What causes plaster to crack?Updated 13 days ago

Plaster can crack due to two main causes: substrate movement or improper application.

Each type has a distinct appearance, timing, and cause. Understanding the differences is key to diagnosing the problem and choosing the appropriate remedy.

1. Cracks from Substrate Movement

   These are the most common type. Movement cracks usually appear after the plaster has cured and are often very fine, running in a straight line. They typically occur in areas of structural stress such as corners of windows and doors, along framing joints, or where ceiling loads are concentrated. These areas experience subtle shifts due to thermal expansion, settling, humidity fluctuations, or structural loading. While these cracks are largely cosmetic, they can reflect underlying stresses in the building. Repair is possible, but if movement continues, cracks may reappear.

2. Cracks from Improper Application

   These cracks appear as soon as the plaster has dried. Most are shrinkage cracks caused by applying plaster too thickly or failing to allow adequate cure time between coats. Poor surface preparation, rapid drying conditions, or mixing errors can also contribute. These cracks often appear as short, branching, Y-shaped or map-like fissures. Unlike movement cracks, they may indicate an issue with the installation method or environmental conditions during application. If left unaddressed, they can lead to delamination, moisture intrusion, or finish failure over time.

Preventing plaster cracks begins with stable substrate construction, proper reinforcement (such as mesh over joints), correct mix ratios, appropriate application thickness, and careful control of drying conditions. When in doubt, consult a skilled plaster professional to evaluate the cracks and recommend a durable repair strategy.