An adaptor is provided that is configured to receive a support assembly for mounting a flexible fire sprinkler fitting in a ceiling support structure formed of parallel, spaced beams. The adaptor includes first and second long sides configured to be secured to each beam of an adjacent pair of beams, and first and second short sides configured to be secured to the support assembly. One end of each long side is joined by the first short side, and an opposed end of each long side is joined by the second short side such that the first and second long sides are parallel to each other and perpendicular to the first and second short sides, whereby the adaptor is a generally elongated, hollow, rectangular structure. The adaptor permits the support assembly to be mounted in a suspended ceiling formed of hat channel or metal stud furring.