C O N T E X T 1 3 4 : M A Y 2 0 1 4 19 TOBY MURPHY and IAIN McCAIG Repairing Harmondsworth Barn Detailed structural assessment influenced the design of repairs to the vast, 15th-century, timber-framed barn that John Betjeman called the Cathedral of Middlesex. The start of work repairing Manor Farm Barn – the ‘Great Barn’ – at Harmondsworth, Middlesex was the culmination of many months of preparation that followed English Heritage’s purchase of the Grade I listed building in 2011 to rescue it from years of neglect.The selection of appropriate remedial measures relies on understanding a building, its history and significance, its condition and how it has responded to environment and use, and the accurate diagnosis of faults. A wide-ranging programme of surveys, research, specialist investigations and feasibility studies have been carried out at Harmondsworth Barn in line with good conservation planning practice. Harmondsworth Barn was built in 1425–7 by Winchester College and remained in agricultural use until the 1970s. A truly impressive structure – Sir John Betjeman called it the ‘Cathedral of Middlesex’ – it is the largest, least-altered, timber-framed barn in England. Exceptionally well preserved, it even retains much of its original oak weatherboarding. The oak frame incorporates a number of features that are experimental, precocious and unusual for the building’s location and date – although not all of these worked out well structurally, as we shall see.The fact that the building has survived less than a mile from Heathrow Airport, in between the M25 and M4, is in itself remarkable. The frame of the barn (58m long x 11.4m wide x 11.8m high) is divided into 12 bays by 13 aisled crossframes, with two small arch-braced trusses at each end of the hipped roof. The walls rest on a low masonry plinth built largely of ferricrete (iron-oxide-cemented gravel) boulders; inside, the arcade posts stand on blocks of Reigate stone. Evidence exists for numerous campaigns of repair carried at different times to roof coverings, timber frame, exterior cladding and plinth walls. Of these the most recent dates from the early 1990s, when the plinth walls were consolidated and partly underpinned by St Blaise, and repairs were carried out to the north wall frame and cladding by McCurdy and Company. At this time, Peter McCurdy undertook a meticulous measured survey of the barn, recording all the carpenter’s marks, and noting the method of conversion and condition of every timber. Earlier in the 20th century the roof had been re-tiled. The weatherboarding and roof coverings in the two southern-most bays of the barn were replaced following fire in the 1970s. Although the frame was charred, it survived. Older repairs are also in evidence, including iron bolts to strengthen arcade post jowls where these had split, and wrought-iron ties and straps to restrain dislocated joints. In 2011–12, condition and structural surveys were carried out by English Heritage’s Building Conservation and Research, and Civil and Structural Engineering Teams. Close visual inspections were made from a cherry picker of each timber and every joint in the frame.Trial pits were dug to examine the foundations of the plinth. Inspections were carried out to assess the extent of timber decay and insect damage. Although the superstructure had been subject to movement and deflection over time, overall it was found to be in remarkably good condition. The interior of Harmondsworth Barn temporary struts nave tie beam aisle brace arcade plate lap dovetail assembly A diagram of the framing in a typical bay
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