A reciprocal roof is type of roof where the individual rafters mutually support each other. They originated in medieval Japan but probably date back further and aspects of reciprocity can be found in the roofs of many early human settlements. Reciprocal roofs are generally found on round, elliptical or polygonal buildings as the weight of the roof can be easily directed downwards via the post and beam henge on which it sits.
We came across the idea and fell in love with this form of roof when we worked with Neil at Earthmovesdesign. When we were designing our house design we decided to incorporate a twin reciprocal roof over our open-plan kitchen and living room to divide the space and to draw light into it via the openings in the roof.
Our design did present a number of engineering problems. The first issue was how to build such a roof on a non-circular henge. We were advised by a specialist engineer, Chris Southgate, to add additional braces at each corner of the henge to deal with the weight of the roof.
The other issue was how to set the roof pitch. Normally the rafters are laid on top of each other in the centre and the pitch is determined by their collective spiral height (= diameter of all rafters at their thinner end). Our spiral height would be between 2.5 and 3 metres which would produce a 45 degree pitch. However as our pitch was set for 30 degrees we had to scribe and scallop out the rafters to achieve this. Having a carpenter on board who was able to deal with the complexity of this design was essential.
The task was very physically demanding as our rafters have a minimum diameter of 200mm and are up to 6m long. We used Tirfor winch and a 4 metre tripod which a local engineering firm, Coughlan Engineering, fabricated for us to lift the rafters into place. We built both roofs on the beams they will be affixed to on the ground, as assembling this at height would be very expensive. We aim to lift of each completed roof and lower it onto the henge when it is erected using a crane.
We were delighted with the finished result which is sculptural and beautiful. We really look forward to gazing at the sky and the stars though our roof.
The next task is to lay-up the north and east post, beams and braces. We will then begin the process of erecting and fixing the frame and roof, which will take 4/5 weeks. We are hopeful of covering the structure before Winter so that we can complete some of the internal jobs before next Spring.