A Note on, 'The Building of the Legionary Fortress of Inchtuthil'

Abstract

Elizabeth A.M. Shirley has recently made a valuable attempt to estimate the amount of material and labour used in the construction of the fortress.107 It is probable that the Roman builders adopted an empirical approach to the design and construction of their buildings, and would quickly have learnt that weather conditions in the North of Britain were different from those in the Mediterranean provinces. Rook108 quotes evidence for a pitch of about 20 degrees for tegula and imbrex roofs in Italy. The pitch of any roof has to take into account the nature of its covering and the exposure rating; usually the more exposed the position and greater the rainfall the steeper the pitch. Roofs steeper than 35 degrees generally present sufficient exposure for there to be positive pressure on the exposed face and suction on the lee face. However, with slopes of less than 30 degrees the windward pitch can be subject to severe suction or negative pressure, and is not less prone to windward uplift as stated by Shirley.109 The other factors to be taken into account are wind speed parameters. These vary considerably over the country and can be 37 m/sec in the South-East of England, to 50 m/sec in South-Eastern Scotland.1 10 There is a little evidence for the pitches of tiled roofs in Roman Britain; the aisled roofs to the building at Meonstoke1 being c. 20 degrees, and the roof to the villa at Redlands Farm1 12 laid to 22.5 degrees, both sites being in sheltered positions in southern England. Evidence from the fourthto fifth-century basilica churches in North Syria,'113 which had tiled roofs, shows a consistent roof pitch of c. 30 degrees. This pitch was used, rather than the lower pitch of c. 15 degrees reflecting that of the Greek pediment which would have conformed with the strong Hellenistic traditions114 inherited by the province. The implication must be that the pitch of the roof was dictated by practical as well as probable cultural considerations. Brodribb has shown that few tegulae had nail-holes formed in their surfaces,115 the great majority having none. The tegulae, therefore, remained in position on a boarded surface by virtue of their own weight.I11