Roofing with a History
Slate and clay tile are historic, durable and sustainable roofing choices.
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Clay tiles in half-round forms, such as these from Ludowici Roof Tile, are most closely associated with historic styles in the Spanish and Mission vein, especially in unglazed terra-cotta. Photo: courtesy of Ludowici Roof Tile
As architectural as they are ancient, nothing in roofing comes close to slate and clay tile. Unmatched for historic authenticity and durability, these two materials now enjoy newfound appreciation as natural, recyclable materials that puts them on today's short list of sustainable building products. Always high end, with a complexity and heft that require skilled installation, slate and clay tile are nonetheless among the most cost effective of roofs due to their legendary longevity – 75 to 100 years or more. With so many similarities, it's tempting for homeowners or specifiers to lump slate and clay tile together as essentially the same roofing, but knowing more about some of their differences can help in choosing the appropriate material for a particular project.
Roofs for the Ages
Though slate and clay tile covered roofs centuries before the settlement of North America, neither of these materials really became widespread in the U.S. until the latter half of the 19th century. Even after the first domestic slate quarry opened in 1785, it took the expansion of railroads after 1850 to make it practical to ship weighty slate beyond markets close to quarries. Equally influential were the immensely popular plan books of Andrew Jackson Downing and his followers that introduced home builders to European styles like the Gothic Revival and Second Empire – both ideally roofed in slate. Clay tile, too, got a tremendous boost from the Industrial Revolution, especially the Centennial Exhibition of 1876, which featured tile-roofed buildings, and the perfection of tile-making machinery in the 1880s. Clay tile in terra-cotta orange became the crowning touch for Richardson Romanesque buildings at the end of the 19th century, but it really came on strong for the new breeds of Arts & Crafts and Mediterranean Revival-style houses of the early 20th century, especially with the advent of multicolored glazes.
Being a natural material, slate varies in color permanence depending upon its source. For example, this recycled Pennsylvania slate from the Durable Slate Company still retains it deep blue-black color. Photo: courtesy of the Durable Slate Company
While you won't find roofing slate or clay tile at a lumberyard, today these materials can be had almost anywhere across the continent through quarry representatives and manufacturer distributors – far from the original sources of raw materials or regional architecture. In fact, some of the same quarries and dealers who sell new slate also stock recycled slate, and not a few businesses deal in both slate and tile. Not surprisingly, historic restoration is a significant segment of the business, and re-roofs with both clay tile and slate remain constant across the country, too, especially, according to Tom Collard of Granville, NY-based Evergreen Slate, "in older sections of towns with slate – basically any place there are high-end houses."
If you're designing or specifying with slate or clay tile, it helps to know what you're dealing with. Slate, of course, is stone – any one of several sedimentary rocks laid down as silt in ancient oceans and mined in mountainous regions called slate belts. Sediment is what gives slate its density and therefore durability. Some historic slates are still going strong at over 175 years of service. Different sediments, as well as diverse compounds that might have found their way into the mix, are also what give slate its color – hues commonly ranging from black, grey and blue-grey to shades of green, brick red and deep purple. As a natural material, slate can exhibit many variations within these categories, as well as aging characteristics that the industry typically classifies as unfading, semi-weathering and weathering. Mark Hewitt, FAIA, of Bernardsville, NJ-based Mark Alan Hewitt Architects recommends checking on lead times for colors and specialty slates. "Some quarries switch between working, say, red and green slate, and there may not be a big inventory of these less-in-demand types," he says.
Deep purple is one of the most popular slate roof colors, along with shades of black and blue-gray. Purple slates, such as these from the Durable Slate Company, are historically from quarries in the New York/Vermont region. Photo: courtesy of the Durable Slate Company
With slate, the material itself is only part of the design of the roof. Historically, slate roofs were divided into three different types. In standard slate roofs, the slates have smooth faces and consistent dimensions that produce a uniform roof. Whether the roof is plain or patterned – say with tails cut in geometric shapes or a mix of colors – it's still a standard roof, by far the most common type. The next type is textural. Here, the slates vary in thickness, have uneven tails and usually combine colors in a seemingly random pattern. Though in fact carefully planned, textural roofs evoke the haphazard accretions of medieval buildings and became very popular for houses in the Tudor and English Revival styles of the early 20th century. The third type is the graduated slate roof. Here the roof is laid with the largest and thickest slates and the most exposure at the eaves, and then the slates diminish in exposure and dimensions as they go up the roof. This produces a foreshortening effect that enhances the drama of a large roof. Some quarries expand this list. "The only other type we would add would be Dutch lap," says Amber Gallihar of the Columbus, OH-based Durable Slate Company, "where the slates lap side-to-side as well as top and bottom."
As dimensions increase, so does weight, and for a dense material like slate, thickness is an important consideration. Depending upon thickness, slate roofs weigh in at 800 to 1,000 lbs. per square (versus 550 lbs. or less for asphalt shingles). Though slates once could be had as thin as 3/16 to ¼ in., they are getting harder to split, and the most common thicknesses are now ¼ to 3/8 in., 3/8 to ½ in. and over ½ in. for graduated and textural roofs. Recently, at least one metal frame system has come on the market to enhance the installation and coverage of slate on a roof and therefore minimize weight. Nonetheless, for projects with ample roof framing and budget, big slates are not an overriding concern. "We've seen a trend towards more graduated slates and thicker slates," notes Collard. "People view it as an added value that contributes to a unique appearance."
Depending upon the source, historic slate roofs have service lives of 60 to 175 years. At Evergreen Slate, all slate is tested for an S1 rating of 75 years. Photo: courtesy of Evergreen Slate
In contrast to flat, stiff slate, clay roof tile is a boundlessly plastic medium that is molded into scores of creative shapes and patterns – a wealth of options that basically divide into two categories: flat tiles and pantiles. Pantiles are generally any tiles with a half-round or barrel-shaped form. Today, the most traditional types, such as Spanish tile or two-piece Mission-style tile, are called profile tiles in much of the industry. While such tiles are part of the architectural image of the Southwest and Florida, Tab Colbert of New Lexington, OH-based Ludowici Roof Tile says that "generally, profile tiles represent less than 50% of today's market."
More common and far more diverse are the flat and interlocking tiles. Patterns range from straightforward, massive slab tiles to traditional English and French styles with sculpted surfaces and interlocking sides and tops. Color, too, is one of the signal beauties of clay tile. Beyond the ubiquitous earthen red-brown of the ceramic base, glazes in red, brown, black, purple and green came on the scene by the late 1890s, bringing with them an expanded range of design possibilities that remain popular today.
Like slate, the density of clay is what makes the material so durable as a roof but formidable as a load. Generally, interlocking types run in the range of 800 lbs. per square, barrel and Mission styles around 900 lbs. per square and flat slabs and many other types at 1000 lbs. and more per square. While tiles in the 800 lb. range are not necessarily too heavy for houses built to recent building codes with beefier framing, some clay tile manufacturers have introduced new lightweight products that at around 600 lbs. per square are the equivalent in weight of an architectural asphalt roof.
The Most Recyclable of Roofs
When it comes to roofing period houses, the challenge is often to match material that's already on the building – whether for a major repair or an addition – and here slate and clay tile present twice the normal sourcing options: recycled material as well as new material.
Traditional clay roofs are composed of two mating barrel-shaped tiles, but one-piece
versions have been common since the late-19th century, and both are available from manufacturers such as Ludowici. Photo: courtesy of Ludowici Roof Tile
Even with the construction boom of the last decade, and the consequent interest in slate imported from Third World countries, domestic quarries remain the best sources for matching historic slates. "It's not any harder to buy or quarry quality slate here in the U.S. than it was 15 years ago, and the quarries we work with really stand behind their products," says Steve Cluxton of Durable Slate. As with clay, the endurance of slate also makes recycled material a practical alternative. A recycled slate roof can last from 50 to 100 years, depending upon the slate type, and may cost 30% less than new material.
Matching clay tile of the past with new tile is generally done on a case-by-case basis, often with the help of design services at major manufacturers that can research former products. "It gets tricky though," says Hewitt, "because decades ago there were more manufacturers, each offering lots of styles." Technology can be a hurdle, too. "Glazes from before the 1920s, for example, were often lead based and cannot be reproduced in the same way today," adds Colbert. Even so, period design characteristics, such as graduating color hues and density up the roof, are still available and often popular again today. Coming as close as possible to historic products is where recycled clay tile can be a boon, offering matches for shape and color that might be otherwise unavailable. "A recycled-tile roof will last 50 to 75 years," says Gallihar, "subject, of course, to the type and the elements it endures."
Clearly, with such extraordinary durability and high value as earth-friendly materials, slate and clay are not only good for the life of a building. In all senses of the phrase, they're also good for the planet.
Gordon Bock, longtime editor of Old-House Journal magazine, is a writer, architectural historian, lecturer and technical consultant who shares information about historic buildings on his blog at www.bocktalk.com.
Click here for a list of suppliers of slate roofing
Click here for a list of suppliers of tile roofing