SOPRALENE® 250 SP 4.0 is a SBS-modified bitumen base ply for use in approved multi-ply membrane and flashing assemblies. SOPRALENE 250 SP 4.0 is composed of a proprietary formulation of elastomeric styrene-butadiene-styrene (SBS) polymer modified bitumen and is reinforced with a tough, dimensionally stable non-woven polyester mat. The topside is surface with fine mineral aggregate and underside is surfaced with polyolefin burn-off film to optimize heat welding.