Roofing: Mastics, Coatings, Adhesives

06/10/2009 | By Richard L. Fricklas

Sustainability of roofing systems requires that the systems be repairable. Any roof system can be punctured if the impact is great enough. Repair materials need to be compatible with the roof system in place; that, in turn, requires the building manager to know what’s overhead.

Commercial (low-slope) roof systems can be classified as:

  • Bituminous built-up, using asphalt or coal tar pitch.
  • Bituminous polymer-modified systems.
  • Sprayed-in-place polyurethane foam with coating.
  • Single-ply non-bituminous.
    • Weldable thermoplastics.
    • Non-weldable elastomeric.
  • Metal roof systems.

Proper repair requires that you know what roof system is in place so that you use the appropriate materials.

The traditional built-up roof consists of multiple layers of bitumen-saturated roofing felts glued to one another with bitumen, with a “flood-coat” of more bitumen into which roofing aggregate has been embedded. Although less common, some built-up roof systems are surfaced with mineral cap sheets or just with paint-type coatings.

Asphalt is derived from petroleum as a heavy residual material (bottom of the barrel) (see Fig. 1). Note that “mineral spirits” is part of the same distillation process. Asphalt is completely compatible and soluble in this “paint thinner.”

When possible, repairs should be made with compatible materials. Asphalt mastic, found on every roofer’s truck, in lumberyards, and at roofing supply yards, is readily available. The ingredients will include petroleum asphalt, mineral spirits, fibers, and fillers. The flash point (the point at which the compound would ignite in the presence of a flame) is generally above 100 degrees F. (38 degrees C.), so the product isn’t designated “red label” or flammable.

For residential use, the mastic is packaged in 1-gallon pails. For commercial use, generally, more than 1 gallon is needed to effect repairs. In many cases, commercial roof mastic is provided in 2.5-gallon pails. Five-gallon pails are available everywhere; however, that’s a rather inconvenient size for a maintenance person to lug around a roof.

Thick or Thin?
Depending upon the proportion of ingredients, these “cut-back” products can be as thin as water (e.g. primer) or heavy enough that, even with a trowel, the products are stiff and require a strong arm to apply. (See Table 1 for a list of products.)

After the patch is made, some loose aggregate from the surrounding area can be sprinkled into the topcoat of fresh mastic. Leaving the aggregate off has the advantage of keeping an eye on the repair to ascertain that it has succeeded, and to help you avoid stepping on a fresh patch.

Figure 2 lists common bituminous cutbacks and coatings.



  • Asphalt
    • ASTM D312, Types I to IV
  • Coal Tar Pitch
    • ASTM D450 Type I


  • Mineral Spirits (Petroleum Distillate), ASTM D235
    • Boiling Range 300-400°F
    • Flash Point 100°F minimum

Asphalt Primer:

  • Solution of mainly solvent with some asphalt ASTM D-41
  • Also Coal-tar based primer ASTM D-43

Pages: 1  2  View All  

Related Coverage