Composition or built-up roofing is a multi-ply system of fabric
or paper, a viscous waterproofing substance, and a mineral aggregate.
Historically, various materials have been used for each of these
components. Paper, pasteboard, canvas, burlap, and felt have all
served as the base membrane. These materials were often dipped
in the waterproofing coating before being applied to the roof.
The availability of felt in rolls facilitated the mechanization
of this saturation process, and felt became the standard base.
Pine tar, natural asphalt, coal tar and asphalt were the
major materials to be effective as the waterproofing that saturated
the base sheets and adhered the layers. Sand, gravel or slag provided
the top surface. Though there is no evidence of their success,
numerous other materials were tried as components, many as part
of patented formulas: woven strips of paper or twine, sawdust,
china clay, plaster of Paris, cattle hair, gum shellac, boiled
linseed oil, boiled fish oil, and blood.
Pine tar and gravel were combined for roofing both in Europe
and the United States in the 1870s. In 1800 pine tar was applied
to canvas at the Octagon House in Washington, D. C., where
it provided the roof covering for seventeen years. Evidence of
similar systems used elsewhere in the early 19th century is very
In the 1840s a method learned from a roofer in Newark became
the basis for a roofing business begun in Cincinnati by Samuel
M. and Cyrus M. Warren. They met with success, as did others
in the northeast, applying roofs of heavy paper, covered with
pine tar and sprinkled with sand. The significance of the Warrens
was the innovation they brought to the system. From experiments
begun in 1847, they successfully replaced increasingly costly
pine pitch with coal tar, a by-product of manufacturing
illuminating gas from coal. Their continued development of the
product and expansion to other cities assured the Warrens' status
as leaders in the industry. They were the first in the 1850s to
distill coal tar, producing a superior refined tar. Later they
found that natural asphalt from Pitch Lake in Trinidad
could produce an easy-to-mix roofing pitch when combined with
petroleum tar, a by-product of oil refining.
The potential of composite roofing was apparent to many, and
the number of related patent applications exploded in the
1860s and 1870s. The developments in composition roofing were
well timed to meet the mid-19th century increase in the use of
flat and low-sloped roofs. The only alternative for such roofs
at that time was metal, which depended on the performance of the
many fabricated seams that joined the small sheets. The relative
merits of metal and composition roofing were constantly debated.
Henry Hudson Holly writes in 1878: "Metals are the
best covering for roofs that are inclined to be flat...composition
such as tar or other materials we would not advise
on good work, as its only merit is its cheapness." Ignition
and spread of fire were important issues in the debate. A 1911
test by Underwriters Laboratories rated the fire retardant qualities
of "good slag and gravel roofs" in a class with inferior
roofing materials. Nevertheless, in 1912 the National Board of
Fire Underwriters considered approved composition roofs together
with metal, slate and tile the best for fireproof construction.
The issue seems to have been settled in1916 when composition roofs
met the requirements of both fireproof and fire retarding classes
of the Board's rating system. Built-up roofing came to dominate
the commercial roofing market and remains in use today.