It means you need to cut joists to their proper length before nailing the floor frame together (for example 8 ft less 2*1.5 = 7' 9 ). Construct the floor frame of your small cabin by nailing joists together with joist hangers. After your small cabin floor frame is built it is time to place the subfloor sheeting. Use 5/8 pressure treated plywood. Three of these 4x8 ft sheets will nicely cover an 8x12 ft cabin floor.