Know Those EVT's


Karen Warseck,  AIA

T he National Roofing Contractor's Association (NRCA) has issued a revised technical bulletin regarding the equiviscous temperature (EVT) of roofing asphalt. Previously, the recommendations were for EVT to be defined at the temperature at which the viscosity of roofing asphalt is 125 centistokes, plus or minus 25 degrees F. Research by NRCA and the Trumbull Asphalt Division of Owens Corning Fiberglas has shown that the viscosity earlier recommended was too high for optimum performance on the roof. Lower viscosity asphalt was shown to be fundamental in assuring even moppings with fewer voids. As a result, NRCA is now recommending a temperature where 75 centipoise is attained as the EVT, plus or minus 25 degrees F.

What this means is that the temperature of the asphalt needs to be higher at the point of application than previously. For example, a sample of Type II asphalt used in the research reached the old definition of EVT at 383 degrees F., under the new definition, the temperature was 417 degrees to meet the EVT requirements. A Type III sample similarly went up from 420 degrees to 475 degrees F.

Corner Detail

In addition, EVT was defined for coal tar products as "the temperature at which a viscosity of 75 centipoise is attained, plus or minus 25 degrees F."

The bulletin also defines EVT as the temperature at the mop bucket or felt layer immediately prior to the application. This means that the kettle temperature must be even hotter to accommodate cooling between the kettle and application. Because of this, the asphalt must be heated to almost the flash point to keep it hot enough. The NRCA recommends that the asphalt be heated to no more than 25 degrees below the flash point to avoid fires and other dangers associated with overheating the material.

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Karen Warseck, AIA. CSI, is president of Building Diagnostics® Associates, a Hollywood. Fla., firm that specializes in the analysis of roofing and waterproofing problems.

This article was reprinted from a 1988 newsletter of the Broward County Chapter, The American Institute of Architects.

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