The Building Conservation Directory 2020

76 T H E B U I L D I N G CO N S E R VAT I O N D I R E C TO R Y 2 0 2 0 C AT H E D R A L COMMU N I C AT I ON S Fig 4: In Rawlinson’s patent the slates fit into rebated rafters set at centers to fit the slate widths. Later examples simply bedded them on top. included slate or metal strips at the tail of each slate which were ‘scalloped to allow ventilation’ (Fig 7). In a further variation of unknown origin used at Colley House in Reigate, (Fig 8) the slate slabs were not overlapped vertically. Instead they were butt jointed over a slate noggin let into the rafters and sealed with putty. This was still in place in 1999 although the bedding had deteriorated resulting in leaks. Rawlinson’s original concept had been for the slates to be screwed to timber rafters positioned to accommodate various widths of slates. However, in Liverpool the ironmaster John Cragg realised that, provided the slate slabs could be made to a specified size, it was ideal as a modular system using cast iron frames. Happily, slate quarries were able to supply 1.5m-wide slates and slabs for the three churches he Fig 2: Single-lap slating in Caithness with the vertical joints protected by under- sealing (Photo: Terry Hughes) Fig 3: In double-lap slating each course overlaps the next-but-one course below, so the perpendicular joints are weathered. In effect the bottom course acts as a soaker for the middle one, and the top course acts as a cover flashing. In contrast, in single-lap slating the perpendicular joints are open and have to be weathered by over-sealing or under-sealing. Fig 5: An example of over-seal slating in Sweden (Photo: Anna Blomberg & Kristina Linscott, Riksantikverieambetet) SOME PATENT SLATED ROOFS Liverpool churches by Thomas Rickman & John Cragg: St George’s church, Everton, 1813–14, St Michael in the Hamlet church, Aigburth, 1815 St Phillip’s church, Hardman Street, 1815–16 (demolished 1882) London buildings Derby Gate (Formerly Whitehall Club), 47 Parliament St, London, 1864–66, Charles Parnell ReformClub, Pall Mall, London, 1837–41, Charles Barry (roof replaced with clay tiles c2005) Others Ashmolean Museum, Oxford, 1841–45, Charles Cockerell Colley House, Reigate, 1840, TR Knowles senior Penoyre House, Nr Brecon, Powys, 1846–48, Anthony Salvin designed withThomas Rickman. Two of these, St Michael’s and St George’s have recently been reslated by Finlason Partnership Architects. The third, St Phillip’s in Hardman Street, was demolished in 1882. At St Michael’s the brick walls were also clad with slate fixed behind mouldings in the iron structure and with cramps and spikes. These were almost all removed subsequently, and the brickwork pointed. CONSERVATION AND REPAIR Rawlinson predicted that the durability of the timber rafters would determine the life of the roofs. Unfortunately, this wasn’t so: it was the aging and embrittlement of the unctuous cement which eventually led to leaks. More often however, the slating has been assumed to have been the cause when in reality the leaks were the result of other defects. In the Liverpool churches leaks were associated with the structural cast iron gutters which stood on top of the walls. These problems have been repeatedly compounded by people walking on the low pitch roofs to try to locate the assumed leaks in the slating and breaking the slates (Fig 1). The Penrhyn slates themselves are very durable and can usually be reused provided they are undamaged. However, if it is necessary to lift any of them, this must be done carefully to avoid damaging the fixing holes. If replacements are needed Welsh Slate Limited can supply them. Where leaks occur in the body of the slating all that is usually needed to cure them is to lift the cover strips and re-bed them with a more durable seal. This was done successfully at Derby Gate in Whitehall in the 1980s (Fig 7), on St Michael’s in 2006 using Arbomeric MP20 modified polymer, and St George’s in 2016 with Compriband TP600 sealing tape. However, the work on the latter two was complicated by the failure of the cast iron gutters which sat on top of the walls, so water was leaking straight into the wall heads. The slate slabs which formed the ceiling were supported by the lower flange of the I -section cast iron rafters, which simply rested on the inner edge of the cast iron gutter. Any leaks or condensation from the rag slates of the roof (carried by the top flange of the rafters) should therefore have been caught by the slabs and discharged into the gutters. Unfortunately the walls had separated, the gutters had cracked in a few places and the joggle joints had leaked.