Slate As A Roofing Material
Background Information
Among the many different types of roofing materials, slate is one of the most durable and aesthetically pleasing of all. In terms of its composition, slate is little different from the clay deposits you might find in a river bed. Through the process of different geological forces of pressure, temperature and time; clay is transformed into the shale and slate that we see. When slate forms, tremendous temperatures and pressures cause the mineral grains to align so that they are parallel to each other. This granular alignment is referred to as cleavage. This cleavage enables quarry workers to split out the thin, flat sheets of stone we see on slate roofs (see Figure 1). Different slates vary in their composition, structure, and durability. Since slate is a natural material, derived from different types of mineral deposits, slate is neither uniform nor consistent.
Figure 1. Splitting slate.
History of Slate
There is evidence to show that slate roofing was used prior to the Revolution. Building ordinances in the seventeenth century recommended the use of slate roofs due to its non-combustible nature. Until 1785 all slate was imported from North Wales. 1785 marked the opening of the first commercial slate quarry in the United States (see Figure 2). By 1876, most roofing slate imports had dried up and the United States became a net exporter of this material. In the early 1900's when the U.S. roofing slate industry was reaching its highest point in distribution, there where over 200 slate quarries operating in over 13 states. In 1915, a decline in the use of U.S. roofing slate resulted from several factors: skilled labor, competition from alternative materials, and cost of installation.
Figure 2. U.S. slate quarry.
Physical Properties
The...
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