"Slanted ceilings" refers to the type of ceilings that are not flat and instead incline or slope upwards, creating an angled or diagonal surface above your head. Full definition
A room with tall ceilings can handle a tall headboard, while a room with slanted ceilings makes an average - sized headboard look larger that normal. (bhg.com)
My biggest dilema is how to store my craft / art / sewing supplies because of the low slanted ceilings in the closets in that room. (theinspiredroom.net)
Awkward slanting ceiling parallel with angle of stairs gives varying, yet tall walls — up to 10 feet high! (theinspiredroom.net)