4421 Bitumen based sheet roofing (33) Membrane roofing, of bitumen or bitumen based sheets to near flat or pitched roofs, fully supported 4422 Synthetic rubber sheet membrane (59) Membrane roofing of all forms of synthetic rubber sheeting, to near flat or pitched roofs, fully supported 4423 Modified bitumen sheet roofing (31) Membrane roofing of modified bitumen (SBS) or (APP) torched onto near flat or pitched roofs, fully supported 4424 Sheet plastics roofing (11) Membrane roofing of plastics sheets to near flat or pitched roofs, fully supported 4427 Sheet Membrane & Integrated Structural Substrate System (0) Sheet membrane(s), usually on insulation on structural profiled metal substrate, possibly with a few interlayers. Specific metal substrate is either provided by membrane supplier or specified by membrane supplier.