Rob - the drum of the Ohio Steel 48T lawn roller is made of 13-gauge steel, which is approximately 0.0897 inches thick. This is classified as a PRO-grade roller, so the steel drum is roughly 20% thicker than the steel on a residential grade lawn roller. It is recommended that these be filled with water - mainly because water displaces very easily within the drum and the drum can be emptied easily and quickly. You can also use a fine sand as well. I would NOT recommend using rocks or stones.