[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.
Marshall,
Philip Cryan and Collins, Allison Brooks. "