"They will build the roof."
Translation:Ili konstruos la tegmenton.
I'd suggest that "oni konstruos la tegmenton" would be a fair translation, since "they" is used in colloquial English to me "somebody" in usages like this.
Mi pensas ke ambaŭ eblas. Mankas kunteksto, do pliaj frazoj, por bone decidi kiu versio pli ĝustas.
If you want to suggest it, feel free to report it using the "report a problem" feature. More context would make the question more clear, but my feeling (given the very small amount of context available) is that the meaning of the sentence refers to an actual "they" who will build the roof. "Oni konsruas tegmenton" is the sort of thing you'd say if you were saying "they often make roofs out of tile in Arizona" -- which is nothing like what's suggested by the (lack of) context.