Our hosue has a detatched third garage with a large atttic that I would like to see if I can remodel to use as an office. The problem is, I don't know if the floor in the attic is strong enough. the joists are 2x12s, set at 16 on center that are 18'6" long. There is no center posts or support for the ceiling, just the floor joists of the attic. The attic floor joists currently have oriented strand board over them. I haven't pulled the floor to see if there is any blocking between them or not, but the floor has a lot of bounce. My intent was to build an exterior staircase, and a small deck at the entrance of the attic, where I would place a door to the attic room, but I can't be sure that the floor will be sturdy enough to hold the weight with just blocking added, or if I will need to sister the joists. The interior garage dimensions are 18' wide by 22' long, and the attic ceiling to the crest of the roof is about 12 feet, so there is a ton of space there, but I am concerned about falling through the floor! Will the flooring be adequate with blocking, or should I sister some, or all of the joists with additional 2x12s?