Architectural Terra Cotta
What is Terra Cotta?
Architectural terra cotta is a fired mixture of clay and water that is pressed into molds and then fired in a kiln where it hardens. Most commonly, a glaze is applied to the surface for protection against the exterior elements. Terra cotta has been used in building construction in the United States since the 1850’s and became popular as a cheaper alternative to decorative natural stone. Its popularity remained until around 1930 when newer mass-produced building materials that were more cost effective became available.
Unfortunately, during the era that terra cotta was a popular, there was not a lot of consideration given towards waterproofing and the protection of embedded structural steel. Water penetration often leads to damage from freeze/thaw cycles and corrosion of structural steel. Terra cotta is strong under compression, but it is weak under tension or shear, making it brittle and prone to crack under stress from freeze/thaw or corroding anchors. Once cracks form, these become gateways for more moisture penetration and deterioration accelerates.
As with all exterior building materials, terra cotta units should be inspected regularly. Projecting units, such as cornices, balconies, water tables, etc., those with structural steel fastenings, should be inspected for structural integrity, to avoid safety hazards below.
How it’s Repaired & Maintained
Spalling of the ceramic glaze is usually an indication of excessive water penetration. These areas can usually be repaired with specialized patching and coating materials that can be tooled and tinted for a reasonable color match. Depending on the structural integrity, cracked units can be patched, or sealed, to prevent water penetration. Broken units, particularly those structurally impaired, or with missing portions, should be replaced with new replicated units. Replacement with actual terra cotta is usually preferred, however long lead times, potentially up to a year, can be problematic. Substitute materials are sometimes necessary for budget and scheduling purposes.
The best line of defense in maintaining architectural terra cotta is to prevent moisture penetration. Typically, this can be accomplished by keeping mortar joints above, in, and around the terra cotta watertight. Sealants/caulking can trap moisture and prevent evaporation of moisture in a wall. Sealants/caulking should only be used in skyward facing joints, such as cornices and water-tables.