Upvote:3
Great Question! One of those things that you just don't think about when you think of the Roman Empire. The search I did brought me to this Wikipedia article about Roman Military Engineering, which should provide you with even more links to give you all the information.
Some funny things I learned:
Looks like they not only made most things locally and on the spot, but also provided roads to quickly retreat and move their heavier equipment. So, it seems they were able to create weapons, forts, roads, etc... on the spot, but also moved equipment around. However, I doubt moving a siege engine from Rome to Germania proved to be very efficient.
Upvote:8
Gaius Gracchus and his reform law, Lex Militaris, made the state responsible for supplying the military with equipment and clothing. It also meant that merchants and craftsmen would be employed providing this equipment and thus receiving a more stable and reliable income from the sale of supplies to the government rather than individuals. Military supply contractors made money off the government even back then.
In some cases, the merchants followed along with the fort/road system and established facilities to smelt iron, work lumber and so forth. The logistical advantage of the Roman road system also made it possible to move supplies over longer distances and required fewer manufacturing areas.