Ceilings are structures appearing in every Super Smash Bros. game. Like walls, ceilings redirect the momentum of characters sent upward and, in Melee and Brawl, can be teched . In many cases, "ceilings" are the underside of a stage.
In Smash 64, there is a small, low ceiling within the "house" on the right-most area of Hyrule Castle. It's useful for some combos. Another ceiling appears in the Mushroom Kingdom stage on the left side, which goes beyond the blast line. In this game only, KO points are not awarded to a character who KOs an opponent that hit the ceiling and then hit the ground again before crossing the blast line.
In Melee, ceilings return, but aren't as common, as very few stages have undersides. A few who do are Temple, Great Bay, Brinstar Depths, and Poké Floats. The most notable ceiling is on Temple's Cave of life in the mid-left section of the stage.
In Brawl, ceilings are more common than in Melee, appearing on such stages as Luigi's Mansion, Spear Pillar, and Mushroomy Kingdom. Given Brawl's physics, which include slower falling speeds for each character, a character getting hit upward onto the undersides of stages is much more likely, even in a regular match on a neutral stage like Battlefield (they're also more likely to recover).