THE CRUCK AND AISLE-FRAMED EXHIBITION HALL
We recently received the article below from the man who built our Great Hall, Adam Heath. It is an interesting read and follows the story of the build.
The first site meeting was in late September 1992. The project was still very much in its infancy then, with an idea for a log style building from round timber or roughly squared beams, of locally sourced pine, larch and douglas fir.
The building would be used for an exhibition about trees in our world.
Part of it needed to be big enough to allow the positions of the exhibits (some of them large) to be arranged and re-arranged as required.
Areas for an office, gift shop, restaurant and toilets also had to be considered. The deadline for completion was the opening date of the building and grounds to the public on the 24th of May 1993.
After much discussion it was agreed that the building should be a traditional timber frame. The first design was a combined aisle-truss and base-cruck structure using Welsh oak. It was to be 10 meters wide by 30 meters long and of 7 bays .The 4 base-cruck trusses would create an open space with plenty of headroom in the 3 bays of the main exhibition hall and the aisle- truss posts would form a division at each end from the exhibition area for the office and other facilities. This was sent to our structural engineer for comment.
The structural engineer told us he would only certify base-cruck trusses if they were placed between conventional, triangulated frames (e.g. aisled) This arrangement would not give the exhibition hall the clear floor space required as the posts of every other truss would land well into the room.
However, the engineer also said we could have any number of open trusses if a collar did not truncate the cruck blades, but rose from ground to ridge in one piece and joined at the apex to make a 3-pin frame.
We had not considered using full crucks initially, mainly because of the short time available in which to find enough suitable timber that was long enough (12 meters) and with the correct deflection to suit the pitch of the roof.
I wanted some historical justification for the design, it was not to be a replica in any way, but something that broadly represented some of the details found in late mediaeval Welsh vernacular architecture.
I had only seen one example of a cruck combined with an aisle -truss, this was the remains of the great Welsh gentry hall house Plas Cadwgan preserved at the Avoncroft Museum of buildings. Here the aisle-truss took the form of a screen, (spere truss) between the cross- passage and the hall. We spoke with the Director of the museum and he suggested we also see Ty-Mawr a base-cruck and aisle hall near Castell Caerinion, Powys. At the time of our visit it was still a ruin shrouded in tin sheeting with an uncertain future.
Intrigued by this ’mixed form of construction ‘ I bought a copy of Peter Smith’s excellent book ‘Houses of The Welsh Countryside’ that illustrates other similar examples from Wales, which was a source of inspiration for the final design of the project.
We needed to find 8 cruck blades, ideally from 4 trees split into pairs to make 4 trusses for the exhibition hall. The remaining 2 internal trusses would be of aisle construction and the gable ends half- hipped.
We asked our local sawmill if they could help. They had some stands of timber in the welsh borders. I went down to Newbridge –on –Wye near Builth Wells to look at some trees on the banks of the River Wye.
Buying timber in the round can be a gamble and I can remember feeling quite nervous about selecting standing trees particularly as they needed the right curve in the right place to make cruck blades. I could only find 1 tree that was large enough to make a pair from. It had a good curve and was just what we were looking for. I chose 5 more trees, they were not as bent as the first one, but as no others were available, and this was our last chance of finding anything, so they were picked.
It was nearly November and by luck we were just entering the felling season, so the trees could be sawn down almost immediately, the last thing we wanted was a delay in the delivery of materials. The timber fallers were instructed to fell the trees and to cut them as close to the ground as possible, to get the maximum length on the bottom of the blades.
Having gone to this trouble, disaster nearly struck when the lorry driver, struggling with the loading of the largest and longest tree, wanted to cut it in half. Only the swift actions of one of the timber extraction crew stopped him putting the chainsaw right through the bend and making two straight lengths!
The trees were to big for our local sawmill to handle so they were taken to another mill up the road that had a large horizontal bandsaw. The cruck blades needed to be 200mm thick and after sawing we got two very good matching blades from the largest tree and from the smaller ones 4 reasonable single blades. These were put together to make pairs. One of the trees was too straight for crucks so we had it cut into beams for the gable end walls. Some of the larger pieces of bough timber were also converted for use in the frames as well as the remainder of the curved material for collars and braces.
The final design was partly dictated by the timber we had, and with no time left to look for anymore trees we decided on a combination of three cruck trusses and four aisle-trusses. It would be 10 meters wide and 30 meters long. The three cruck frames and a wide aisle-truss would go over the exhibition area at the East end. Two narrower frames would form the entrance foyer, gift shop and toilet area in the middle of the building with one more aisle-truss at the west end over the kitchen and restaurant. As a contribution to the project the cruck trees were supplied at a reduced fee.
When the timber finally arrived at our yard we were faced with a new problem, our workshop was not wide enough to accommodate the full span of the cruck trusses. The blades had to project out of the building onto sloping ground while they were being framed up. As we only had very limited use of a borrowed forklift everything was moved around on rollers, which made handling the cruck blades difficult.
We made up a large softwood template, which was laid on to each cruck blade and the positions of the collar, purlins and ridge marked. Then the blades, collar and braces were scribed and a pulley block used to turn the crucks on to their backs so we could cut the mortices and tenons. They were bridle jointed at the apex with a housing for the ridge beam. Where the cruck blade was low and could not directly support the purlins, tapered principal rafters supported on blocks were made to carry them.
By Christmas 1992 we had completed the three cruck trusses but there was still a lot to do. The two narrow aisle-trusses for the entrance foyer were framed up with cranked collars and arched braces sawn from natural bends. The arch braces were shouldered back into the aisle posts so that we could run a moulding all the way round. We chose an ovolo profile and this was worked out of the solid.
The last two aisle-trusses were started early in the new year, the one at the East end of the exhibition hall had simple arcading between a broad tie-beam and collar and was moulded. (photo 9)
The framing of the truss over the kitchen and restaurant was in straight timber and after working with all the bent stuff was a relief in some ways! Its completion also meant that we were close to finishing the framing.
The raising of the frame went very well considering the fact that it took place during February in Snowdonia . The foundations, and pad with pockets for each of the cruck feet were ready, and set out perfectly, despite the fact that the weather was against the main contractor for most of the time.
The half hipped gable was assembled at the East end and the first aisle-truss was lifted up with the crane and propped. The next truss was cruck framed. When this was upright and held securely with its feet located in the pockets the ridge beam was dropped on and the tapered principals were fitted. These were joined to a spur notched into the face of each cruck blade and tenoned to an oak post. Then both sets of purlins were cogged and pegged. The rest of the cruck trusses were raised in the same way. The last three aisle-trusses were craned up and then the wind braces were fitted.
With the raftering of the roof and the framing of the walls in softwood done our part of the project was finished. From the design stage to completion of the timber frame had taken 5 months. Given the short time available it would not have been possible to do without the hard work of everybody involved.
The Green Wood Center was opened as scheduled on the 24th of May 1993 by Dafydd Wigley MP and the project was awarded a Prince of Wales award.
Architect, Designer and Carpenter: Robin Heath
Architect, Designer and Carpenter: Trevor Stevens
Timber Frame Designer and Carpenter: Adam Heath
Main Contractor: Mike Smylie