The timber roofs above the main vaults of the abbey, coeval with the parts of the building they cover, are illustrated in J. P. Neale and E. W. Brayley, History and Antiquities of the Abbey Church of St. Peter, Westminster (1823), ii. These important medieval roofs are described only very briefly in an addendum to the Inventory of Westminster Abbey by the Royal Commission on Historical Monuments published in 1924 (entry no. 22a, slip p. 58), and no detailed description of them has so far been published. They underwent heavy restoration at the beginning of the eighteenth century, but survived in great part until the present repairs under the direction of Mr. S. E. Dykes Bower were put in hand. By 1964 the whole of the nave roof had been reconstructed and the roof over the south transept had been taken down and reconstruction was in progress. No major repairs to the roofs of the presbytery or the north transept had been started. At this stage, by kind permission of the Dean and Chapter, the Royal Commission was given facilities to make detailed records of the surviving medieval structures. In the following account the documentary evidence relating to their building and repair is outlined, their construction is described in some detail, and, finally, their historical context and their setting in the general development of roof construction are discussed. The photographs were taken by Mr. J. Parkinson, and the work has been co-ordinated by Mr. A. R. Dufty, Secretary to the Commission.