Reciprocal roof

Hey everyone.
We've already built a reciprocal roof for our cobhouse and it's fantastic. Now we want to build another one for the outdoor kitchen. However the outdoor kitchen is rectangular (3m x 4.5m) and not round like the cobhouse, so the reciprocal roof would be elongated with different lengths of poles and the weight might not be distributed evenly. Would this be an issue?

This is an interesting question, in part because I have not heard of it before.
I found this info about designing reciprocal roofs, but no mention of rectangular buildings.
how to design a reciprocal roof

YES, they can be built, this link speaks of the chinese bridges built using the RF principle, and has drawings of more complex systems covering rectangular structures
Use of reciprocal roof frames
From the above paper,
"Typically, in simple inclined RF configurations loads are transferred by combined axial and bending action, while in flat assembles they work mainly in bending. In both the flat and inclined RF configurations there is shear at the point where the RF beams rest on each other. RF structures are less efficient compared to gridshell structures, which work mainly through axial action.
Despite this, there are some important benefits of using RFs:
RFs are formed by short members;
In complex RFs there is a large degree of built-in redundancy;
In symmetrical configurations all joints are identical;
In symmetrical configurations all members are identical.
These advantages, together with the extremely large scope of possibilities for creating new spatial configurations, make them a structural system unlike any other.
RFs are varied in the configurations that are possible and as such are an interesting system to use for building applications. The different structural forms offer possibilities for different applications.
This paper discusses some explorations with full-scale models as well as innovative applications of RFs recently constructed."

Hi Seb - welcome to Permies.

Have you considered building a round roof anyway? You would have to build up the walls following the parabolic slice through the cone you create. It would require a bit of maths to figure out. Potentially you could have some big eves, so I guess it depends on what’s around the building.

Edward Norton wrote:Have you considered building a round roof anyway? You would have to build up the walls following the parabolic slice through the cone you create. It would require a bit of maths to figure out. Potentially you could have some big eves, so I guess it depends on what’s around the building.

OK, I don't get the parabolic bit unless you're referring to height, however my initial reaction considering I live on the Wet Coast, was to build it round with posts to support its round foot-print and end up with some nice outdoor work/play zones that are protected from the rain. That will require extra material, but if the outdoor zones would be useful to you, it may be worth it.

Yes, you can do a reciprocal roof on a rectangular structure.

The "rectangular" reciprocal concept that I remembered seeing is very different in nature to the circular ones. The first link below was specifically contemplating its use for an earthen roof. At least one post in that thread suggests that the footings would need to be very secure for it to be reliably safe. That might mean that you do as suggested up post and in effect, build a rectangular building under a secure bridge structure, and if the abutments aren't part of the end walls, you would have some outside covered storage for all the things that benefit from that.

https://permies.com/t/155262/Leonardo-da-Vinci-bridge-design

https://permies.com/t/34585/years-wooden-bridge-stands

Your plan, Seb, is intriguing. Please post pictures of your existing structure and any plans you have for the new structure and report back on what your final decision is. Whatever you decide, please do so with care as we don't want anything bad to happen.