Reciprocal roof


This frame gets pride of place, though small, is the most exciting to date…   A ‘Reciprocal roof’ describes a structure of mutually supporting beams in a closed circuit. This frame consists of eight green oak timbers scribed together to create an elegant roof. We have experience with reciprocal timber frame construction, but only planar varieties, such as the Serlio floor. This pitched roof frame design takes it to a whole new level! The basic principle of a spiraling layer of timber beams predates that of Villard-de-Honnecourt to the late twelfth century in Japan. The Buddhist monk Chogen used this technique to construct temples and shrines. Hence the technique here has been used in this modern day Buddhist  Ashram. After much discussion and refining, the final design was concluded as thus; Eight primary rafters converging to create a 1m opening for the ‘lantern’. Additional sprocket feet for exterior aesthetics and to take a curved fascia board. The lantern has eight sides, each with a window and capped with eight curved rafters. Reciprocal.1.COtf The wall construction is cob and round with a diameter of ~6m. Given the building is classified as a shed 🙂 ,  it did not need planning permission or be subject to building control. This is no bad thing as the design is subject to progressive collapse, meaning there is no inherent redundancy in the structure if one timber failed! Therefore certain constraints, such as ridge height, limit the size of the structure. Since the walls were built to a height of 2.4m, the pitch of roof needed to be around 15° to keep the top under 4m. One of the design briefs was to keep the design as clean and simple as possible. Having no timbers simply rested upon one another, scribed flush fitting joints is a key design feature. This makes for complex joinery and involves complex mathematics; I use a system based upon boolean logic 😉 . There are many factors that alter the detail at the jointing of the primary rafters, such as pitch of roof, opening size for lantern and size of timbers. The CAD from inside: Reciprocal.2.COtf Due to the complexity of joinery the frame had to be constructed as a whole unit in the workshop. Typically we only assemble elevations, such as a wall or roof plane, rarely as a whole. Unfortunately there are no photographs of this stage, only of the timbers stacked and being used as a bench whilst making the lantern!. Note the angles involved in the housings for rafter to rafter jointing. workshop The most challenging factor is accommodating for the variation in timber sizes, twisting and trapezoid effects that exist within rough sawn green oak. Each timber is scribed to it’s partner and numbered accordingly. Fortunately everything went as planned and the joints fitted perfectly. The whole structure was then disassembled and taken to site for assembly. Using our shear legs we were able to carefully lift each timber into place, most tricky was the first as there is no point of reference. The first timber is supported from below, in this case and adjustable acro prop, also temporarily braced to the sides.  Shown here is the second timber being located.


Note that the foot of each rafter has an oblique birds mouth joint to the wall plate. The wall plate is made up of four layers of 18mm structural plywood, this was deemed to be the strongest and simplest solution. It will eventually be hidden behind cob.  The very ends of each rafter is finished with an obliquely cut curved detail, mirrored on the additional sprockets.


The same process of lifting each timber individually was repeated until the penultimate one was located. Though time consuming, this was relatively simple.


In this image you may notice grooves along the length of one side of the timbers. This is to take the oak tongue and groove boarding to be used to finish the ceiling, giving a flatter final finish.

The final piece is the key to the whole structure, and unlike the others cannot be dropped into place. The timber resting upon it is in the way and therefore has to come in at an angle from underneath. With a wall plate in the way it is theoretically impossible to insert! Though very awkward, we managed to fit this final piece and remove the supporting prop. Each of the joints were pegged (hidden from top) and pegged through wall plate into the upper layer of cob.


The lantern was already assembled, therefore could simply (eh!) be dropped onto the top.


And from a different perspective.


The following picture was taken before adding the additional sprockets and clearly there is much left to do. The front has a entrance section, with additional ‘floating’ spine beam. This will help support the heavy living green roof that will cover the entire structure.


We hope to get some more pictures as this fantastic project progresses.  (October 2014).