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Fastening the bottoms of the cross bridging is an important step in ensuring the structural integrity of the floor system. The correct timing for fastening the bottoms of the cross bridging is typically after the floor sheathing has been installed and the joists have been secured, which aligns with the choice that emphasizes the benefits of doing it at that stage.

When the floor sheathing is complete, it adds stability to the entire floor assembly, ensuring that the joists remain aligned and straight. This is crucial because cross bridging is used to prevent lateral movement and provide support to the joists. Fastening them at this point allows for a more accurate positioning since the floor system has been adequately installed and has had time to settle.

The timing prior to the involvement of other trades, while necessary for overall workflow considerations, does not guarantee that the floor will have settled properly, which is vital for ensuring straight and secure fastening. Therefore, fastening the bottoms of the cross bridging once the floor is sheathed helps maintain the overall straightness of the joists and the effectiveness of the cross bridging in maintaining the integrity of the floor.