Puede que sean iguales o puede que sean diferentes dependiendo de cómo lo analices. Antiguamente todas las casas tenían "tejas" en la parte exterior del techo, y "el techo" se refería exclusivamente a la parte interior de la casa, pero hoy día creo que el uso es intercambiable digo yo.

Many thanks - this makes sense and is helpful.

They are not! "Tejado" is exclusive used for a roof made of tiles. If you live in an apartment on the first floor I'm sure you don't have a "tejado". Even if your house do have a "tejado", if you are inside and are looking up, you can't actually see the "tejado" (roof tiles). What you see is the "techo" (ceiling).

They are interchangeable when we speak, but if we want to make a difference, "techo" can be translated to ceiling and "tejado" is translated to roof, actually a tile roof, it comes from "teja" that is the tile.