Plaster Definition

• Plaster is defined as material used to cover mudbrick or bare stone walls on the interior or exterior of a house or commercial building. It is referred to as stucco when used on the outside of a building. Plaster is created by burning large pieces of marble, travertine or limestone in a lime kiln, which results in the formation of lime. Lime is then mixed with water to make plaster. The stuff, which is then applied to walls when wet in a paint-like fashion, becomes hard when dry. "Fresco painting" is a term to describe the painting and decorating of plaster.
  
  

Horizontal Cracks

• Horizontal cracks in plaster generally occur while the plaster is being applied. Wood laths, or wooden walls that feature wood beams in a horizontal structure called lath lines, tend to shrink and move as plaster is being applied, which can result in cracks along lath lines once the plaster dries. The plaster can also crack along joints and intersections if a wood board product is used. Unless the plaster is loose, horizontal cracks are generally stable and are of little concern.
  
  

Vertical and Diagonal Cracks

• Vertical and diagonal cracks are indicative of structural issues. The building may be moving if vertical or diagonal cracks in plaster form, and if you follow the crack to its origin, you will probably find the structural shift that is causing the crack(s). Since something has occurred within the building to move the plaster, vertical and diagonal cracks must be remedied to avoid further movement, which can result in more plaster moving and becoming loose.
  
  

Other Types of Plaster Cracks

• Cracks, such as those that form in irregular lines and patterns, can appear on plastered walls for reasons other than structural. Also known as map cracking or alligatoring, these cracks occur due to poor application or simply old age. Old plaster can crack along beams, lath lines, studs and intersections. Sometimes old or poor-quality applications of plaster will crack along the finishing coat of plaster only.