Seems this roof thing is quite abstract matter. I see it as roof (or ceiling) as it doesn´t change on which part of the earth you are standing. Sky is always above of us. We can go to other side of the earth and there too sky is above and we don´t stand there our heads down or think that sky is like the floor. Anyways, its meaning is to protect us. Also that Allah is the one who has created it like He has created the whole world, universe (and maybe more which we don´t know). The Arabic word there as "saqf" - سقف means roof, ceiling, roofing, overheads (I used google translate). What anything else this word might describe, you better looking for more specific explanation from someone else than me. I am just a simple sister without very much knowledge.