Not all slate is created equal. Being a natural stone mined from the Earth, slate comes in many types with many different characteristics. In the old days, when roofing stone was wrestled from deep quarry holes and dark mines using hand tools and beasts of burden, the splitting of roofing shingles from rock was an arduous and exacting art. Many slate veins contained very hard, rough textured rock strata that could not easily be split into the uniform, thin sheets required for shingles. For the sake of efficiency, the stone was split into the largest slate shingles possible, creating a supply of coarse shingles that varied considerably in size some larger, some smaller. In order to make good use of all of these slates, a certain style of roof was developed of necessity graduated slate roofs. In this style of roofing, the largest stones, sometimes massive, are installed at the bottom of the roof. This allows for the heavier weight of these large slates, perhaps 30” long and an inch thick, to be born by the wall of the building. It also relieves the roofer of having to carry the flat stones, which may weigh 200 pounds, to the very top of the roof. Furthermore, the bottom of the roof is exposed to more water than any other part of the roof and heavier slates are more apt to withstand the excess erosion and weathering that occurs at the drip edge. As the roof installer progresses up the roof, smaller and smaller slates are used, with the smallest slates, perhaps only 12” long, fastened near the top. The result is a roof that “graduates” in size from large at the bottom to small at the top, yielding an architectural style that is utterly unique and quite pleasing to the eye. Traditional graduated roofs also utilize random width slates. There are still many of these roofs in good condition scattered throughout the United States and Europe, yet the art of installing graduated slate roofs is a disappearing one. This article focuses on this art and is intended to preserve some knowledge and, hopefully, revive some interest in one of our architectural treasures — graduated slate roofs. A good place to look at the long history of graduated slate roofs is in Scotland, England and Wales. Scottish slate tends to be a rugged, coarse, and extremely durable material. Unable to split large, uniformly thin slates from the raw material available in Scotland, the Scots created a distinctive roofing style with a rough texture in keeping with the stone architecture so characteristic of Scotland's traditional buildings. This graduated slating style was also popular in England and Wales for the same reason — it allowed for the efficient use of a stubborn material. Each country had its own installation peculiarities, however. Scottish slate, for example, was “head nailed,” with a single nail hole at the top center of the slate, and nailed into solid wood board decking referred to as “sarking,” as opposed to strips of wood known as “slating lath.” Although slating lath was popular in Wales and England, Scottish slates were so random in lengths, widths and thicknesses that a solid wood deck was desired in order to make it easier to nail the shingles to the roof. Scottish slates are also “shouldered” their top corners are knocked off, a practice still in use today in much of Europe. Because the Scottish slates were nailed at the top, there was a risk of gale winds lifting the bottom of the slates and blowing them off the roof. The Scots, however, utilized rather thick slates, the weight of which more than compensated for the weakening effect of head nailing. An interesting characteristic of Scottish slate roofs is that the slates can be swiveled on the nail, to one side or another, in order to clear the way for replacing a slate underneath. This swivel effect is aided by the shouldering of the slate. A slate ripper, America’s foremost slate repair tool, is rarely needed when removing a damaged slate in Scotland. The English and Welsh also once used a head fastening technique involving the use of a wood dowel instead of an iron nail. The dowel was driven through a hole in the top center of the slate, then the shingle was hung over a thin hardwood lath strip that had been hand split from a log. This practice eventually gave way to what is called “center nailing,” a nailing style used in the US today in which the slates are nailed with two iron or copper slating nails, one on each side, situated about 1/3 of the way down from the top of the shingle. The center nailed slates in the UK are usually nailed to sawn lath strips, perhaps an inch thick and two inches wide — a method of nailing still prevalent in Europe today and a carry over from the days when all stone roofs were graduated stone roofs. This slating method differs from today’s standard US techniques in that a Scottish style continuous board deck is preferred in the US rather than the lath strips more common in England. Incidentally, graduated slate roofs are known as “diminishing course” roofs in the UK, while uniform slate roofing is known there as “tally” roofing. Courtesy of: JW Roofing the best Miami Roofing company and widely held as the best Miami Roofer offering, Tile, Slate, shingle and flat roofs to South Florida
Please take a look at Flat roofs down under