Abstract: An adaptor assembly makes it possible to mount an electric fixture, such as a ceiling fan or electric lighting fixture, to a ceiling with a slope greater than 32.degree.. The adaptor assembly includes a slope angle extension cover formed as a truncated conical surface with a bottom end lying in a first plane adapted for attachment to a standard fixture canopy and a top end lying in a second plane at an angle of 26.degree. to the first plane and adapted to be attached through an adaptor mounting plate to a junction box mounted in a recess in the ceiling.The mounting plate has a plurality of radial slots and is rotated to bring a pair of slots in alignment with screw holes in mounting lugs on the junction box for attachment with screws. The mounting plate has a pair of deformations providing flat wall portions which, when the mounting plate is installed are in parallel to the bottom end of the extension cover.