Harmondsworth in Middlesex, a village seventeen miles from London, is best known as the original home of Penguin Books, but is also celebrated for its surviving medieval barn, 192 feet long and as magnificent, as John Betjeman said, as a cathedral. It is not, as he thought, the longest medieval barn known in England, but is placed at thirteenth in the league table, and fifth among those still standing. It deserves to be well known, but unfortunately is only likely to become famous as it stands only 150 metres from the end of the projected third runway for Heathrow airport.

The building has lasted well but it was in danger towards the end of the last century, when it was acquired by English Heritage. Over a number of years sums in excess of £600,000 were spent on its repair and restoration. Historic England, who have succeeded English Heritage, were inspired to publish this thoughtful and thoroughly researched account of the architecture and history of the barn in a well-illustrated book. It will be read with profit by those interested in timber buildings and medieval agriculture.

Like many other imposing barns, Harmondsworth's was built for a manor on the estate of a great church institution, in this case Winchester College. The manor had belonged to an ‘alien priory’ attached to a monastery in Normandy, and the building of the barn in 1426–7 was part of the College's programme of investment and improvement of its relatively new acquisitions. The Winchester documents throw light on the use of the barn, but not on the main phase of construction. The date comes from dendrochronology with support from the documents. The surviving structure itself tells us about the decisions made by the builders, and also reveals some errors. It was built of good quality materials, and in particular the timbers were probably selected in a distant wood, though we do not know the precise source. The carpenter in charge of the project was evidently not in the first rank of his craft, and may have lacked experience. Some of the jointing methods he used are described as ‘inept’. The masons with whom the carpenters had to cooperate did not do their work very competently, and the sill walls were poorly levelled. In a remarkable failure of communication, the masons built the sill walls with spaces for doors, but the timbers that were cut and made ready for assembly away from the site were designed with the doors in different positions. The mistake was corrected when the timbers were reared into place with a certain amount of ingenious botching, but these and many other minor errors in assembly make the whole story a useful corrective to the uncritical admiration in which medieval craftsmen are commonly regarded.

The documents enable a good account to be given of the use of the building, from the carting of the crops in the harvest through the threshing process and the disposal of the grain. Much of the crop would have been sold in London. The barn's capacity seems very large in view of the likely volume of the harvested demesne crops. After the king took over the manor in 1543 the building was owned and managed by a succession of laymen, and one wonders at the good luck that prevented its decay or demolition through the centuries. We can only hope, now that the history of its construction and use has been presented with such skill, that it escapes the new threat to its survival.