The drywall shortage of 1999 is history. Four new drywall plants have been producing material since last fall, and six more are expected to come on line within the coming year. "Last year, domestic manufacturers turned out 28 or 29 billion square feet of drywall, when builders actually needed 31 billion feet," says Craig Weisbrouch, vice president of sales and marketing with National Gypsum. "Demand should be about the same this year, but production should be up to 33 or 34 billion feet," he says. Jerry Walker is the executive director of the Gypsum Association, an industry trade group. "If someone still isn't getting enough drywall now," Walker says, "there's something wrong