Introduction to the Reciprocal Roof

Hobbit House Exterior

Simon Dell's Hobbit House

Several weeks ago, I was reading a blog on natural building a particular phrase caught my attention… A Hobbit House.   Mind you, my admiration of J.R.R. Tolkien started in 4th grade when I read “The Hobbit” and has continued unabated since.   So I read on about the natural-built home of Simon Dell and his family in Wales and was rewarded with another intellectual curiosity new to me…  the Reciprocal Roof.

The pictures of the roof frame his project immediately caught my attention… a set rustic  poles arranged radially around a round skylight.  A quick search on the web led me many other projects, mostly owner-builder homes, using a similar roof system.   This led to a quick self-education on the idea of the reciprocal roof.

What is a Reciprocal Roof?
Hobbit House Roof

Simon Dell's Hobbit House Roof

A Reciprocal Roof is a special space framing system where framing members are arranged in a  pattern such that the system is self-supporting by means of each members is supported and support one of more other members.   The Reciprocal Roof is sometimes called a Mandala Roof or can be more generally called a Reciprocal Frame, which includes bridges or other types of construction. The roof frame creates a conical roof form over a circular, or oval, building space.

To get a better visual of what we are talking about here are images searches for Reciprocal Roof ImagesMandala Roof Images and Reciprocal Frame Images.

The system has been used in very natural construction using timbers or poles at rafters that are not necessarily straight or even debarked.   For the roof to be durable, the poles are bound together.  Binding strategies include simple tied rope, mechanical fasteners such as lag screws or bolts or highly designed and crafted timber frame creations.    It is not required that a reciprocal roof be built from wood, or bamboo; however, aside from aesthetic appeal, one main advantage of the system is to build a roof span wider than the length of any of the framing materials used in the construction.  If one were construction a similar frame from a man made product, such as steel, the intent would likely be to duplicate the architectural form.

Now looking at this system from an engineering perspective, I see several items of note.  Obviously it can be low-tech in both building materials and building methods.   The roof can span a space wider than the length of the rafters.   Less obvious is that much of the strength is developed through flexural bending of the members near inner apex and a tight connection between rafters will be valuable to keep the roof from settling excessively after construction.     On one site, I saw mention that the system does not create an outward thrust on the supports, which I believe may be an oversimplification of the real statics of the

In a future post, I’ll look into how the system works, from a more detailed structural engineering perspective.    Until then here are some various links to appease your curiosity.

Build Well…

Scott