A roof support for an underground mine which has a hydraulic cylinder positioned between a roof engaging support and a ground engaging base and has a flexible gas barrier or enclosure surrounding surfaces of said hydraulic cylinder which are subject to corrosion by gases present in the underground mine. Continuous flowing non-corrosive gas such as nitrogen is supplied to a space between the gas barrier and the hydraulic cylinder for preventing corrosion of cylinder surfaces by corrosive gases present in the mine which may include hydrogen sulfide.