"The roof of that house is red."
Translation:El techo de esa casa es rojo.
The verb tejar means to cover with tiles. I have never seen a car with roof tiles so you are correct. Techo can be the roof or ceiling of anything. In rural Spain it is not unusual to see older buildings where the underside of the roof is also the ceiling of any rooms on the top floor. I think "tejado" actually means something tiled, but that so often refers to a roof that the word is used to refer to any tiled roof.
From the "Gran Diccionario Oxford, Ingles-Español, Oxford Press, 2008:
TECHO: masculine ceiling  (Latin America) roof. ////////////////////TEJADO: masculine (Especially in Spain) roof