I used 2x6s but had to rip them all down about 3/4″ so that they would be flush with the top of the trailer. I joined two 8′ pieces together and, since they were pretty bowed, the piece on the left is temporarily holding them straight until I got all the joists in. Because of the location of the cross ribs around the axles, I used joist hangers on two joists. Since the frame is upside down to receive the undercarriage, I had to remember to put the hangers on upside down!