Modified bitumen systems were developed in Europe during the 1960's and were brought to the U.S. marketplace in the early to mid-1970's. The essence of the membranes is in the controlled manufacture of a heavy roofing sheet (usually 120 to 180 mils thick) employing multiple layers of asphalt and reinforcers around a core of plastic (APP) or rubber (SBS) modifiers. These systems are then installed by heat welding, using special torching apparatus or hot mopped into place using new or conventional methods of asphalt application.

The special characteristics of mod-bit systems are in their toughness, light-weight loading, and resistance to wind and fire. They also provide an easily inspected surface and long-term service expectancy. Because the multiple layers of roofing are built into one sheet, a great deal of the quality control is manufactured in the plant. However, good design and good roofing practices are still essential to the performance of this roofing system.

In effect, mod-bits exhibit properties of both built-up and single-ply roofing, yet it is a system which is complimentary in fulfilling specific roofing requirements that may not be met by B.U.R. or single-ply systems alone.