On the third week, we put up a 2"X12" frame on top of the first floor framework. We put up 2"X12" boards, 20 feet long (minus the width of the frame boards,) 16" on-center, attached to the outside 2"X12" frame. In the middle of the joist span, Stephen put in little sections of 2"X12" from one joist to the next. 20 foot long 2"X12"s are very heavy, and getting them up was cumbersome.
Then, we put down the floor sheathing on top.