plafono

The false seam here is -on-, the fraction suffix. Duono is a half, triono a third, kvarono a quarter, and a learner who has just learned that will look at plafono and try to make it a fraction of something called a plaf. There is no plaf. The root is plafon- in one piece, borrowed from French, and the -on in it is the tail end of fond, a bottom - which is a nice detail, because a ceiling really is the flat bottom of whatever is above you, seen from underneath. The French built the compound; Esperanto took delivery of it whole.

The room's three surfaces are worth memorising as a set, because two of them are easy to muddle: planko is the floor, plafono is the ceiling, muro is the wall. Planko and plafono start with the same four letters and mean opposite ends of the room, so say them out loud a few times now rather than in somebody's house later.

And keep plafono apart from tegmento. A tegmento is the roof: the outside of the building, the thing the rain lands on. A plafono is the inside: the surface you see from your bed. A tall building has one tegmento and a great many plafonoj. The figurative use follows the same picture - a plafono is an upper limit, prezoplafono a price ceiling - and it is understood exactly as in English.