The adaptation of slate for roofing purposes is inextricably
linked to its genesis. The manufacturing processes of nature have
endowed slate with certain commercially amenable properties which
have had a profound influence on the methods by which slate is
quarried and fabricated, as well as its suitability for use
as a roofing tile.
Slate roofing tiles are still manufactured by hand using traditional
methods in a five step process:
- trimming, and
- hole punching.
In the manufacturing process, large, irregular blocks taken
from the quarry are first cut with a saw across the
grain in sections slightly longer than the length of the finished
roofing slate. The blocks are next sculped (???), or split
along the grain of the slate, to widths slightly larger than the
widths of finished slates. Sculping is generally accomplished
with a mallet and a broad-faced chisel, although some types of
slate must be cut along their grain. In the splitting area, the
slightly oversized blocks are split along their cleavage planes
to the desired shingle thickness. The splitter's tools consist
of a wooden mallet and two splitting chisels used for prying
the block into halves and repeating this process until the
desired thinness is reached. The last two steps involve trimming
the tile to the desired size and then punching two nail holes
toward the top of the slate using a formula based on the size
and exposure of the slate.
Minerals, the building blocks of rocks, through their characteristic
crystalline structures define the physical properties of the rocks
which they compose. Slate consists of minerals that are stable
and resistant to weathering and is, therefore, generally of high
strength, low porosity, and low absorption. The low porosity
and low absorption of slate mitigate the deleterious action of
frost on the stone and make it well adapted for roofing purposes.
The two most important structural properties of slate are cleavage
The metamorphic processes of geologic change necessary to produce
slate are dependent upon movements in the earth's crust and the
heat and pressure generated thereby. For this reason, slate is
found only in certain mountainous regions. The most economically
important slate deposits in this country lie in the Mid Atlantic
and Northeastern states transversed by or bordering on the
Appalachian Mountain chain. Variations in local chemistry and
conditions under which the slate was formed have produced a wide
range of colors and qualities and ultimately determine the character
of the slate found in these areas.
Slate is available in a variety of colors. The most common are
grey, blue-grey, black, various shades of green, deep purple,
brick red, and mottled varieties. The presence of carbonaceous
matter, derived from the decay of marine organisms on ancient
sea floors, gives rise to the black colored slates. Compounds
of iron generate the red, purple, and green colored slates.
Generally, the slates of Maine, Virginia, and the Peach Bottom
district of York County, Pennsylvania are deep blue-black in color.
Those of Virginia have a distinctive lustrous appearance as well
due to their high mica content. The slates of Lehigh and Northampton
Counties, Pennsylvania, are grayish-black in color. Green, red,
purple, and mottled slates derive from the New York-Vermont district.
The slate producing region of New York, which centers around Granville
and Middle Granville, is particularly important because it contains
one of the few commercial deposits of red slate in the world.
Slates are also classified as fading or unfading according
to their color stability. Fading slates change to new shades or
may streak within a short time after exposure to the atmosphere
due to the presence of finegrained disseminated pyrite. For example,
the "weathering green" or "seagreen" slates
of New York and Vermont are grayish green when freshly quarried.
Upon exposure, from 20% to 60% of the slates typically weather
to soft tones of orange-brown, buff, and gray while the others
retain their original shade. Slates designated as unfading maintain
their original colors for many years.
Color permanence generally provides no indication of the durability
of slate. Rather, time has shown that the Vermont and New York
slates will last about 125 years; Buckingham Virginia slates 175
years or more; and Pennsylvania SoftVein slates in excess of 60
years; Pennsylvania HardVein slates and Peach Bottom slates, neither
of which is still quarried, had life spans of roughly 100 and
at least 200 years respectively. The life spans provided should
be used only as a general guide in determining whether or not
an existing slate roof is nearing the end of its serviceable life.
Ribbons are visible as bands on the cleavage face of slate
and represent geologic periods during which greater amounts of
carbonaceous matter, calcite, or coarse quartz particles were
present in the sediment from which the slate was formed. Ribbons
typically weather more and were most common in Pennsylvania slate
quarries. As they were not as durable as clear slates, ribbon
slate is no longer manufactured for roofing purposes. Mottled
grey slates from Vermont are the closest match for Pennsylvania
ribbon slate available today.
In recent years, slates from China, Africa, Spain and
other countries have begun to be imported into the United States,
primarily for distribution on the West Coast. The use of imported
slates should probably be limited to new construction since their
colors and textures often do not match those of U.S. slate.