Plaster as an Architectural Specification

From its origin as a British colony, the United States inherited a fully developed plaster tradition that would expand considerably from the mid-19th century until the 1940s. The fine craftsmanship can still be enjoyed in public settings such as period railway stations, banks, courthouses and capitals across the nation. Many fine plaster ceilings enduringly grace private residences in historic neighborhoods such as Brooklyn Heights and Peninsular Charleston.

However, in the decades following World War II plaster ceased being specified and plastering rapidly diminished as a trade. What happened? Two distinct movements figured prominently: the ascendancy of architectural Modernism and cheap, industrialized residential construction.

Modernist architectural programs were no longer teaching the traditional language of ornament. By contrast, students were learning that ornament was born of a superstitious and deceitful past, craft was a criminal enterprise injurious to the human spirit and that industry and technology were to be embraced as the basis for a new, purer aesthetic. At the same time, prefabricated temporary housing developed for the military during the war was being modified by developers for residential use. Factory produced construction systems were designed to be assembled by unskilled, replaceable laborers. Part of the package was to replace traditional plastering with nascent “dry” wall systems. By the 1980s there were practically no traditional plaster apprenticeship programs, little opportunity for training, by all appearances the trade had died.

With the destruction of New York’s Penn Station in 1963 and the proposed destruction of Grand Central Station in the 1970s, an architectural consciousness began to arise among the public at large, a widespread awareness of how much of value from our traditions was being lost. This coalesced both into an historical preservation movement and a renewed interest in traditional architectural design. By the 1990s plaster was being once again, if cautiously, specified in new construction. For several generations of architects plaster has become somewhat of a mystery not understood or taught by architectural programs. Many architects would like to specify plaster, yet being unfamiliar with the medium are concerned with exposing themselves to risk or appearing irresponsible with their client’s budget. Let’s see if we can’t ease that trepidation by considering some of plaster’s strengths and its most practical, effective uses in contemporary architectural specification.

Traditional Plastering

Gypsum drywall has largely displaced traditional plastering for interior walls and ceilings. Many homeowners are surprised to discover that the vast majority of the gypsum used for drywall is the waste byproduct of coal-fired power plants pollution control systems. This is in contrast to gypsum, lime and other plaster binder materials that are mined from naturally occurring deposits. I would like to highlight a few specifications where traditional plasters should be considered as a practical alternative to drywall:

1. Monolithic substrates
2. Curvilinear surfaces
3. High durability

Monolithic substrates (as opposed to cavity wall systems) are solid substrates, common examples being brick masonry, cast concrete or CMU blocks. The maturing natural building market also typically use monolithic substrates such as straw bale, adobe and compressed earth blocks, cob, rammed earth, hemp lime, etc. Exterior plaster or “stucco” will invariably be a practical solution in the exterior. However, interior plaster applied directly to the substrate is likewise a practical alternative to furring strips and drywall. With a carefully selected binder (clay, gypsum, lime, hydraulic lime or cement) a plaster can be formulated that closely matches the thermal, expansive, permeability and other characteristics of the substrate resulting in a far superior surface that is completely integrated into the substrate.

Curvilinear surfaces which may include walls but are often horizontal ceiling surface such as domes, vaults and the underside of staircases are a logical consideration for plaster specification. I’ve been on many projects where vaults are painstakingly framed out with what is commonly called “ship hull framing” to receive multiple layers of quarter-inch drywall that has to be cut into small strips, soaked and scored in the back to adjust to the curvature. This is completely unnecessary and an inferior construction to traditional plaster over lath, requiring only nominal framing.

High impact and abrasion resistance may not be expectations of walls surfaces for a residential home; however, for commercial and institutional specifications of hotels, museums, shopping centers, universities, airports and similar settings, long term durability and low maintenance are concerns to balance against initial cost. Gypsum, lime and cement plasters are often prudent investments that can also provide a pleasing aesthetic.

Cast Mouldings and Ornament

As the millwork industry became increasingly sophisticated, soft “paint grade” woods such as pine and poplar began to displace plaster as the economic plain moulding specification. The integration of ornament enrichment into mouldings slowed this transition until ornament itself was largely stripped from architectural design in the mid-20th century. Nevertheless, there are many strong arguments for specifying plaster mouldings with the following specifications often being competitive or less expensive:

1. Medium to large curvilinear profiles
2. Non-radial curvilinear profiles
3. Large, complex crown mouldings
4. Curvilinear oriented mouldings
5. Ornament
6. Low maintenance

Straight mouldings can be produced just fine in plaster but it excels like no other medium in being able to contour to curvilinear shapes. I say “curvilinear” as opposed to “radial” because plaster is not constrained physically or economically to arcs of circles but can readily accommodate ellipses, hyperbolae, or free formed curves. The process helps to explain this property. The first step in creating a plaster moulding is hand-cutting a reverse metal profile from a template. The profile is mounted on a jig and the plaster is built up in successive layers on a table or ramp. For plaster it matters very little if the profile is large or small, very complex or composed of non-radial curvilinear elements. In fact, the moulding itself can be curvilinear such as vertically for architraves surrounding arches, or horizontally as for moulding applied against a curvilinear surface or even complex helix shapes as sometimes encountered in the stringers of descending staircases.

With a return to interest in traditional architecture there has been a corresponding interest in ornamentation. As with plaster moulding profiles, the work of plaster ornamentation is by hand so there are no mechanical constraints. Often much time is invested in a single model of high quality from which a mould is produced. Plaster is an inexpensive material and multiple casts can be easily and economically produced. The more detailed the enrichment and the more units produced the more value can be attained with ornamentation.

Particularly when large or ornate mouldings are specified I have found clients concerned about maintenance becoming an issue. The coefficient of expansion of soft woods is relatively high with changes of temperature and especially humidity. This is exacerbated by the reality that wood mouldings are typically affixed mechanically against drywall or plaster materials that have a very low coefficient of expansion. The wood moulding moves, the wall does not and cracks develop quickly between the disparate materials that are either addressed with caulk or lived with. Alternatively, plaster mouldings are affixed with plaster to a plaster (or drywall) substrate. The result is a monolithic system, the bond is so strong that the mouldings literally become a part of the wall. Most plasterers will guarantee that aside from structural movement their work will not crack, ever.

This article is a brief summary of a subject that can become very specific for a given project. As a technical consultant for plaster materials and application, I provide services to architects helping them properly specify plaster and plaster systems. I also work with plaster contractors providing training and onsite consultation services as needed.