Our property has an annex to the side with a flat roof which we would like to build a room on. Different builders tell us different approaches to do this. Two of them say the whole flat roof has to be taken out first and the building won't be water tight until the room on the upper level is built and roof put on. One builder say he would build the room on the upper level first, then work on the flat roof (floor of upper level). This sounds the least disruptive but is this possible? I mean will the builders be able to build on existing flat roof first? What if they need to change the joists?