanted to thank u for all the explanations. We’re going to try too build a low impact house in Romania, and I find your posts very useful.

David Rivera Ospina June 4, 2009 at 9:23 pm

Thank you for their generosity, first time that I see in detail the process to reproduce it.(Gracias por su generosidad, primera vez que veo en detalle el proceso para reproducirlo)

Lucille June 6, 2009 at 2:03 pm

Hi, I was just wondering what you meant by “you should have buried strong dead men in your walls with wire to attach the rafter.” What exactly is dead men? I am interested in building my own cob/strawbale home, and was wondering how the roof stays in place without knocking the walls down?

ziggy June 10, 2009 at 8:59 pm

Deadmen are simply pieces of wood (in this case, about 10 inch long pieces of roundwood) buried in the cob wall. Wire is wrapped around the wood to attach to the rafters.

Cob is incredibly strong so there’s little worry about the walls getting damaged. (My roof must weight several tons and it’s simply sitting on the walls without any damage.)

Oliver November 18, 2009 at 8:43 am

Hi,

Just wanted to tell you about an opportunity to join Tony Wrench building a low impact cordwood round house with a reciprocal green roof. The workshop is in July 2010 in Poland. Details are here http://greenhamlet.com?whatson

We will be making a documentary of the build which we will publish on Youtube. If you would like to be told when the film is available please email me.

Oliver

Niels Kloppenburg November 30, 2009 at 12:11 pm

Hi ziggy,

I love the looks of the reciprocal roof, but I have some remarks. First, this kind of construction, or arranging, has been around for a long time. A teepee or teepee-like structure does indeed use it, though you’d find it hard to recognize because of the small inner circle (basically none) and verticallity of the structure. It seems Graham Brown invented it independently, though.

Now for structure. I know it looks organic, and I love organic as well, but that’s no reason to toss reason out the window. This kind of construction does NOT differ in any structural way from a center ring with spoke like rafters extending to and beyond the wall. Actually, depending on your wire connection, it’s more prone to slip out of shape. It DOES transfer horizontal loads to your walls, which you haven’t stopped with a tension tolerant ring beam of sorts, nor with butresses. In your case, no biggie. BUT, that’s in YOUR case. This kind of construction CAN go horribly wrong, especially when the living roof gets loaded to heavily.

On a side note, what helps diminish (or actually, counteract) the outward horizontal forces is the similar heavy loading of your eaves, which creates an inward horizontal force. Another traditional solution is to make your rafters (or a more easily replaced extension of them) extend to the ground.

All of this might be important when you decide to go for a wall of less thickness, for instance one that functions mainly as thermal mass within a straw bale building. It’s definitely good practice to constrain horizontal forces on your walls.

Anyway, I’m reading all of your blog with a lot of attention. As a somewhat more professional builder I’m looking for ways to structurally improve (for second stories, etc, etc) and speed up these kinds of design, without introducing heavy machinery or prefab/industrial building materials. Keep up the good work and keep people inspired.

Regards,

Niels

ziggy December 8, 2009 at 3:27 pm

Niels: Thanks for the detailed reply. The whole structural thing really interests me, and I must admit I don’t have a strong understanding of the thing!

Gary Richardson May 22, 2010 at 2:39 pm

ziggy,
I want to call to your attention what appears to be a somewhat definitive study of the subject: “Reciprocal Frame Architecture” by Olga Popovic Larsen (2008, Architectural Press). I’ve just begun reading the book after you turned me on to your reciprocal frame roof, which I think is the answer for my oval-ish 200 square-foot cob meditation hut. I have 34 9′ x 2.5″ to 3″ lodgepoles that I’m hoping to make into a complex RF roof.

ziggy May 22, 2010 at 10:17 pm

Gary: Wow, thanks. I can’t believe I’ve never come across this book in all of my research about reciprocal roofs! I’d love to get a look at this book somehow…

ziggy May 22, 2010 at 10:18 pm

One more thing: are you saying your rafters 2.5-3″ in diameter? Sounds kinda small….!

Rushin July 12, 2010 at 11:14 pm

Hello ziggy,

How do these roofs perform with load applied as in green roofs? Are there any examples of such work?

Great information.

Anything you would advise to pay attention to when building such roof in sandbag construction?

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