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If you already have plaster walls and ceilings, adding new plaster costs $2 to $10 per square foot. For an average-sized room, that would come out to $1,200 to $10,000. However, the cost to replace plaster with drywall is higher—running between $720 and $1,250 for a 10-by-8-foot wall or ceiling (not including drywall finishing).

All plaster walls will crack over time due to normal house settling and expansion and contraction from temperature and moisture changes. Small, thin hairline cracks are generally not serious and are mostly a cosmetic issue. However, larger or more significant cracks can indicate structural problems.

Here's how to tell the difference:

• Cosmetic Cracks: Cracks that are 0.5 cm (0.2 inches) or thinner are typically cosmetic.

• Structural Cracks: Cracks wider than 0.5 cm, excessive or deep cracks, multiple cracks in the same area, or cracks that span around corners can signify a structural or foundation issue. Cracks along the joints where walls meet the ceiling also warrant attention.

For cracks wider than 0.5 cm, structural repair is needed. If a crack is 2.5 cm (1 inch) or wider, or if you notice any signs of a structural problem, it is best to contact a plaster repair professional to assess the damage.

While a normal moisture level for drywall is between 5% and 12%, replacement is recommended if you get consistent readings above 17%. At this level, the drywall is likely too compromised by extensive moisture damage to be considered safe. You can replace either the entire sheet or just the affected section. If you are unsure after a visual inspection, it is best to have a contractor or water restoration provider use a professional moisture meter to get an accurate reading.

Yes, you can install drywall over plaster, but there are important factors to consider. While it can create a smooth, flat surface, it is often not the best approach. Installing drywall over plaster will increase the wall's thickness, which can cause issues with existing window and door frames that may no longer fit. You might also need to adjust power outlets to keep them accessible. A better, though more expensive, option is to remove the old plaster and lath entirely before installing new drywall. Drywalling directly over plaster seals in the old material, and if any problems arise later, you will need to remove both layers down to the structural beams for repairs. Given the potential complications, this job is best handled by professionals.