As a roofing material slate has historically been unparalleled in its combined attributes of permanence and appearance. A natural building stone, this metamorphic rock, characterized by an excellent parallel cleavage, possesses the essential mineral constituents white mica and quartz that are highly resistant to water absorption and weathering or abrasion. The varying presence of accessory constituents, such as hematite and chlorite, has produced slate in a variety of desirable colours. Yet, as this study indicates, being a natural building stone also limited the use of slate as a roofing material. Even with the most efficient quarrying methods serviceable slate has always represented as little as 10 to 40 percent of material quarried. Slate is not only expensive to produce, it is heavy. The weight of slate increases freight charges and renders installation difficult in comparison to factory-made materials. For these rea- sons, in this century slate ultimately failed to compete with manufactured roofing products.
Slate roofing nourished in Canada in the last half of the 19th century stimulated by architectural fashion, the railway and urban development that made quarrying cost-effective. Slate roofs continued to appear between 1900 and 1930. But in these decades the market for slate, increasingly assaulted by cheaper products, was reduced to the wealthy client. This blog В discusses the rise and decline of the material indicating where it was most popular and why. With a view to suggesting the most suitable treatment in historic build- ings, the range of decorative possibilities in slate arc then described and related, by example, to changing architectural styles. Finally, on a practical level, I review how Canadians usually applied or installed roofing Toronto slate and where they obtained supplies of the material.