I was asking this same question not too long ago. I came across the concept while looking for people who had built small strawbale houses themselves on the internet. One of the case studies I read had used a reciprocal roof and it looked really neat. So I researched them a bit more (read “I frantically scoured the internet for anything and everything I could find about this way of building a roof”). After doing the research, it looked like a great way to do the roof on the cottage we’re planning to build on the farm.
A reciprocal roof uses an interlocking pattern of timbers to create a clear span (no support poles in the middle) roof design. The poles are arranged such that each one supports and is supported by another pole in the system. This arrangement effectively transfers loads among all the poles and creates a very strong and rigid roof structure. It’s also fascinatingly gorgeous. I like that using this design, the octagonal space in the main living area will be mirrored in an octagon at the peak of the roof visible from below. The timbers themselves will also be visible which should create a really nice effect similar to this picture from http://www.simondale.net/house/index.htm .
To get a feel for how one of these is constructed, we built a model. Our model has 6 sides, so its not completely accurate for ours which will have 8 sides, but it gets the idea across. First you prop up the first pole (or popsicle stick as the case may be). Then you start laying out the next poles one by one. Each one rests on the one before it and gets notched so that it can’t slip. In the real thing, these will also get pegged or bolted together so that they can’t pull apart. The last pole is inserted under the first and on top of the prior pole so that all the poles are locked together. Then the support is removed and the roof bears its own weight. You can read more about building a reciprocal roof here (http://small-scale.net/yearofmud/2008/11/26/how-to-build-a-reciprocal-roof-frame/ ).
Model pieces notched and ready for assembly
Support the first rafter
Stacking the rafters
Finished Model
This model is strong enough to support a plate easily. When you play with it, it’s easy enough to see how the loads are transferred through the individual members such that the majority of the weight becomes tension along the grain (the direction that wood is strongest). A small amount remains sheer where the timbers meet at the peak of the roof so the timbers are sized to handle that load. We found the calculation for that in one of our timber framing books. Additional math for the reciprocal roof can be found on this website (http://www.mts.net/~sabanski/pavilion/pavilion_design.htm ).
There are a couple of challenges in this type of roof though. The first is that you’re putting the roofing material on a bunch of triangles instead of a bunch of squares or rectangles. Our modern roofing systems aren’t really built for that, so it will involve a lot of customization (read “labor and frustration”) to get everything to fit correctly. The second challenge is what to do with the hole in the middle? Some designs leave this open, some use a skylight there. Leaving it open isn’t really an option – I don’t like it to rain in my house. The skylight is interesting because it would let in a lot of natural light. However, I have yet to find anyone whose skylights didn’t leak at some point and I don’t want to deal with that either. Because of that, we’re planning to build a cupola to sit over this spot and allow for some airflow and maybe some light. The current thought is to size it so that we can put small windows in it to allow in daylight and have vents (or make the windows where we can open them from below) so that it will create air flow through the house. The finer details of how all that will work are still in the design stage, so stay tuned!
I agree with your final thoughts. Many houses, especially in urban areas, are not designed to handle reciprocal roofs, so you have to do some planning first. It might be a bit challenging, but reciprocal roofs are very economical and require less labor than other roof types. Can’t wait to see what you’ve done! :D
ReplyDeleteTerry Arnold
Thanks for the encouragement! We've ended up planning on a more traditional shape (rectangle) for the first structure, but we're still planning to do a Reciprocal roofed studio in the future. The ability to more easily build in stages pulled us away from the RR for the first building. I love them though and I can't wait to build one!
DeleteGreat Roof ideas for a house
ReplyDeleteThe 3-notched model sticks use the same mechanism of the 'Fireman's Forearm Carry', where there are 3 points of support for each member: the elbow(up), the wrist (down), and the hand grip(up). I know from experience that this works, but is very strenuous. However that is NOT the case for the illustrating roof which has no notches, apparently depending on torsional friction of the members and rigidity of the sheathing to prevent them from sliding away from the opening.
ReplyDeleteAs for the notched stick model, there should be some calculation of the splitting strength of the top notch, and the calculated roofing load. I can't seem to find any.
Additionally, it looks as if failure of the weakest such member means catastrophic failure of the roof.
You are correct that failure of any one member means failure of the entire roof. Fundamentally, this is a very simple truss with very few members bearing the weight. Because of that, the individual members must be sized to carry the load. To determine the minimum beam size (or to evaluate if the beams you have are sufficient), you would begin by calculating the total roof load. That would be the weight of the roofing and decking material you plan to use. They you would add the "live load" or snow load for your area (usually available in the local building code). That force (usually in psi or lbs/ft^2) then gets multiplied by the area of your roof. The result is divided by the number of members in the reciprocal roof to get the load born by each member. The point of greatest force in the truss is the point where the next member rests on the prior one, so we do the sizing calculation there. You would take the cross sectional area of that plane (so the cross sectional area of the beam minus any notch cut into it) and multiply that by the shear strength (usually in PSI) of the material. Note that all wood species have different sheer strengths so you have to find one for the type of wood you're using. If the total sheer strength of the cross section is greater than its share of the roof load plus a factor of safety, then you're good to go. Make sense?
DeleteAs far as the force on the points in the truss, you are correct that it is much higher in this truss structure than in a standard triangle filled truss. Fewer members means more force concentrated on each one. That significantly limits the clear span of these structures. It can, however, hold a significant weight.