Situated in the Cambridgeshire countryside near to the village of Bourn, Bourn Mill stands as one of England’s most ancient surviving windmills.
It was built in the reign of King Henry VIII with its vast oak timbers and beams dating back to 1500.
In the spring of 2022, Bakers of Danbury embarked on the restoration works.
The workings of the mill
With a design that dates to medieval times, the mill operates via the body of the mill, known as the buck, being able to rotate around a huge central oak post to allow the 4 sails to face the wind and turn to drive the machinery that grinds the wheat.
The buck is supported by a vast wooden trestle of 4 oak beams; the horizontal cross trees and angular quarter bars. These are suspended and supported on four brick piers. Following a routine inspection of the mill at the beginning of 2020, it was these beams where extensive decay was discovered raising a real concern about the future of the mill.
Historic England were enlisted, and formal investigations began in 2020 to clearly understand the issues being faced. It was found that decay to the trestle beams had previously been addressed using a resin and sand-based filler forming blocks which were tied to the sound timber with stainless steel rods.
The timber had so significantly deteriorated that it became alarmingly apparent that these beams were providing little structural integrity to support the weight of the buck. The rotten timbers were also suffering from wet rot displaying cubing and a white bloom and although dry rot was not in evidence, the timbers were at the stage where this could occur, the resulting spores of which would be catastrophic to the timbers of the mill.
Found also was that the surface of the original timbers and main support column had undergone multiple applications of a black finish over the years. Identifying the specific coatings proved challenging but the presence of coal tar was suspected as a contributor to the deterioration of the timbers.
The Restoration Programme - Raising of the mill
The initial phase centred on raising the main body of the mill to allow the detachment and removal of the rotten supporting timber beams. Five hydraulic jacks were placed at specific points: four on the outer corners of the upper and lower cross tresses, and one under the central cross point main post, all supported by suitable base pads.
The existing side screw jacks supporting the structure were carefully loosened and a team of four operatives then lifted the entire trestle with the jacks under the buck being progressively tightened at 50mm intervals. Once the windmill was raised to the specified height of 150mm, the jacks under the buck and side supports were tensioned. The trestle was then lowered and detached from the buck and pivot plate. Internal timber supports were added. Throughout the process a spotter was present to monitor and ensure stability.
Replacing the oak beams and reconstruction
The trestle was dismantled in sections. Extreme care was taken with lifting equipment used to reduce manual handling. All existing joints were recorded wherever access allowed, and the old beams were removed and analysed for the patterns to be replicated in the new oak beams.
Each new oak beam of colossal weight was firstly dry fitted before the final fix. The beam ends were treated in a beeswax oil then held in position with stainless steel bolts & brackets and bolted to the brick piers. The existing wedges between the cross trees and the main post were retained and reused.
The brick piers providing the support for the beams were also reconstructed using lime mortar to match the original. There was a 30-day drying period to allow the lime to harden before the lowering process and the reconnection of the pivot plate and buck to the new trestle could take place.