Is it possible to melt down plastic bags or bottles to make transparent sheets for a house's roof?
In practice no - at least not on a domestic scale. The structure of most plastics would break down and the material would weak, become brittle or even carbonised, at least locally. Your chances are negligible of controlling the process (temperature) accurately enough (for those few plastics for which it may be possible) to get a viable material. Try it with a few small samples. Then just buy some plastic sheeting.
can and it very easy. melted the plastic on the temperature 105°C to 138°C (depending on the type of plastic used) then moulded by the colded mold (so that the plastic is not attached to the mould)
possible? probably. but you will need large flat areas, AND probably something to contain fumes. Consider instead using plastic bottles that are cut in half ( top to bottom) and overlap, like some roof tiles do.