Coal Tar Pitch Roofing Explained

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Coal Tar Pitch Roofing Explained

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What is Coal Tar Pitch Roofing?

Coal tar pitch roofing is a type of built-up roofing in which coal tar pitch forms the waterproofing component of the roof membrane.

Coal tar pitch roofing is the original “tar and gravel” roof.

It was so common and used on so many flat roofs for such a long time that even today, when most built-up roofs are asphalt-based and use no “tar”, they are still called “tar and gravel”.

Coal tar pitch is very rarely used today, due to the health precautions which must be taken when working with it, as well as the large number of cheaper roofing alternatives now on the market.

How Coal Tar Pitch is Made

Coal tar pitch is produced by distilling or heat-treating coal tar, which is a by-product of the production of coke or coal gas.

Although it is technically a fluid, coal tar pitch is extremely viscous, and can be considered a solid for practical purposes at normal ambient temperatures.

How Coal Tar Pitch Roofs are “Built Up”

When used in roofing, coal tar pitch is melted on-site in an industrial kettle and applied to the roof (normally with mops) as a hot liquid.

Pitch-impregnated roofing felts are installed in overlapping courses during the application of the hot coal tar pitch to reinforce and strengthen the roof membrane.

A layer of coal tar pitch that has been applied to the roof and the layer of reinforcing felts that are set into it is called a “ply”. One ply in a finished roof is usually an eighth to a quarter of an inch thick.

Typical coal tar pitch built-up roofs have four or five plies. At the end of the installation, gravel is typically set into an additional surface layer of hot liquid coal tar pitch (called a flood coat).

The gravel surface provides fire-resistance and protects the coal tar pitch membrane from UV rays, foot traffic, and hail damage.

“Self-Healing” Coal Tar Pitch Membranes

Coal tar pitch roofs are noted for their ability to “self-heal”: a coal tar pitch roof membrane will slowly fill in small punctures or stress cracks all by itself due to its fluidity.

Related Pages

1. General: See this informative, in-depth general article about various roof systems at the Whole Building Design Guide site, which is maintained by the National Institute of Building Sciences.

2. General: A good short introductory article on coal tar pitch roofing, “Coal-Tar Pitch Update” by roof expert Dick Fricklas, is archived on the website. The article is from 2009.

3. General: The Pitch Drop Experiment. Check out the decades-long experiment that’s still going on at Australia’s University of Queensland, intended to demonstrate the fluidity of pitch. You can watch the pitch in real time as it drips once every 13 years or so. This is at room temperature; it gets much hotter on a roof, decreasing viscosity, and producing the “self-healing” quality for which pitch roofs are esteemed.   

4. Historic Roofing: For a look at a coal tar pitch roofing guide from 1896, see Barrett’s Handbook on Roofing and Waterproofing for Architects, Engineers, and Builders – Barrett Manufacturing Co. – 1896.

5. Safety: Learn about health risks related to coal tar pitch exposure on the website of the National Center for Biotechnology Information. Safety precautions are important when working with this material.

6. Safety: The Coal Tar Pitch Volatiles page at the OSHA website.  

7. Safety: See this material safety data sheet for health and safety information about coal tar pitch at the Durapax website. This is only an example and other products and brands may be different.

8. Technical: See this coal tar membrane system specification, as well as Coal Tar Application and Design Considerations for insight into how a coal tar pitch roof system is put together. Both documents are made available by Durapax.

9. Technical: See this product data sheet for the technical characteristics of coal tar pitch at the Durapax website. This is only an example and other products and brands may be different.

10. Manufacturers: See this list of coal tar pitch roofing manufacturers.