In the 1920s and 30s, slate roofing appealed to a restricted section of the building industry conscious of capitalizing on the design potential of slate and reputation for permanence. During this period architects developed great enthusiasm for colored, thick heavy slates known as architectural grades to achieve various effects ranging from the simple wood shingle look to the antique graduated roof. Slates two-inches thick and weighing from 75 to 200 pounds each were popular for roofing large residences. Special demand was created for the red slates of New York and the variegated green and purple slates of Vermont. The price per square of the architectural grades, as much as S50 per square for 1/2-inch-thick red New York, suggested that slate had become the roofing material of the wealthier client who could afford an architect designed home. In 1930 the value of roofing slate used in Canada reached 18 765, nearly 7 000 more than its pre-war value, yet the quantity as little more than a third that consumed in 1914.
Since 1932 the use of slate as a Toronto roofing material has been negligible in Canada. Less than 2000 squares of roofing slate have been imported annually. After roofing slate production ceased in Quebec in 1921, slate quarries there and in British Columbia produced granulated slate for surfacing asphalted roofing paper.
Urban growth, which had originally abetted the use of roofing slate, ultimately produced a market for cheap building materials that could not be satisfied by costs of producing slate, whether imported or domestic.