Single Ply Fully Adhered Partially Bonded Built-Up Felt Systems. Flat Roof MG is a high performance Polyisocyanurate with mineral coated glass facers suitable for use below single ply waterproofing systems and partially bonded built-up felt. Flat Roof MG is a high performance Polyisocyanurate with mineral coated glass facers suitable for use below single ply waterproofing systems and partially bonded built-up felt.