Just to add up - I think "techo" means more like "something which provides a shade". So "ceiling" actually should be much proper word for "techo". But in some cases a roof can actually be a ceiling as well. Let's say in some old house or shop or houses in which poor people live. I don't you would know it if you are an American. But here in India we have a lot of poor people living or working in houses/buildings which don't have any ceiling at all, but roofs used as ceilings
This is the reason why I love reading the comments so much. It not only helps understanding and clearing up things but also when someone comes up with such nice, curt and humorous remarks it makes learning much more fun. In a way these comments are my motivation for learning the language. And that's why Duolingo rocks. You won't find learning a language on your own so much fun elsewhere.