Once the support structure was in place, logs were laid across the piers and then boards were used to form the roadway.
I estimate that the piers were about twenty feet apart so a thousand foot river span would have required 50 piers. The bridge was built in ten days which meant workers would have placed five piers per day assuming the roadway was built as a parallel process over piers already completed.
The Romans had a variety of familiar tools at their disposal, including adzes, hammers, saws for cutting boards, and block and tackle. They used 9 inch nails to fasten boards together.
Once the bridge was complete, the army crossed over the Rhine. They spent 18 days in the land of the Sugambri, burning abandoned villages because the enemy had fled their homeland in fear of the Roman juggernaut. After treating with the Ubii, Caesar crossed back over the Rhine and destroyed the bridge behind him. Ten days in the making, Caesar’s Rhine bridge lived for eighteen days.