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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.

A common cause of separation between a wall and the ceiling is truss lift—this can happen to the ceiling of rooms directly under the attic when the seasons change and wood building materials dry out, shrink, and shift. Roof trusses often dry out at different speeds, and if the bottom pieces dry out faster than the top pieces, they can shrink and pull the attached drywall away from ceilings and walls, causing cracks. Fortunately, truss lift is only a cosmetic problem, not a structural problem, and you can fix it by separating the ceiling drywall from the roof trusses, and patching the cracks.

You should wait at least 24 hours for drywall mud to dry before sanding or applying another coat. However, the exact drying time depends on several factors, including the thickness of the application, room temperature, humidity, and the type of compound used. Thicker layers, or projects in high humidity, may require 48 hours or more. Lightweight compounds tend to dry faster than heavier, all-purpose ones. You'll know the mud is ready when it feels completely dry and firm to the touch and has changed to a consistent light color. Sanding too early can cause smudging or gouges, so patience is essential for a smooth finish.

You’ll want to inspect your plaster walls for signs of damage before painting. If you notice soft spots, deep cracks, bubbling, or areas where the plaster is crumbling or pulling away from the wall, it likely needs repair. While small hairline cracks are common and likely cosmetic, you should address larger issues before painting to ensure a quality, smooth finish.