Foundations of Construction: Tarred roofing felt cushioned graceful slate tiles   

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The Church of the Good Thief, built 1892-1894 in Kingston, Ontario. The elegant slate roof incorporated ecclesiastical design, still vivid today. Susanna McLeod, May l0, 2022.

By Susanna McLeod

It happens frequently in industry. Older materials and designs are pushed aside for new technologies. Modern innovations have advantages of price and speedier manufacturing, while older materials have substance and longevity. The long history of slate roofing proves its strength.

Wood shingles, clay tiles, tin, and galvanized sheet metal provided roof coverings since the 1800s and earlier in Canada. For a time, slate was regarded as the royalty of roofing materials. “A slate roof is extremely resistant to the elements, including debris impacts and chemical degradation,” said Heather & Little Limited. “It is completely immune to many factors that would destroy other types of tiles, like fire.” Such reliability came at a steep price, beyond the average homeowner.

A foliated metamorphic rock composed of chlorite, muscovite, quartz, hematite, and other minerals, plus a bit of clay, slate formed under high temperatures and compression. The fine-grained rock is hard and brittle; the smooth layers can be split into smooth sheets for classroom blackboards, writing slates, and roofing materials.

Initially importing expensive slate from British and European quarries, builders changed suppliers when the railway made purchasing from North American sources more cost-effective. From the 1850s through the 1860s, slate was imported without tariffs from the Vermont, Pennsylvania, and other eastern states under the Reciprocity Treaty of 1855.

As the Ontario population grew rapidly, the construction boom required an abundance of slate to erect government buildings, universities, and churches. To supply the growing demand in the mid-1800s, several quarries opened in Quebec. The quality of the stone belts ranged from poor to excellent condition.

“Roofing slate was produced in Quebec practically continuously from 1854… until 1921,” stated H.W. McGerrigle’s report for Quebec Department of Mines in 1938. “Maximum production was in 1889: 695 tons, valued at $119,160.” Colours varied by the area’s mineral composition, from grey and dark purplish grey to light green, red, and shades of blue. Opening in 1857, the quarry at Kingsley, east of Sherbrooke, produced slate belonging “to the unfading class. The deposit is large and apparently the working conditions are favourable,” The railway was only about 6.4 kms away.

“A variety of sizes was produced to sell at an average price of $3.80 a square (the amount needed to cover 100 square feet of roof surface with a three-inch lap),” wrote Mary Cullen in Slate Roofing in Canada (Environment Canada 1990).. “Specimens of roofing slate from Kingsey, Frampton, Melbourne, Shipton and Tring were shown at world industrial exhibitions in London (1851) and Paris (1855).” Two of the quarries in the Eastern Townships “supplied most of the needs of the Canadian market until 1900.”

Established at Saint-Marc-du-Lac-Long in 1995, Glendyne is regarded as North America’s largest slate quarry. Near several older, closed quarries, the site is producing dark grey roofing tiles from about 40 m-thick bed of slate.

Using specific tools to strike the slate parallel to the foliation, smooth sheets of stone were separated from the quarried rock. The slates were cut to the desired size and shape, and holes were made by the installer’s hammer at the site or by a special hole-punching machine. Heavier slates had holes drilled at the quarry.

The building’s steep roof was prepared for the tiles. Usually, the 7/8” to 1” thick board sheathing was installed along with tarred roofing felt. Providing a modicum of insulation, “the purpose of the felt was to protect the roof while slates were being laid and to form a cushion for the slate,” added Cullen. The tiles were secured to the roof with two nails at the head of the slate. Attached with galvanized or copper nails of 1-1/2” or 2” length, noted Cullen, the government standard for roofing slate was 1-3/4” galvanized nails.

Flashings were added at chimneys and walls, and at roof intersections. Copper, sheet lead or galvanized iron formed the flashing to encourage drainage and prevent water intrusion. “The generous valley flashings for Canadian slate roofs ranged from 12 inches to 24 inches wide with about 15 inches the most popular.”

“The 1867 Parliament Buildings set the tone, embodying design elements of both [Gothic Revival and Second Empire] styles and emphasizing the picturesque effect of the roof by the use of polychromatic (variously coloured), slate-covered mansard roofs and towers,” said Cullen. Choosing Second Empire style after Confederation, the Department of Public Works used the design “in an ambitious building program for post offices and custom houses throughout the country.” The style featured mansard roofs “with slate arranged in decorative patterns on the slopes.”

By the 1930s, slate lost its noble status to asphalt shingles and manufactured roofing materials, easier to install and more affordable. Undergoing rehabilitation projects, the aging slate of Parliament buildings is gradually being replaced with copper.

But wait… for weather resistance, longevity, and elegance, slate is again among roofing options.

© 2022 Susanna McLeod. McLeod is a Kingston-based freelance writer who specializes in Canadian History.

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