It's critical to get secure bonding of the roofing felts (plies) using bitumen. To achieve this bond the roofing contractor applies thin, uniform moppings of bitumen. This waterproofs the system and ensures proper adhesion for fusing the membrane system together. The temperature of the bitumen is critical. By heating it to the proper temperature the roofing contractor gets the right viscosity for proper mopping. The contractor heats the bitumen to its EVT or Equiviscous Temperature, the temperature at which it can be most effectively mopped into uniform layers. Each batch of bitumen should be labeled by the supplier with its EVT.
Once felts are rolled into place on the heated bitumen applicators pull brooms or squeegees over the felt or use some other method to make sure that its embedded in the bitumen.
The strength of the membrane depends on the type of felt used, the number of plies, overall ply construction, and the lapping of the overlaying felts. Typically, membrane ply construction is defined by headlap, endlap, and sidelap. Headlap is the distance of the overlap that exists between the lowermost and the uppermost plies of the shingled portion of the roof membrane when measured perpendicular to the long dimension of the membrane. Endlap is the overlap distance that is measured from where one roll of felt ends to where another begins. Sidelap is the overlap distance along the length of the felt where one roll of felt overlaps the adjacent overlying felt.
The application of Built-Up Roofing systems is detailed work, but the professional who pays particular attention to those details such as curbs, walls, flashings or other projections that interrupt the membrane, achieves a quality, efficient, long-lasting product for the building owner.
ARMA offers many sources of information to enable the roofing professional to do just that.