The Building Conservation Directory 2020

75 C AT H E D R A L COMMU N I C AT I O N S T H E B U I L D I N G CON S E R VAT I ON D I R E C TO R Y 2 0 2 0 ROOFING 3.1 PATENT SLATING TERRY HUGHES W HEN, IN 1772, Charles Rawlinson of Lostwithiel in Cornwall was granted Letters Patent for a New Invented Method of Covering Roofs with Slates he may have believed he had invented something new. No doubt he was unaware that the same system had a long history in Scandinavia, Caithness, Orkney, Ireland and probably many other places. Not only is it a vernacular system but it also came to be re-invented many times after Rawlinson. THE PATENT SYSTEM Patent slating is single lapped, so each course of slate only overlaps the course immediately below, like pan tiles (Fig 1). However, single lapped slates do not overlap sideways as pan tiles do, so the perpendicular joints are open and would leak unless they are weathered in some way. Rawlinson chose to do this by bedding narrow slate strips onto the butt joints. This is known today as over-sealing. The alternative is under-sealing (Fig 2). In the more conventional double-lap system of slating the perpendicular joint issue doesn’t arise because each course of slates overlaps the next-but-one course below, so that the overlying course acts as a cover flashing and the underlying one as a soaker (Fig 3). It is interesting to speculate how Rawlinson came to devise his method as he is unlikely to have known about vernacular single-lap slating. He would certainly have known rag slating2 as he refers to ‘slates or rags’ in the patent. This method which is common in Cornwall and Devon uses very large slates of up to 1.3m wide which are nailed directly to rafters without battens. As a result, when fixed in the normal double-lap method they have very large side laps, far wider than necessary. Generally, side laps of no more than 160 mm are needed to prevent leaks in even the most severely exposed situations. As the rags are random sized the actual side lap depends on how the slates are laid together, but even so it would be obvious to any roofer or quarry operator that the width of lap would far exceed what’s necessary. So, wherever large slates or stones were available – from south west England to the far north of Scotland and Scandinavia – single lapping was adopted (Fig 5) and the side gaps were weather-sealed in one way or another. Rawlinson slating adopts the same general approach, with rag slates supported between the rafters on either side, into which they were rebated (Fig 4). In practice, the rag slates needed to be available in, or sorted into, sets of equal width sufficient to cover a rafter length, but there could be sets of different widths for each run. Once the widths were known, the rebated rafters were fixed at centres to support the runs of adjacent slates. To seal the butt joints the slates were bedded in ‘unctuous cement’ and screwed to the rafters. Capping slates were bedded and screw-fixed over the perpendicular joints. Rawlinson provided recipes for cement for more and less exposed areas (see box on this page). HISTORY OF USE As far as is known Rawlinson’s system wasn’t used extensively for roofing in his hometown or in the south-west generally where it is only known (at least to this author) as wall cladding (Fig 6). However, by 1788 patent slates were being made at Penrhyn Quarry in North Wales (as referenced in Welsh Slate by D Gwynn, p44). This was during the time when the Wyatt family of architects was influential in the quarry. Samuel Wyatt was given the job of modernising the medieval hall at the Penrhyn estate and his brother, Benjamin (1745–1818), was appointed general agent to the estate. From this a specific arrangement was established with Lord Penrhyn allowing Samuel and his other brother James (1746–1813) all the slate they required before any was supplied to his Liverpool agents. They promoted many uses for it including patent roofing4. It isn’t known whether Samuel and James Wyatt had seen Rawlinson’s patent or whether the system was invented independently but it was promoted by James and adopted by many architects on buildings in London and further afield (see box on following page) in the first half of the 19th century. In 1843 another patent was granted to William North which incorporated what today would be called interlocks or water bars made of slate or metal to weather the lateral butt joints rather than using slate rolls. It also Fig 1 : Rawlinson’s system of patent slating on St George’s Church, Everton. Left: the roof before repair showing a run of slates on the right which have all been cracked, probably by someone walking on them. Right: the same roof during stripping, showing how wide slates were but-jointed over the iron rafters and the gaps oversealed with narrow slates. In the foreground the slabs which form the ceiling can be seen, supported by the iron rafter’s lower flange. (Photos: Terry Hughes) RAWLINSON’S RECIPE FOR UNCTUOUS CEMENT A cement for near the sea coasts where the rain penetrates and is driven with great violence. To make a hundred weight, take 66lb of best whiting, 12lb of sea coal or wood ashes sifted very fine, 10lb of hard brick or tile sifted fine, 6lb of white lead; put all into an iron pot, and heat it over the fire, till the moisture is entirely evaporated. Keep it stirring all the time; and when well dried, put to it three quarts of boiled, and six quarts of raw, linseed oil. Mix the above with the hand in a tray, or on a board; then well beat it till it becomes tough and soft; mould it thoroughly with the hand, and examine whether all the parts are well incorporated: if they are, it is fit for use.