Keeping on Top of It

BUILDINGS - Smarter Facilities Management


Keeping on Top of It

Coal tar built-up roof maintenance and repair


Coal Tar vs. Asphalt

Before performing any maintenance procedures on a built-up roof, you must first determine the type of roof system, coal tar vs. asphalt. Three main methods to differentiate between the two systems are:

  • Odor. When heated, asphalt has a strong odor characteristic of fuel oil. Hot coal tar has an odor similar to creosote wood preservative.
  • Solubility Test. Asphalt is readily soluble in petroleum solvents such as gasoline, kerosene, and lighter fluid. Coal tar is not readily soluble in these solvents. With a cloth that has been soaked in a petroleum-based solvent, rub vigorously. Asphalt will transfer a heavy residue to the cloth and make it tacky to the touch. Coal tar will become discolored but will not transfer a heavy film and will not be tacky to the touch.
  • Laboratory I.D. In a beaker at a ratio of 2 grams of sample to 20 grams of mineral spirits, asphalt will quickly turn the color to deep brown. Coal tar will gradually turn to yellow.

Coal tar built-up roofing has proven superior performance for more than 140 years. However, any material exposed to environmental and human abuse will need attention from time to time. Therefore, be aware of the most commonly encountered maintenance requirements of a coal tar membrane system. Be sure to first determine the type of roof system - coal tar or asphalt - you have.

When using asphalts and coal tars in conjunction with each other, you should note that the two systems' chemical compositions are sufficiently different and will not mix. Asphalt and coal tar can only be used together successfully if recommended procedures are employed. Remember the following four rules:

  1. Do not try to mix asphalt and coal tar in a kettle; they will not mix.
  2. Do not use the same tools to apply asphalt and coal tar unless they are clean.
  3. Do not "gravel-in" a coal tar and felt roof with any asphalt material.
  4. Repair coal tar roofs with coal tar-based repair compounds and asphalt roof membranes with asphalt-based materials.

Choice of Repair Methods
An important advantage of coal tar is its "cold flow" characteristics, which allows it to self-heal small fractures before more serious damage can occur. Coal tar will change its shape due to the actions of gravity alone. When the rooftop temperature reaches about 70 degrees, coal tar roofing pitch begins to flow. This flow will fill cracks or fissures in the waterproofing barrier and restore the system's integrity.

There are times, however, when there may be physical or deteriorated membrane damage to the roof that needs repair. Three approaches to making repairs include:

  1. Use traditional hot-applied coal tar pitch with either organic or inorganic reinforcement membranes (typically requires a professional roofing contractor).
  2. Use cold-applied coal tar repair compounds with fiberglass or polyester reinforcements.
  3. Use the newest innovation in coal tar roof membrane repair materials - a polymer-modified coal tar membrane. These new products can be hot-mop-applied; cold-applied with a polymer modified cold-applied adhesive/mastic; or hot-air welded, which makes repairs faster, easier, and more economical.

Typical deterioration or damage to a coal tar roof might include:

  • Punctures and Ruptures, which are the result of any number of occurrences including falling objects (wind-toppled equipment), dropped objects (tool boxes), fasteners backing out of the deck, ruptured blisters from foot traffic, etc.
  • Bare Spots, which are areas that lack surface bitumen and aggregate, leaving the felts exposed to the elements.
  • Deteriorated Metal Edge Stripping, which employs metal flanges normally installed over the roofing membrane at the roof edge that are stripped with felts to keep them watertight. Expansion and contraction of the metal will often break the stripping at the joints or laps in the metal. Any rupture or deterioration in the felt stripping could allow water entry beneath the flange and eventually find its way underneath the roof system and into the building.

For more complete specifics on repair and maintenance aspects of coal tar roofs, visit, access August 2001 Menu and click on Smarter Buildings.

Bob Tomlinson is application technology leader at Honeywell Commercial Roofing, Cary, NC, (800) 221-6490.



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