No Sag Drywall USG has recently introduced a 1/2-inch sag-resistant drywall — Sheetrock Ceiling Board — similar to Domtar CD Ceiling Board (see "For What It's Worth," 7/90). The ceiling boards are reportedly as stiff as 5/8-inch drywall, but as light as conventional 1/2-inch material. USG would not tell us how their board is made, except to say it has a "specially formulated core." Domtar has a three-layer "controlled density" core. The gypsum near each face is concentrated to increase the board's strength. Both 1/2-inch ceiling boards are designed for 24-inch on-center framing, where extra insulation is required, or where the moisture in heavy-textured finishes might weaken ordinary drywall. For more information, contact United States Gypsum, 101 S. Wacker Drive, Chicago, IL 60606, 800/621-9622; or