ANCILLARY ROOFING ELEMENTS
In the preceding chapters an attempt has been made to give a general view of Scottish vernacular roofing of slates and tiles. This chapter discusses the use of elements associated with slate and tile roofs. 4.1
Fixings
The traditional hardwood roof structures which developed through time relied on ropes and pegs, etc, or heavy ballasting of vegetable material to hold the whole assembly together. Where buildings required more substantial roofs with the cross-pieces supporting the roof covering comprising cleft laths or sarking boards, nails to fix these to the rafters or cabers were required. Throughout time the nail-making blacksmith has provided these in sizes to suit the particular job, but so fragmented has this trade been in former times that very little has been recorded, rather it has been taken for granted as a basic fact of life. Nowadays we buy machine-made nails, more or less on demand, without thinking of the way things used to be done. In former times iron would have been transported to the blacksmith and nails would have been a local product. At about the same time as the industrialisation of slate quarrying took place, a steady concentration of nail-making by traditional methods led to the nails being taken from industrial centres to the place of use in finished form. By the end of the Victorian period, the making of nails had changed completely. Unlike modern nails which are made from wire of a particular metal the traditional nail was made by cutting a tapering piece from a plate of grey iron. Other metals, mainly copper, were introduced towards the end of the 19th century. Some nails at this time were cast and coated with zinc but these were used for larger slates. Although ferrous products can oxidise or rust, wrought cut nails have had a long life on the roof, both slate and board nails having served for a century or more. Modem ferrous wire nails do not have the same durability and are best used with a zinc coating either hot dipped galvanised or sprayed, sheradised or pour coated. These nails have good durability when used to nail lath, batten or board to rafter, where the zinc coating will not be damaged. They are less successful where the zinc may be damaged by the slate or tile being fixed. The most successful nail for fixing slates is the copper wire clout head nail. Aluminium alloy nails are more often used with tiles with a shorter
durability. At the end of the traditional nail period, large quantities of cut copper clout head nails were used for slating and, whilst these proved durable, their tapered shape reduced their nail withdrawal resistance. This was not a problem with similarly shaped wroughtiron nails due to surface oxidation providing a better grip. At the present time restorers are seeking to use stainless steel nails in the hope that durability will be good. This is a gamble for a number of reasons, not least for those set out towards the end of Chapter 1. The most important issue regarding nails is the adequacy of sizes. The diameter of the nail and the length of penetration into the board or batten are important to provide withdrawal resistance. With board and batten nails, ring shank nails can be used to increase the resistance. With slate or tile nails, smooth shank nails of adequate size are preferable to the unforgiving ring shank nail which can cause broken tiles and slates during any roof settlement that may take place.
4.2
Mortar
Another item of major expense for the traditional roof was that of building lime. The process of producing slaked or putty lime for building is well-known and is not repeated here. Not all areas had lime available at a convenient distance and it is interesting to note how this influenced building methods, such as the use of moss for draughtproofing, etc. Development of building was held back in areas where lime was scarce or too costly. Mention has been made of the use of lime with sand to make a gap filling mortar suitable, for the most part, as a draught excluder and as a supplementary fixing to hold the heads of the slates or tiles firm, the precondition being that the lime mortar did not become wet on a regular basis. Normal lime mortar used for bedding is not frost-resistant and therefore has to be kept dry. In the case of slates and tiles, This means it cannot be used in the covering assembly where rain water regularly flows. In other aspects of traditional building, lime mortar had to be protected from getting wet, as at abutments for example, by being protected with a generous coating of limewash or whitewash, itself made from neat slaked lime. This had to be reapplied regularly to provide the necessary protection.