Flaws in the ceiling drywall are typically very easy to notice. Light from ceiling fixtures and windows can exaggerate any deviations in the ceiling surface, especially in large, expansive areas, which are common in many of today's homes.
Sagging causes a ceiling to appear wavy. Typically, high humidity during installation, heavy insulation from overfilling the cavity above, improperly spaced fasteners, or the wrong type or thickness of drywall can cause a ceiling to sag. For instance, high humidity can soften drywall's gypsum core, causing it to sag between framing members.
Cause 1:
Incorrect drywall selection for roof trusses
Regular drywall isn't appropriate for a 24" o.c. roof truss ceiling. Regular drywall will eventually sag, since the framing members are spaced too far apart to support the drywall.
Solution
Install sag-resistant drywall for all ceiling applications. It has a stiffer gypsum core that resists sagging at 24" o.c. framing. Sag-resistant drywall can span 24" o.c. framing with ½" sheets. If using regular drywall, use 5/8" sheets to increase sag resistance.
Cause 2:
Poor interior environment during installation
Humidity and contact with water can soften the gypsum core of drywall and expand its paper face, causing the drywall to sag between framing members. Portable space heaters generate large amounts of water vapor, dramatically raising the humidity level in the home. Drywall can absorb this moisture, potentially causing the ceiling to sag.
Solution
Before the hanging crew begins, maintain the inside air temperature and framing surface temperature at a minimum of 50°F for at least 48 hours. Ensure that they drywall is dry and is maintained at a minimum of 50°F for 48 hours prior to hanging. Throughout hanging, finishing, and painting, keep the inside air temperature at a minimum of 55°F while the crews are working and 50°F at all other times. Drywall should be installed in conditions as close to normal occupied temperature and humidity levels.
Install the furnace prior to finishing the drywall, isolating the return air plenum from the system. If portable space heaters have to be used, use them in conjunction with adequate ventilation by opening the windows about ½". Space heaters are strongly discouraged, though, due to the water vapor they generate.
Cause 3:
Improperly spaced fasteners
The drywall and its fastening schedule are designed to support a certain amount of weight. When fasteners are spaced too far apart, the weight of the drywall between the fasteners is enough to cause it to sag.
Solution
Follow the recommended fastening schedule, which varies depending upon sheet type, sheet thickness, and fastener type.
ProTip!For ceilings, the fastening schedules also vary depending on whether the drywall is installed parallel or perpendicular to the framing.
