cutback

Cutback is bitumen or asphalt dissolved in a volatile solvent, typically naphtha or kerosene, to lower its viscosity at room temperature. The solvent makes the binder flowable enough to apply and work without heat, which is its defining characteristic and primary advantage over straight bitumen that requires heating to 300, 350°F before use.

Cutbacks are classified by viscosity grade and evaporation rate. The most common grades are rapid curing (RC), medium curing (MC), and slow curing (SC), where the designation refers to how fast the solvent evaporates after application. RC cutback thins quickly and is used for tack coats and thin spray applications; MC is standard for many roofing adhesives and cements; SC cutback evaporates slowly and is chosen when working conditions are cool or when longer open time is needed before assembly.

The typical composition is 65, 85% bitumen by weight with the balance being solvent. As the solvent volatilizes, the bitumen hardens in place, developing waterproofing and adhesive properties. This curing process may take hours to days depending on temperature, humidity, air movement, and the cutback grade selected.

Use and limitations

Cutback is applied by trowel, squeegee, spray, or brush for roof membranes, flashing cement, pipe joint coating, and as a bonding agent in roofing and waterproofing work. The absence of heat makes it safer to handle than hot bitumen and eliminates equipment investment in kettles and hoses, but solvent fumes present inhalation hazards and cutback is less durable than hot-applied bitumen in high-UV or high-traffic areas.

The name derives from the process of cutting back, or diluting, raw bitumen with solvent. Environmental regulations in many jurisdictions have restricted the use of cutback roofing products because of volatile organic compound emissions during curing. Cold-applied bitumen emulsions, which use water instead of organic solvent, have displaced cutback in many applications, though cutback remains standard in specialty cements and where working conditions or substrate compatibility favor solvent-based products.