Slate is a very old roofing material. Its’ first known use as such dates back several centuries. But the material itself is far, far older. It takes a long time for the slate material to form and there is no artificial process that can duplicate it.
Slate is unique among roofing materials in that is not manufactured. It occurs naturally as a result of geological processes which take many millions of years. Formation begins with very fine-grained sediment accumulating on the ocean floor. Over time this sediment compresses and solidifies, forming the sedimentary rock called shale.
The next phase of slate formation involves tectonic plates, the continent-sized pieces of earth’s crust. As these plates shift position segments of crust crash into each other. During these tectonic collisions, massive portions of rock get buried and squeezed together, subjecting them to high pressures and temperatures. When shale is subjected to tectonic collision it undergoes a transformation to become slate. This transformation, called metamorphism, takes millions of years.
Slate has layering called foliation. Foliation forms perpendicular to the direction of compression and is the result of the increased pressure shale experienced during metamorphism. Foliation matters. When slate roofing is struck parallel to the direction of foliation it will split smoothly and neatly. This is the whole reason why slate works as roofing material. After minimal quarrying and cutting, a chisel and a single hammer blow produce a roofing tile in an instant. If slate required extensive sawing, smoothing and polishing, as is the case for many other rocks used in architecture, the economics would be impractical.
Another key economic factor is that slate is abundant. In the United States alone there are several large deposits in Virginia, Pennsylvania, New York and Vermont. The Newmont Slate Company, located in Pawlet, Vermont next to the New York border, is the largest roofing slate producer in the country. It’s not a coincidence that so many slate deposits occur along the Appalachian Mountain chain. This portion of the Eastern United States was, hundreds of millions of years ago, seafloor. Sediment on this seafloor solidified to make shale which was then metamorphosed during the tectonic collisions that created the Appalachian Mountains, beginning about 480 million years ago.
Think about that the next time you see a slate roof. Asphalt shingles and metal roofing can be churned out quickly in factories. But slate cannot. The slate that sits atop roofs worldwide began as little more than marine mud. After millions of years of compressing it turned to rock and after millions of more years sitting in pressure cooker-like conditions its’ structure and composition was altered into a near-perfect roofing material. If you want a roof with character, the slate roof is tough to beat.
Please contact us to discuss which slate roofing option is best for your home or business. We would be delighted to assist with all your roofing and general contracting needs.