Twenty-five years ago, NRCA adopted the equiviscous temperature
(EVT) concept for the application of interply mopping asphalt in
built-up roof (BUR) systems. Since then, this concept has become
widely accepted and adopted by the U.S. roofing industry. However,
it sometimes is incorrectly applied to purposes other than those
for which it was intended.
If you install hot-applied, bitumen-based roof systems, it would
be useful to review the EVT concept and know when it applies.
In 1977, after more than four years of research and experiments,
NRCA adopted the EVT concept and issued its Technical Bulletin
The research revealed that the critical property for mopping
asphalt is the flow of the asphalt at the application point. The
experiments proved that the ideal flow, adhesion and mopping weight
for asphalt for BUR systems are achieved when the asphalt's
viscosity is about 125 centistokes (0.125 Pa•s) immediately
before application, such as in a mop...
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