One of our regular commercial clients, was in the process of replacing their furnaces, and discovered a sagging floor. Timing was crucial, so we went in Friday night (after the furnaces were removed), and were done by Sunday, so the new furnaces could be installed on Monday.
We took out the existing sub-floor, reinforced the joists, put in new sub-flooring (filling in previous holes no longer needed), then added a new one piece vinyl flooring to spruce the room up a bit.