Abstract: A method of installing a gas service line in a building or the like is described, in which a flexible sleeve elbow unit is snaked through a small hole in the building ground slab, downwardly through a narrow passage dug out from the earthen fill beneath the slab, and whereupon it is flexed at approximately 90.degree. to pass horizontally through an aperture in the building foundation wall, to where it is connected to the underground gas service line exterior of the building. The flexible sleeve elbow unit has an inner, gas carrier pipe of corrugated stainless steel, and an outer corrugated stainless steel casing pipe surrounding the corrugated length portion of the gas carrier pipe, providing an annular space therebetween to contain any gas leaking through the carrier.