Cast iron ballast that adds to an existing umbrella base rather than replacing it. Rooftops, piers and open pool decks routinely need more holding weight than a standard base provides, and this is how to get it without rebuying hardware already in service. Twelve inches across and two inches high, 33 pounds as shipped. Facilities teams add them when umbrellas move to breezier positions, and hotels, marinas and beach clubs keep spares on hand for peak season.