That's curious, I would never say "techo" for roof, but "tejado". Techo: ceiling, roof: tejado. However, I would say "una cabaña/choza con el techo de paja".
I should think about it.
And about "cielo", I've heard "el cielo de la boca" (curiously, roof of the mouth in English), but never "el cielo del coche", but "el techo del coche".