The roof or your mouth should really be called the ceiling of your mouth

roof

| ro͞of, ro͝of |
noun (plural roofs | rufs, rʊfs |)

1. the structure forming the upper covering of a building or vehicle.
2. the top inner surface of a covered area or space; the ceiling: the roof of the cave fell in.
3. used to signify a house or other building, especially in the context of hospitality or shelter: helping those without a roof over their heads | they slept under the same roof.