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Slated for Preservation
Marshall, Philip Cryan and Collins, Allison Brooks. "Slated for Preservation," presented at The Roofing Conference and Exposition for Historic Buildings, Philadelphia, Pennsylvania, March 17-19, 1999.

Slated for Preservation, a compliation of many resources pertinent to roofing slate, has been developed as a web site by Allison Brooks Collins, as a Senior Project toward completion of her B.S. in Historic Preservation from Roger Williams University.


[Submitted to the The Roofing Conference and Exposition for Historic Buildings.]

Roofing slate has a long history, written in stone millions of years old. This summary briefly describes the geological formation of slate; its discovery and production; its application as a roofing material for various architectural styles; and the industry today.

The use of roofing slate is determined by several interrelated factors: its geologic properties (the composition, metamorphism and structural deformation of parent rock); extraction and finishing techniques at the quarry and mill; and a range of cultural, economic and technical determinants. All these factors influence slate's selection, use and performance as a roofing material.

Inherent limitations of pre-industrial age (here, pre-1840s) transportation, coupled with the weight of slate, resulted in its limited, local use inland and the import of European slate (mainly Welsh) to coastal cities. Most roofing slate employed in industrial-age (post-1840s) American construction is from Wales (with imports remaining strong up to World War I), Maine, Vermont, New York, Pennsylvania, Maryland and Virginia. Much of this slate was formed from mudstone and shale deposited along a continuous offshore environment that extended from the present-day Appalachian Mountains, to Canada (which now exports slate from Quebec and Newfoundland) and Wales. After deposition, continental collision and its associated orogenic (mountain-building) processes caused low-grade metamorphism, involving recrystallization and reorientation of minerals to form slate: a fine-grained metamorphic rock characterized by excellent foliation, or slaty cleavage, produced by the parallel alignment of microscopic platy minerals such as mica and chlorite, with quartz imparting added strength and secondary minerals contributing to color and weathering characteristics. Subsequent continental drift separated these land masses to form the Atlantic Ocean.

These rocks from Wales and the Appalachians were reunited, at least in small measure, when Welsh imports were shipped to America beginning in the early 18th century. Later, Welshmen discovered slate and opened quarries in all the major districts: Peach Bottom, in Pennsylvania and Maryland (quarries opened in 1785); Lehigh Northampton, Pennsylvania district (1812), including the Lehigh River (1846) and Bangor (1853); Vermont-New York (1845); and Monson, Maine (1870).

Quarrying and manufacturing operations have remained largely unchanged for the last one hundred years. The industry sustains skilled workers in this traditional trade, providing a valued insight into the industrial archaeology of our heritage, while meeting the market demands of today. Blocks of slate are hoisted from the quarry, transported to the splitting shed, sawn parallel to grain (a structural feature perpendicular to cleavage), cut into oversized blocks that are hand split (with standard slates 3/16" to 1/4" thick) and trimmed (employing a rotary blade much like a hand lawn mower) to form shingles to specific dimensions, shaped with a pattern at the butt end, if wanted, hand punched or drilled to provide nail holes, and stacked on pallets for delivery.

An inherent range of compositional and structural properties affects quarrying, splitting, grading, use, and performance. The best roofing slate, defined as Grade S1 in ASTM C 406, provides a uniform shingle with high tensile strength, durability, low water absorption, and high acid resistance. Color is determined by chlorite and secondary minerals (including hematite, carbon) to produce black, brownish-black, blue-black, gray, green, purple, red and streaked or mottled slate. Some minerals oxidize and change color after quarrying--as with 'Vermont Weathering Sea Green'. Carbon ribbons, occur in some Pennsylvania slate. Large mica imparts a luster in 'Monson Black' from Maine and a sparkle in 'Buckingham Black' from Virginia. Surfaces range from smooth to rough, clear to textured.

Use of roofing slate was well documented by the late 17th century. For example, in Philadelphia historians record the use of 'tile-stone' on Pennsbury House by Governor Penn and refer to the Samuel Carpenter House (ca. 1687-1699) as 'The Slate House,' suggesting the roofing was exceptional. Following several major fires, building ordinances in 17th-century cities required fireproof construction, with specific attention to roofing. Slate met code requirements, while providing a durable material, resistant to urban pollution.

Except for monumental structures, roofing material was not generally considered a distinguished feature of many early buildings. In part, this was largely because roofs themselves were neither highly visible nor intentionally-decorated architectural features until the mid-19th century. While steep-pitched Medieval roofs of 17th century America were prominent, there were few cladding options, other than wood shingles. By the time a selection of roofing material was available, the shallow-pitched Georgian hip and Federal gable roof had all but disappeared behind balustrades. Greek Revival gabled roofs maintained a low pitch, favoring to hide behind a solid parapet or attic, if possible, or at least be subordinate to the greater, rationalist emphasis on planar wall surfaces and geometry.

By the mid-19th century, a well-developed transportation network was in place carrying a mobile society and products, including slate from newly-opened quarries charged with meeting the rising demands of construction. In reaction to this industrialization, Americans chose to flee to the countryside (suburbs) and adopt picturesque landscaping and architectural styles, constructed with material produced by the same industries these residents abhorred.

Slate was well suited to meet these needs. To begin with, architectural styles were justified on grounds of ethics as much as aesthetics. Promoted by A. J. Downing and others, Christian, or pointed, architecture of the Gothic style demanded truthfulness of materials and design, including a steep-pitched roof. Other styles were more calculated, though less moralistic, in their use of roofs and roofing materials. Mansard roofs were prominent, at times even serving as billboards. H. H. Richardson specified Monson slate as roofing for many of his Romanesque masonry buildings. On the homefront, writers attested to the purity of water delivered from slate roofs (though likely through lead pipes). By design, slate roofs predominated on these picturesque Victorian piles.

Roofing materials were finally considered an integral part of exterior decorative treatments. Slate shingles served these purposes well, providing a natural stone, with inherent color variation (even when using stone from only one district, notably Vermont-New York) and patterns (achieved by trimming). Constructional polychromy of the High Victorian Gothic raised the ante by demanding a calculated use of natural materials, preferably stone, achieved for roofing by specifying slate from various districts — with a polychromatic slate roof likely displaying purple and green shingles from Vermont, red from New York and black from Maine. For Queen Anne houses, the appearance of a slated roof was considered when developing a polychromatic paint scheme of complimentary tertiary colors.

Following a drop during the Panic of 1893, the production of roofing slate ascended from 621 thousand squares (slate sufficient to cover 100 square feet, with a three-inch lap) in 1893 to 1.44 million in 1902. At this time Georgian, Late Gothic and Jacobethan Revival structures displayed prominent roofs clad with domestic slate shingles. Steep pitched Tudor houses supported heavy, graduated (ranging from large, two-inch thick slates at the eaves, to small pieces at the ridge), highly textured, multicolored slate roofs, giving a new meaning to the term 'Tudor hardtop'. At times flat — usually Neo Classical — roofs were covered with slate tiles over a built-up system.

Domestic roofing production plummeted from its high in 1902 to about 325 thousand squares in 1920, then went incrementally down to less than 50 thousand squares in the 1980s. This decline, which began during World War I, was hastened by postwar production of new roofing products and a changing economy that favored short-term interests over long-term investments in architecture and amenities for the public good. Witness the construction of International style PSFS Building (1929-1932) in Philadelphia that, excepting its penthouse signature, remains an anonymous modern structure, seemingly decapitated for lack of any visible roof. By the late 20th century, a domino effect produced similar buildings nationwide, including monotonous midtown Manhattan megaliths where no roofs were needed as the sky was the limit. In this climate the down-to-earth, generations-old slate industry barely kept its ground.

In the 1990s, there has been a resurgence in the use of slate roofing spurred on, no doubt, by good economic times but also caused by an increased demand for slate in preservation and new construction. Quarries that remained open are working to capacity; others are being reopened or developed. Slates are being imported from historic and new sources. Virtually any historic slate can be obtained or matched. Property owners realize the long-term benefit of a maintained or new slate roof, architects explore the full design potential of slate, and companies that provide full scope roofing services can do justice to slate by taking a systems approach to its use and installation. The industry and these roofs — old and new — are indeed slated for preservation.