In Spain is called 'tejado' the highest zone of the houses viewed from the outside, and as you say one of the two main kind of roof. Certainly there is another kind, 'la azotea' but is more normal to say:
'Una bonita vista de los tejados de la ciudad' que 'Una bonita vista de las azoteas de la ciudad'.
The technical term for the topmost of a building is 'cubierta' but we use 'tejado' ;-)
There may be geographic differences, but "techo" can mean either ceiling or roof. http://www.wordreference.com/es/en/translation.asp?spen=techo
I said, "Can you get ON the roof?" and it said "I used the wrong word, and should have said "Can you get TO the roof?""
Is there a difference?
If I am standing outside of my house and the float on my tinaco is stuck and a neighborhood kid passes by is it not perfectly okay to ask him "Puedes alcanzar el techo para mi?"
Please let me know if I am wrong.
There's a couple of things. Firstly, by what native speakers have said above, it seems "techo" is primarily used for the inside of a roof, like the roof of a cave, or ceiling of a house, and getting on this type of roof would defy gravity.
That said, it seems it is possible to use "techo" for an exterior roof, and getting on this type of roof would be fine. So, the problem probably lies with "alcanzar." It means reach, achieve etc., but not necessarily "get on." Perhaps "subir" would be best for that.