Slate is a fine-grained, homogeneous, metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering. It is mainly composed of quartz and muscovite, often along with biotite, chlorite, hematite, and pyrite along with, less frequently, apatite, graphite, kaolin, magnetite, tourmaline, or zircon as well as feldspar.

Every slate roof looks terrific when it's wet - unless you're seeing leaks inside. There are, fortunately, some better ways to look at and think about this material. Slate roof failures result from breakdown of the material itself, from poor installation, or from poor maintenance. Evaluation of the condition of slate roofing, and estimating its remaining life considers at least these factors: type of slate - durability of the material itself, age and condition of the slates - where are we in the expected life of the material and what damage is visible, installation quality, fasteners used, evidence of repair history, quality of maintenance work, leaks - a telltale which could point to any or all of the above. Variations occur in color, thickness, surface texture, graduated sizes and varying widths. Standard slate roofs use slates 3/16″ to 1/4″ thick of one uniform length and width with square tails laid to a line in a conventional shingling pattern. Textural style roofs use rough-textured slates with uneven butts and varied thickness. Different colors and varying sizes are often used. The resulting roof has a very rustic look. Graduated slate roofs use slates which vary in size and often in thickness, with larger slates at the eaves, smaller and with less shingle exposure at the ridge. Original work graduated roofs show smooth transition among the sizes from larger to smaller as each course of slates approaches the ridge line. Where lots of repair work was done this effect may have been lost. This roof style makes the roof and building look larger and taller than it actually is.