What is the function of a 1/4" DensDeck underlayment?

When you use DensDeck immediately above a steel deck underneath insulation, it is referred to as an underlayment. There are two major functions of 1/4" DensDeck underlayment. First, it acts as a thermal barrier. If a fire breaks out inside the building, the 1/4" DensDeck underlayment will delay the transmission of temperature to the roof insulation for at least 15 minutes. If there is no thermal barrier, the insulation likely will melt and drip inside the building, adding fuel to the fire. Second, a 1/4" DensDeck underlayment provides a strong and versatile surface for the application of insulation. DensDeck offers a suitable surface for subsequent roofing operations such as hot mopping, torch application, mechanical or adhesive application of vapor barriers, insulation, and/or membranes. UL 1256 is the qualifying test for DensDeck over the steel deck. If the flute span is more than 2 5/8" and less than 5" or if the deck is plywood, use 1/2" DensDeck.  more