Why Does Plaster Crack?

Cracks in plaster can result from shrinkage, an over-thick coat, weak adhesion, moisture or movement in the substrate. A small stable defect in the finish is assessed differently from a crack that grows, crosses several layers or appears with displacement. Diagnose the location and behaviour before covering it.

Causes within the plaster system

• The material dried unevenly or was applied outside the thickness allowed for that product.
• The substrate was dusty, weak or unevenly absorbent.
• The plaster and the underlying surface were not a compatible system.
• The layer dried too quickly for the conditions or retained moisture in thicker areas.

Look at the crack, not only the line

Record its length, direction and relation to corners, openings and changes between materials. Check for a step between the sides, loose fragments, dampness, new branches or a repeated crack after repair. These observations do not prove a structural cause, but they show when a simple cosmetic repair is not enough.

When a local repair may be reasonable

A small, stable defect limited to a sound plaster layer can be repaired with a compatible system after the cause has been addressed. Remove loose material, prepare the base and follow the product method instead of hiding the crack beneath paint.

When to ask for an assessment

Arrange qualified assessment for a growing or through-crack, visible displacement, distorted openings, several related diagonal cracks, persistent dampness or signs that the wall is moving. Do not widen, drill or otherwise open a suspicious crack yourself.

Reduce the chance of recurrence

Match the plaster system to the substrate, use the measured average thickness, prepare the surface and respect the drying conditions. The plaster calculator estimates material from area, average thickness and product coverage; it cannot identify why a crack formed.