Photo: Eric J. Nordstrom
Photo: Steve Hall & Kendall Ristau Photographers, LTD
Terra cotta commonly comprises clay-based materials, sand, and grog (in architectural terra cotta) that have been pressed and fired to create a unit masonry material. Grog is previously fired clay pieces that are integrated into the mix as “aggregates” to help stabilize the clay materials and limit shrinkage for larger units. Terra cotta units are architectural terra cotta, hollow clay tile, guastavino tiles, roof and coping tiles, and interior tile. In general, terra cotta materials use more refined and denser clay body mixtures than clay masonry brick.
The manufacturing process varies based on how the units are formed, including hand pressing, extrusion, slip casting, and ram pressing. Because clay has high moisture content when excavated, there are several steps in the manufacturing process that allow the materials to dry, including air drying prior to mixing in a pug mill, and after forming. If the clay units dry too quickly, they are prone to shrinkage cracking. Architectural terra cotta and decorative guastavino tiles are finished with a glaze, which can be a slip finish, matt, or high gloss and can come in a variety of colors and designs. Hollow clay tile and structural guastavino tiles are unfinished.
The information on this page primarily focuses on architectural terra cotta. Although these units are strong and dense, their vulnerability is how they are anchored and connected to the wall assembly. Where there are individual terra cotta units within a brick masonry facade, they are often set in with the mass masonry wall assembly. However, architectural terra cotta’s popularity grew between the end of the 1800s to early 1900s and were largely used as full facade cladding systems anchored back to a steel frame structure with infill walls. Historic Architectural Terra Cotta Standard Construction by the National Terra Cotta Society (1914 and 1927 editions) graphically illustrate details of how terra cotta units were commonly assembled and anchored.
For successful historic and existing projects, it is important to prioritize repairs based on the structure’s conditions and project team goals. Project strategies differ per project and can be selected from a variety of repair options.
Refer to our general restoration best practices page for additional information, details, and resources. Below, find terra cotta repair and restoration options for consideration on your project.
Terra cotta unit damage, such as spalling and cracking, that is too great to repair in situ may need full replacement. Replacement terra cotta units should have similar compatible properties to the original terra cotta. Replacement units that are stronger and denser, or have lower porosity and water absorption, or different thermal characteristics can cause premature future failures of the historic or existing terra cotta wall assembly. It’s generally recommended where terra cotta units are individually replaced in the main body of the facade that the replacement unit(s) be terra cotta.
Alternative replacement materials for terra cotta options include glass fiber reinforced concrete (GFRC), cast stone, fiber reinforced polymer (FRP), limestone, and metal. Each of these have different thermal coefficients and general properties that are different from terra cotta. Clay-based materials tend to expand slightly in place, where cementitious-based material tend to shrink. These materials are appropriate as replacements when an entire system of the wall is being replaced, such as a cornice, parapet, or belt course.
Most terra cotta facades were originally installed without flashings or water management. They also used mild steel anchors, which are prone to corrosion. For system replacement, it’s critical to integrate new stainless-steel anchors, steel protection (for example, a corrosion inhibiting coating), and a new flashing system that includes termination bar, membrane, drip edge, and weeps.
Check out IMI’s terra cotta detail series for examples of window sills and mullions, window lintels, and building cornices and parapets. Each set includes a depiction of original construction, pressed terra cotta replacement units, and extruded terra cotta replacement units.
IMI’s free project support, technical assistance, and education is here to help you at any stage in your building’s lifecycle.
Our multidisciplinary team draws on decades of experience developing solutions for high-performing masonry and tile projects.
Why settle for anything less than the best when it comes to the installers on your project? BAC craftworkers train throughout their careers to become building enclosure experts and masters of their craft.
When you want to have confidence that you’re working with qualified, experienced crews on your project, you can specify for well-trained craftworkers.
Here are some of the training, certificate, and certification programs you may want to specify for terra cotta construction.
Traditional craft skills and contemporary repair techniques are critical to the preservation of historic buildings and structures. This in-depth certificate gives BAC craftworkers integrated knowledge of historic masonry preservation.
The skill of qualified craftworkers is essential for a successful historic and existing project. This program provides an in-depth look at terra cotta as an architectural cladding material, its manufacturing, overview of its performance, and methods of repair.
Here are some additional resources that focus on terra cotta and restoration. For a more comprehensive list of repair and restoration resources, please refer to the restoration page. For additional guidance, contact IMI.
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