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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).

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.

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.

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.