A polyolefin composition suitable for preparing single ply roofing membranes, comprising: (A) from about 15 to about 40% by weight of a crystalline copolymer of propylene with at least one alpha-olefin of formula H2CnullCHR1, where R1 is H or a C2null8 linear or branched alkyl, containing at least about 90% by weight of propylene, having solubility in xylene at room temperature lower than about 15% by weight; (B) from about 60 to about 85% by weight of an elastomeric fraction comprising: (1) a copolymer of propylene with ethylene containing about 20 to about 35% by weight ethylene, and having solubility in xylene at room temperature greater than about 70% by weight, the intrinsic viscosity of the xylene soluble fraction being higher than about 3.0 dl/g up to about 6.0 dl/g; and (2) a copolymer of ethylene with at least one alpha-olefin of formula H2CnullCHR2, where R2 is a C2null8 linear or branched alkyl, containing about 15% to 40% by weight alpha-olefin, and having solubility in xylene at room temperature greater than about 25% by weight, the intrinsic viscosity of the xylene soluble fraction ranging from about 0.5 to about 5.0 dl/g; the (1)/(2) weight ratio ranging from about 1:5 to about 5:1. The polyolefin composition of the invention, preferably prepared by sequential polymerization in at least three stages, has tensile strength at break null25 MPa, elongation at break null700 MPa and toughness null150 MPa.