Structural Terra Cotta

William Kibbel III, The Home Inspector

Dear Home Inspector: When we purchased our stucco coated home, we assumed it was constructed of brick. But in the eaves off the third floor, I found that the exterior walls are actually built of orange colored blocks with grooves, much larger than bricks. What are these blocks?

You've probably discovered that your home is constructed with hollow, unglazed clay blocks commonly called structural terra cotta blocks. Advertisements and architecture guides from the teens through the thirties also refer to it as hollow structural tile, hollow tile block, hollow building tile, structural clay tile and structural clay load-bearing wall tile.

Many people are more familiar with terra cotta for its decorative use in mid-rise city buildings. Still others associate terra cotta with plant pots and garden statuary. Structural terra cotta is the plain step-sister of the ornamental glazed architectural terra cotta. Architectural terra cotta tiles are either cast from molds or hand carved, then glazed, to create the multiple forms, textures and colors. From residential brownstones to commercial Beaux Arts Style buildings, New York and Chicago have many examples of architectural terra cotta.

Remarkably little has been written about structural terra cotta, by contrast. I've seen just enough of it used for foundations, and entire buildings, to be convinced that these simple blocks of hard clay are an exceptional product. I'm puzzled as to why it wasn't more widely used.

Structural terra cotta blocks are made from natural clay, or clay produced from pulverized shale, that is extruded through a form (like children's play clay spaghetti). The clay is then baked in a kiln to create a hard building block. The hollow interior is divided into cells by a web which gives it strength. The grooves, or ribbing, on all four sides help mortar, plaster and stucco adhere to the surface.

Above grade, plaster is applied directly to the interior side of the structural terra cotta blocks. The exterior is often coated with stucco. Terra cotta blocks can also be used as a structural wall behind anchored brick veneer. One building I inspected omitted the stucco, creating an interesting texture. I don't know if it was left exposed because the builder liked the appearance, or if he ran out of funds.

terra cotta block
A cross section diagram of a terra cotta block shows the cells divided by the web inside the shell.

terra cotta block
Ribs on the four sides of a terra cotta block help mortar, stucco and plaster adhere.

Although manufactured as early as the 1890s, structural terra cotta blocks were most common in the first quarter of the 20th century. Stucco was the usual exterior cover, hence the popularity of the building material in Mission and Mediterranean Revival style homes. The blocks can also be found in military buildings and gas stations built as late as 1940.

Hollow tile blocks have also been used in commercial style buildings, but usually not as a component of a structural wall over three stories. Here the blocks are often used as fill between steel structural elements and for fire resistant wall construction, including interior partitions.

The only unfavorable trait of the blocks is that they are vulnerable to damage, even fragile, under certain circumstances. I've seen damage to individual units from mishandling or mistreatment after installation. Drilling into the blocks to anchor another material can be difficult. It often results in a blow-out of an area of much larger diameter than the intended hole. The brittle nature of structural terra cotta should probably be a concern for people living in an area with the potential for a seismic event.

Fortunately, the compressive strength of the shells and webs of these terra cotta blocks are much stronger than what you would expect from a building material made from the same stuff as flower pots. In many buildings, the blocks have lasted 75 years or more without problems, and can be expected to last for many more decades.

About the Author
William Kibbel III is a home inspector and restoration consultant specializing in historic residential and commercial buildings. He is vice president of Tri-County Inspection Company, serving Southeastern Pennsylvania and Central New Jersey.

Search Improvement Project