Cracks at Interior Drywall

Drywall, also called sheetrock or gypsum board, is widely used for interior walls and ceilings because it

• provides a smooth surface that is easy to paint
• is widely available
• is relatively inexpensive and easy to install
• provides excellent fire resistance

A potential disadvantage of drywall is that it is easily damaged by moisture.  Common causes of such moisture include roof, plumbing, or window leaks. Drywall exposed to significant moisture may stain, crumble or even collapse. COMPOSITION Drywall is composed primarily of the mineral gypsum.  In making drywall, the gypsum is mixed with starch (for paper adhesion), paper pulp (for tensile strength) and a thickening agent.  This core mixture is placed between paper sheets, compacted, and oven dried.  Each molecule of gypsum (or dihydrous calcium sulfate) is composed of two molecules of water (H₂0) and one molecule of calcium sulfate (CaSO₄).  Gypsum is approximately 21% water by mass and nearly 50% water by volume.  When heated to 212 degree Farenheit (such as during a fire), the water (in crystalline form) changes to vapor, thus keeping the drywall relatively cool while crystalline water remains within the exposed drywall. INSTALLATION Basic drywall installation includes the following: sheets are screwed or nailed directly to wall or ceiling studs or joists, with or without adhesive between the supporting member and the sheet; joint compound is applied at the joints, followed by tape and more applications of compound; compound is also applied in layers over the fastener heads, which are slightly recessed into the board; corner strips, or 'beads', are installed at outside corners with several layers of compound; the compound is then sanded, followed by texturing and/or painting. Drywall is typically available and installed as 4'x8', 4'x10' or 4'x12' sheets.  Typical drywall thicknesses are 1/4”,  3/8", 1/2", and 5/8", with 1/2" thick probably the most commonly installed at the interior walls of single family homes. DRYWALL CRACKS Not all drywall cracks are a cause for structural concern. In fact, most are not.  Drywall is susceptible to cracking due to improper support preparation, installation, loss of support, or movement of the supporting structure.  Improper support may be loose or misaligned studs or other framing members. Problems with installation include improper fastener spacing, not enough or improperly mixed compound, etc.  Movement of the supporting structure may occur due to normal flexing of the structure under service loads. Drywall cracking movement may also be caused by normal thermal or moisture-related shrinkage or swelling of the wood studs or other support members relative to the drywall itself. Drywall cracks also occur due to significant movement of a wall, ceiling, or other structure and may be an indication of a structural problem. Such cracks are usually wider and/or have significant differential movement across the crack. Cracks that exist with matching profile at both the inside and outside (exterior CMU, stucco, etc.) of a wall may also be cause for concern.   ADDITIONAL RESOURCES: Drywall Finishing Council - Problems and Solutions