Plaster and Stucco for new work

Plaster

For clarity, we draw a distinction between interior "plaster" and "stucco."  Stucco is not strictly for exterior uses, but was used heavily on interiors up until the beginning of the twentieth century.  Today it is making a comeback due to its unique properties.  Plaster is calcium sulphate or, alternatively, sulfate of lime.  It sets by absorption of moisture.  It exists in many forms and hardnesses, but generally speaking it sets hard within hours, allowing rapid completion of jobs.  Its weakness is water, which over time will destroy it.  Generally, solid lath and plaster will hold up much better to water than drywall or modern gypsum lath-based systems, but cannot handle water in the long term.  Its setting properties make it the material of choice for ornamental work.

Stucco

Today the word "stucco" usually refers to a Portland cement or acrylic-based exterior coating.  Traditionally, the term referred to lime stuccos, which are our preferred material.  Lime stuccos generally require more labor than plaster, and are much slower to set, requiring "rests" between coats.  However, the material is very versatile, and can be used both inside and out.  It is moisture-permeable, allowing walls to "breathe," which is especially important for older buildings, as these were typically not designed for waterproof coatings.  Ornament is possible in lime stucco, though it is not quite so easy as with gypsum.  Lime was used heavily prior to the twentieth century since it was more economical than gypsum.  Sometimes clay or other still-cheaper materials were mixed in to further economize, or were used in areas that were not exposed.  For more information on the advantages of lime stucco, please see our Lime Stucco page.

For new construction, we generally recommend gypsum when:

• the surrounding material is gypsum based
• the surface is to be decorated, with either paint or wallpaper (although integral color coats work well in gypsum)
• mouldings are present, especially those with strongly projecting members

We recommend a lime stucco when:

• the surrounding material is lime stucco
• the surface contains an integral color coat, such as a marmorino
• for the insides of exterior walls that are not lathed and furred

The two materials are not mutually exclusive, and can be blended, depending on the particularities of the installation.

Below are some images (click to enlarge) of an example of three-coat gypsum over metal lath.  This system (three-coat) is excellent for producing curved surfaces:

In this instance, two helical ceilings and one dome were executed.  The walls, done by other workers in drywall, have a somewhat irregular curve to them, rather than a proper cylindrical surface.  Neither drywall nor veneer plasters can be used to replicate the shapes shown here.  In fact, there is really no alternative to installing lath and applying three coats as we did for these specialized shapes.  We oculd also have done these in lime stucco in three coats, but we chose gypsum since the surroundings were gypsum (drywall) and the surfaces were to be painted.

Stucco Classico: Plaster Repair for Pennsylvania