The FR Base Sheet is a tough non-asphaltic fiber glass based underlayment. In conjunction with the Hot Mopped System, the FR Base Sheet can be incorporated to provide a suitable substrate for deck types requiring a fastened base. FR Base Sheet is the ideal substrate for direct applications of the Hot Mopped System or Mechanically Attached System over decks manufactured from wood, tectum, gypsum, and lightweight concrete. Required fasteners for each deck type as well as the fastening density is outlined in Versico's Specification.