Track 2: Materials over Time: Points of Change
3 - Taking an Alternate Approach – Replacing Architectural Terra Cotta
Wednesday, September 26
8:30 AM - 10:00 AM
Location: HYATT-Grand EFG
Architectural terra cotta was developed as the durable and affordable 19th century alternative to carved stone, with initial uses that mimicked the appearance of natural stones. Early anchorage details were also inspired by similar stone anchorages and suffered similar limitations of deterioration due to water infiltration, corrosion, and unaccommodated differential movement. Terra cotta design and detailing evolved as the industry matured with better quality production, improved anchorage details, and designs and patterns that moved away from just mimicking stone. Modern rehabilitation projects that replace deteriorated terra cotta similarly need to evolve to consider both alternate materials and anchorages. Using The First Church of Christ Scientist Mother Church terra cotta replacement project as a case study, this presentation will examine how the design team evaluated alternatives for both materials and anchorages to find the most appropriate approach considering durability, historical appropriateness, constructability, schedule, and cost.
A significant extension was added to the Mother Church at the turn of the 20th century, which features carved granite and limestone facades with terra cotta domes, semi-domes, and smaller roofs. The current multi-year restoration project includes replacement of the two semi-domes, which have a history of ongoing leakage and deterioration that a previous replacement project, within 15 years of the original construction, did not address. Currently mortar set over a membrane waterproofing on a Gustavino thin-shell structure, the semi-dome terra cotta suffers from water infiltration, freeze/thaw deterioration, and cracking due to unaccommodated movement. As currently designed, there is also no easy way to repair or address hidden defects. It was the design teams charge to develop a system that is durable for 75 to 100 years, salvages the Gustavino structure below, provides reliable waterproofing, and can be maintained and worked within the Church’s budget. This presentation will discuss our analysis and options considered in arriving at our design.
Within the narrative of this project, the presentation will review:
• Causes of deterioration of the existing terra cotta roof system
• Evaluation of the Gustavino sub-structure and impacts of roof alternates on its function.
• Anchorage and waterproofing considerations to prolong durability and allow for maintenance, including the selected unique cable attachment system design
• Replacement material options, including machined stone and replacement in-kind. The analysis will present our material testing and evaluation to compare durability, constructability, and maintenance aspects of the material options.
- Identify causes of terra cotta deterioration.
- Evaluate alternative materials for terra cotta replacement options.
- Evaluate terra cotta waterproofing anchorage options.
- Compare durability, constructability, and maintenance considerations of the material and anchorage options.