A grout tillable prop assembly for supporting a ceiling such as the ceiling of an underground mine. The prop assembly comprising an inner member and an outer member adapted for movement between an extended use configuration and a retracted configuration, a unidirectional grout inlet in an upper part of a sidewall of the outer member to allow grout to pass into the prop, the inner member and the outer member having wall portions and being self-supporting, an air conduit through the prop having an opening adjacent an upper limit of the prop, a pressure relief valve coupled to the air conduit whereby the prop is be brought from the retracted configuration to the extended configuration by application of pressurized air through the air conduit and whereby the extended prop is filled with grout via the unidirectional grout inlet.