Walls and ceilings don’t need constant inspection but it’s a good idea to know what’s going on in your home. Cracks in the ceiling or interior walls are NEVER good. The type of material used in your ceiling and on the walls will vary depending on the age of the house. Older houses normally had plaster-and-lathe walls and ceilings. Narrow boards were nailed to the studs and filled with plaster. Some of the older plaster had horsehair embedded in it. More modern homes usually have sheetrock or drywall on both the walls and ceilings.
Cracks in the ceiling or interior walls should be monitored. There are a variety of reason why the drywall or plaster might crack. Temperature and humidity cause the drywall or plaster to dampen and dry out, sometimes dozens of times per year. It’s not unusual for the inside humidity level to be 80% in the summer and 25% in the winter. Purchase a decent whole-house humidifier and dehumidifier. They are not part of your home’s heating or air conditioning systems.
Most modern homes, especially ones built by contractors and subcontractors are often built with lumber that has not been fully seasoned. Most lumber is dried in a kiln but still has a rather high water content when shipped to retail stores. It sometimes takes years for wood to fully dry out. Wood shrinks as it dries. If you notice your doors not fitting right, windows that seem out of whack, or boards on your deck warping, chances are the wood was wet when it was installed.
Drying lumber causes a number of issues both inside and outside a home. Cracks on the walls or ceiling could be from the studs or joists drying out and pulling the drywall or plaster with it. If you suddenly notice a thin crack on a wall or ceiling, just keep an eye on it.