Building molds
An important consideration - before you even begin building a mold - is that you have solid support for the mold and the concrete YOU will pour into it. Another is that the molding table be level. If there is not enough Support and the floor or table flexes, the concrete in the mold will reflect that. If the table is not level, the thickness of the countertop will not be consistent from one end to the other.

Experienced contractors will also tell you that now is when You have to consider the size of the mold. As Rhodes says, "One thing about precast is you have to think about moving these things and placing seams at seven feet or so. The weight is a consideration when you have to move 'it up stairs or over hill and dale."

Gobillot explains that making sections to fit on standard 4-foot by 8-foot plywood sheets is common in the industry, and that's Stone Soup Concrete's preference. "Anything beyond that becomes a special order," he says, but he adds they have never turned anything away as too large. "It takes more money, but we haven't had a project too large to make." The largest project his company has tackled to-date was a countertop 12 feet long, 5 feet wide, with two different levels and an integral sink, and which weighed more than 1,400 pounds.

What you use to build your forms depends on the casting and finishing technique you plan to use. Jeffrey Girard, president of The Concrete Countertop Institute, explains, "If the concrete is to be minimally processed (very little grinding), then the forms need to be much more meticulously prepared than if the concrete is to be ground and grouted. Either way, the forms must be dimensionally accurate in order to cast slabs that will be the right size and shape. Therefore, sturdy materials like steel and melamine are good."