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The short answer is yes, a sagging plaster ceiling is dangerous. Sagging plaster on a wall is something you should fix immediately, but it isn’t a major concern. If the same type of sagging occurs on a ceiling, however, you could be in danger of your ceiling falling in large chunks. When plaster sags, it loses its strength and ability to hold itself up. The bonds that hold it together are strong, though, so if a sagging area falls, it will take large pieces down with it.

The time it takes for thinset to dry is determined by your room’s ambient temperature and humidity levels. Assuming the average humidity level is around 40% with an average temperature of 70 degrees Fahrenheit, thinset can dry within 48 hours. Lower temperatures and higher humidity can extend the curing process beyond 72 hours, while high heat and zero humidity can shorten the curing process. However, thinset can crack if it dries too fast.

Plastering lasts for around 15 to 20 years without needing repairs or maintenance in most cases, and with proper upkeep, plastering can last a lifetime—sometimes more than 100 years. Plaster repair costs between $1 and $3 per square foot, and you may need these services once every 15 to 20 years or so unless you have a harsh climate where humidity changes are a constant issue. You can also opt to skim-coat plaster for between $0.90 and $1.30 per square foot, which may serve you better in the long run.

Plaster walls typically last between 30 and 70 years, but depending on the quality of the materials and the moisture levels in the building, they can last for over 100 years. With expert application and consistent maintenance, plaster can even last for centuries. To extend its lifespan, it is important to have the plaster professionally installed, address any water damage promptly, and repair small cracks as they appear. Regular inspections by a professional can help identify and address concerning cracks before they become significant issues.