When I asked around for some roofing myths to bust, I was a little surprised when the majority of responses centered around this one heard in the field: “When I’m hot mopping asphalt, the trick is to overheat the asphalt a little so that by the time it hits the roof, it’ll be at the right temperature”. It’s good logic, but here’s how the practice can backfire for both the contractor and the customer.
The belief is that by exceeding the asphalt’s application temperature, also known as its equiviscous temperature, inside the kettle, by the time it is pumped to the work area and applied to the roof, the asphalt has cooled to the correct application temperature. This approach sounds logical, but it is not recommended. In the short term, overheating above the recommended application temperature causes no immediate damage to the asphalt. In the long though, usually within hours of use, overheating does more harm that good.
Overheated asphalt eventually loses some of its binding qualities and becomes brittle because oils within the asphalt are flashed off easily the farther past the equiviscous temperature it is and the degradation that takes place is bitumen-specific (depending on the grade used) and occurs with little warning once reached. The asphalt’s viscosity can degrade to that of a substandard grade in that both its coefficient of thermal expansion and softening point are diminished to a point where shrinkage cracks and alligatoring occur readily. Additionally, overheating can lead to increased fume generation (manifested by the appearance of blue smoke) which increases the health hazards for the roof applicators and building occupants. Furthermore, because the flash point of bitumen is near or slightly above the suggested critical temperature, kettle fires can ignite with the obvious consequence to the roofer and building
Keep asphalt temperatures within the recommended temperature ranges to the moment it is applied to the roof to avoid issues later.
For more information, visit this Institute for Research in Construction seminar article.