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Materials in America > Metal
for Historic Buildings, NPS
The Asbestos Shingle, Slate, and Sheathing in Ambler, PA
In the United States, mechanized production of asbestos-cement
shingles began in the first decades of the 20th century, following
Austrian Ludwig Hatschek's invention of a process in 1900
to manufacture rolled and pressed asbestos-cement sheets. Hatschek's
patent, reissued in United Stated in 1907, led to a rapid proliferation
of the new shingles. One early American manufacturer, Eternit,
took their company name from the title Hatschek had given his
Made from asbestos, an inorganic, fibrous mineral, and Portland
or hydraulic cement, asbestos shingles were lightweight, economical,
and fireproof. Manufacturers promoted their shingles as substitutes
for traditional roofing materials such as slate, wood, and clay.
The Asbestos Shingle, Slate and Sheathing Company proclaimed
in 1910: " ...these Asbestos Singles or Slates are so immeasurably
superior in point of practical merit to that of any natural slating
that nothing remains to be said."
A variety of shingle colors could be created by adding pigments
to the wet mix before pressing or by rolling pigments onto the
surfaces of shingles. Colors imitating slates, including Indian
Red and Newport Gray, were common, but many other colors were
available. Manufacturers assured potential customers that their
shingles were colorfast. Unfortunately, many early asbestos-cement
shingles faded over time, causing Columbia professor H. Vandervoort
Walsh to exclaim in 1922: "For this reason we see on every
hand red asbestos-shingle roofs which have bleached to sickly
and thirsty pinks."
The hydraulic pressing process enabled the shingles to be given
a texture, such as a rough rustic surface or one imitating weathered
wood. The many styles and sizes of asbestos-cement shingles available,
made possible roofs laid in various methods including American,
Dutch Lap, and French (known in several variants as hexagonal,
honeycomb or diamond). The French method was particularly popular
for asbestos roofing, capitalizing on the economy of the material
itself by laying it in an efficient manner requiring minimal overlap.
Installation of asbestos shingles was similar to slate. Shingles
could be punched, filed, or trimmed to size in the field by roofing
contractors. Companies such as behemoth Johns-Manville
and The Asbestos Shingle, Slate, and Sheathing in Ambler
in Pennsylvania promoted asbestos shingles not only for new construction
but also for roofing over existing roofs.
In addition to shingles, corrugated asbestos-cement sheathing,
sometimes called asbestos building lumber, was produced by many
manufacturers as a substitute for corrugated iron roofing. Used
principally for industrial applications, corrugated asbestos could
be laid directly on steel roof purlins. Industrial buildings in
particular benefited from the fireproofing qualities of asbestos-cement.
Both asbestos-cement shingles and siding were produced
into the 1980s, testimony to their popularity and affordability.
The countless buildings with this roofing material also attest
to the durability of the product.
What is asbestos?
Asbestos is the name for a group of naturally occurring
silicate minerals that can be separated into fibers. The
fibers are strong, durable, and resistant to heat and fire.
They are also long, thin and flexible, so that they can
even be woven into cloth.
Because of these qualities, asbestos has been used in thousands
of consumer, industrial, maritime, automotive, scientific
and building products. During the twentieth century, some
30 million tons of asbestos were used in industrial sites,
homes, schools, shipyards and commercial buildings in the
There are several types of asbestos fibers, of which three
have been used for commercial applications:
Chrysotile, or white asbestos, comes mainly from Canada, and
has been very widely used in the US. It is white-gray in color
and found in serpentine rock.
- Amosite, or brown asbestos, comes from southern Africa.
- Crocidolite, or blue asbestos, comes from southern Africa
- Amosite and crocidolite are called amphiboles. This
term refers to the nature of their geologic formation.
Other asbestos fibers that have not been used commercially
are tremolite, actinolite and anthophyllite, although they
are sometimes contaminants in asbestos-containing products.
It should be noted that there are non-fibrous, or non-asbestiform,
variants of tremolite, anthophylite and actinolite, which
do not have the adverse health consequences that result
from exposure to commercial forms of asbestos.
Unless the fibers are completely encapsulated, they tend
to break down into a microscopic dust. A single fiber of
asbestos magnified 1,000 times looks slightly larger than
a strand of human hair. Because of their size and shape,
asbestos fibers can remain suspended in the air for long
periods of time, and therefore they can be inhaled.