Techniques > Briefs > Roofing for Historic Buildings > Historic Roofing Materials in America > Metal > Other

Source: Composition, Roofing for Historic Buildings, NPS


Table of ingredients
Italianate roof, flat
Commercial use.
Water, IR thermography, cores.


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 limited.

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.