Geology of Roof Slate
Slate is formed through metamorphism, a process in which the tiny elements that make up a rock are pushed closer together to form a crystalline structure, also known as recrystallization of the stone. This all occurs while the stone is in a solid state, and the materials that make up the stone are not changed, they are merely repositioned to be closer to one another in a crystal pattern. Metamorphism can be caused by heat or pressure. The materials which undergo this transformation to form slate are generally a shale type sedimentary rock.
Slate forms in layers that easily cleave off in thin lines, generally known as Slaty Cleavage. During formation the original sedimentary rocks are forced down into these plains due to immense pressure. The resulting slate can be comprised of many different beds of sedimentary rock, all of which are now reoriented in the same direction.
While slate is generally a grey or grey-blue color, it can also have multicolored effects in its surface. These come from the various different beds of sedimentary rock from which it is derived, often making this material a contrast of hues.
Slate is most commonly used for roofing because it can be sliced into thin layers. It is also very popular in flooring, counter tops, and architectural applications, due to its durability, its natural beauty, and it’s relatively low cost. Its clefted surface makes it slip resistant, causing it to be popular in kitchens, bathrooms, and restaurants, as well as public hallways and other high traffic areas. It is also used in decorative home furnishings, fish tanks, and even in novel accessories such as board games.
Slate is found naturally in mountains. When it is quarried, it is usually done so in large blocks known as slabs. These blocks are then chopped up into smaller slabs for distribution. The size of the slab they can make is dependent on the relative strength of the type of slate. The inherent layering of this material makes some rather large sizes difficult to maintain and keep from cracking.
In stores, slate is generally available in tiles of various sizes including 16X16, 12X12, 8X8 as well as in slabs of various sizes, usually running to about 80 inches in length. Installations are generally done with mortar backing to even out varying thicknesses. Then grout lines are run along the joints of the stones. It is important to adequately seal and protect all tiles before grouting them.