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 physical makeup of slate consist of a fine grained, crystalline rock derived from clay and fine silt deposits at the bottom of the ocean. As more and more materials would overlay these sedimentary particles, they would form bedded deposits of shale. During this process, intense heat and pressure from the earth’s movement would change original clay into minerals such as mica, chlorite, and quartz. Through this mechanical and chemical process, bedded clays were transformed into the slate we see today. Since the production of slate is almost entirely a result of natural forces and materials, slates vary in composition, structure, durability and color.

The color of slate depends on its chemical and mineral makeup. Different locations in which slate is quarried produce different colors. The most common colors are grey, blue-grey, black, shades of green, purple, brick red, and speckled varieties. Different compounds of iron generate the reddish and purplish colors. The grey shades of slate are quarried in eastern Pennsylvania. The red and green tones are found along the Vermont-New York border. The different colored slated are further classified as either unfading or weathering. Some slates will change color over time, those labeled as unfading will not. In addition to a large variety of colors, standard slate comes in two thicknesses: 3/16” and 3/8”.

Although the color of slate represents its mineral make-up, one color of slate will not necessarily last longer than another. Slate has been documented to last anywhere from 60 to 200 years. This documented life span of slate should only be used as a reference in determining whether or not any given slate is nearing the end of its serviceable life. With the proper inspection and given age of a slate roof, it can be easily determined how much more life is left in the slate. Just like any other system, roofing systems are like a chain. In order for them to last and provide years and years of protection from the elements there are several links involved; not just the slate.

Since slate consists of minerals that are stable and extremely resistant to harsh weather conditions, it was and still is preferred as an ideal roofing material. Due to its chemical makeup, it is generally of high strength, low porosity and low absorption. For a roofing materials, this means that slate will last without breaking down or deteriorating due to natural forces and elements. The two most important qualities which isolate slate from most roofing materials are cleavage and grain.

Distribution of slate

Like any other roofing material, slates are sold