Replacing Damaged Terra Cotta Units
- Procedure code:
- Developed For HSPG (NPS - Southeast Regional Office)
- Terra Cotta Unit Masonry
- Last Modified:
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Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.
We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.
PREFACE: This procedure should be performed under the direction of an historical architect or engineer to determine the most efficient and least destructive manner for executing the work. Approval from the regional historic preservation officer is required.
- This procedure includes guidance on removing and replacing severely damaged ceramic veneer, both adhesion type and anchor type. Sculptural elements are not covered in this procedure.
- See "General Project Guidelines" for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
- Safety Precautions
- Historic Structures Precautions
- Quality Assurance
- Delivery, Storage and Handling
- Project/Site Conditions
- Sequencing and Scheduling
- General Protection (Surface and Surrounding)
These guidelines should be reviewed prior to performing this procedure and should be followed, when applicable, along with recommendations from the Regional Historic Preservation Officer (RHPO)
- American Society for Testing and Materials (ASTM),
- Shop Drawings:
- The historical architect shall furnish all drawings, details and other information, including structural drawings, necessary for the manufacture of replacement material.
- Shop drawings showing details of construction and jointing of replacement material shall be submitted by the manufacturer to the contractor for approval by the RHPO before any of the material is processed. Drawings shall show sections, dimensions, and the connection with other work. (If anchored, the typical and special anchors, grooves, etc., shall be shown.)
- The manufacturer shall complete setting drawings, numbered and marked to correspond with numbers and other marks on the replacement pieces. These drawings shall also show all other necessary data to properly and efficiently set the material, including dimensions and sizes of joints, anchors, hangers, expansion and control or pressure-relieving joints, etc., which shall be furnished by others.
- Boston Valley Terra Cotta
- Gladding, McBean & Co.
- Ludowici Celadon
- The Sherwin Williams Co.
- Studio S Pottery
- Superior Clay Corp.
- Terra Cotta Replacement Units (ceramic veneer)
NOTE: ALL REPLACEMENT UNITS SHALL MATCH ORIGINAL IN SIZE APPEARANCE, COLOR AND FINISH.
- All materials shall be in accordance with the architect's drawings and details.
- All pieces shall be of even thicknessthroughout, and shall be true and even on each face.
- Each piece shall be thoroughly burned and free of cracks, air bubbles and other defects.
- All edges and details shall be crisp and sharp.
- Adhesion Type Ceramic Veneer: Approximately 1" thick, with ribs, grooves or dovetails on back. Individual units shall not have a surface area greater than 720 square feet.
- Anchored Type Ceramic Veneer: Machine or hand made, with a wall thickness of approximately 1-5/8" with dovetail ribs projecting on the back. Handmade units shall have profiles as detailed by the architect.
- Anchoring system pieces as required - either stainlesssteel, AISI (American Iron and Steel Institute) Type 316, polyester resin, or as recommended by the ceramic veneer manufacturer.
- "Zinc Chromate Primer" (Sherwin Williams), or approved equal.
- Top coat(s): Number and type of paint as recommended by manufacturer of primer. Paint shall be water tolerable and made by the same Manufacturer should be same as the primer.
- Setting Mortar:
- 1 cu. ft. Portland Cement, ASTM C 150-67, Type I or Type II
- 1/2 cu. ft. approved high calcium slaked lime putty, screened and aged at least 20 days, containing no more than 4% magnesium oxide-OR- ,
- 1/2 cu. ft. hydrated lime, ASTM C 206 or C 207, Type S
- 4 cu. ft. clean, sharp siliceous sand graded from fine to very fine, ASTM C 144
- 1 qt. "Hydratite Liquid" or approved equal
- Pointing Mortar:
- 1 vol. portland cement as above.
- 1/4 vol. aged high calcium lime putty as above, or
- 1/4 vol. hydrated lime as above.
- 3 vols. very fine sand as above.
- Gauge above with mix of one volume "Konset" thoroughly mixed in six volumes water, or of "Anti-Hydro"or "Suconem Red Label" or "Tricosal" or approved equal used according to manufacturer's directions.
- Grout Mix #1: For all vertical voids back of anchor-type ceramic veneer and handmade ceramic veneer shall be wet and sloppy composed of the following:
- 1 vol. portland cement as above.
- 3 vols. clean sharp sand as above.
- 2 vols. top gravel, well graded from fine to coarse.
- Grout Mix #2: For all handmade ceramic veneer sills, copings, balusters, and rails, cornices, etc., shall be lean and composed of the following:
- 1 vol. portland cement as above.
- 4 vols. clean sharp sand as above.
- 3 vols. top gravel, well graded from fine to coarse.
- Clean, clear potable water
- Paint brushes
- Hawks and trowels as required
- Hose and spray nozzle
- Grouting equipment
- Surface Preparation:
- Soak replacement pieces in a vat or box of clean water for one hour or more just prior to installation. The pieces should be noticeably damp at the time of setting. Drain pieces sufficiently to eliminate surface water.
- At the beginning of setting each day, soak all walls to be faced with clean water. Apply water with a hose and spray nozzle. Soak again with water not more than one hour before setting of replacement pieces.
- Removing Damaged Terra Cotta Units:
- Remove mortar from around damaged units by hand being careful not to chip the edges of adjacent sound units. Insert wedges as required as mortar removal progresses to hold damaged units in place until they can be removed.
- Carefully remove damaged units and all loose grout in cavity behind unit.
- Installing Anchored Type Ceramic Veneer:
- Preparing the anchoring system:
- Inspect the anchors and reinforcing armature thoroughly for any sign of corrosion.
- Remove any rust from anchors which can be reused. Coat with one coat of a rust inhibiting primer and top coat(s) of water tolerant paint as recommended by the primer manufacturer.
- Install new anchors as required and as recommended by the ceramic veneer manufacturer.
- Setting anchors and grooves:
- The general contractor shall be guided by the drawings for the placing of all loop-dowel anchors or other anchors, and the provision for the 1" x 6" horizontal grooves in the concrete to permit the proper anchoring and grouting of the ceramic veneer.
- Grooves shall be 12" on vertical centers in all spandrels and 30" elsewhere.
- Alternate detail: 1/4" x 6" grooves approximately 12" on vertical centers accomplished by the use of alternate surfaced and unsurfaced form lumber.
- The loop-dowel anchors, or approved equal anchors, shall be installed in vertical alignment on about 12" centers; and shall be spaced on equal horizontal centers of approximately 12" but not greater than 18", and so located, where practical, that a vertical line of loop-dowel anchors will be directly in back of each vertical joint of ceramic veneer. Keep loop-dowel anchors out of 1" x 6" horizontal grooves.
- Dimension from face of masonry or concrete wall to face of anchored veneer shall be a nominal 4".
- Installing pencil rods:
- Install 1/4" pencil rods vertically through loops of loop-dowel anchors.
- Install approximately 1" away from the face of the wall.
- Above openings, the lower end of the pencil rods should have 4" or more bent at right angles over ceramic veneer soffits, and should lie loosely thereon.
- Installing loose anchors:
- All loose anchors shall be located in bed joints and/or interior webs on 12" to 18" horizontal centers.
- One end of each anchor shall be securely hooked around a pencil rod, dowel rod, or structural steel member and the other end securely hooked into the ceramic veneer.
- Loose anchors shall hook around 1/4" dowel pins in joints of soffit pieces. These anchors shall pass through loops and hook around loop-dowel anchors.
- Special loop-dowel anchors:
- In deep ceramic veneer anchored soffits the special loop-dowel anchors shall extend vertically down to within 1" of the ceramic veneer soffit, and shall be threaded horizontally with 1/4 pencil rods.
- The long dowel end of the special loop-dowel anchor shall have 2" or more bent at right angles and lay flat against the top of the ceramic veneer soffit.
- The loop ends of the special loop-dowel anchors shall extend down vertically at least 5" from the concrete soffit.
- Special loop-dowel anchors are to be of the same material as the loop-dowel anchors.
- Dimension from face of concrete soffit to face of anchored veneer shall be a nominal 8".
- All anchored ceramic veneer shall be set in a full bed of mortar and all voids back of the ceramic veneer shall be filled solid with grout as specified under Grout Mix #1, and shall be well rodded to eliminate all air pockets.
- Where possible to do so, grout should be poured to a height of 6" only behind each continuous course of anchored ceramic veneer and the grout allowed to set sufficiently to "freeze" the piece in place before pouring the balance. Grout shall have sufficient plasticity to fill the joints from the rear.
- All copings and cornices of handmade ceramic veneer shall have filler pieces (grout plugs) at joints.
- Before being set in place, all handmade ceramic veneer copings, etc., shall be turned bottom side up and filled solid with a lean wet grout as specified under Grout Mix #2, andshall later be set in place on the wall and anchored in vertical joints as indicated on setting drawing.
- Vertical joints shall be grouted solid through openings at joints, provided in ceramic veneer before grout plugs are placed.
- This pre-grouting procedure is also to be used in grilles, etc., as directed and whererequired to solidly fill all voids.
- Preparing the anchoring system:
- Installing Adhesion Type Ceramic Veneer:
- Just prior to application of mortar coats, a limited area of the wall and the entire back of the piece of soaked ceramic veneer about to be set shall be given a brush coat of neat portland cement and water.
- Immediately thereafter spread on-half of th mortar coat on a limited area of the wall and the other half over the back of the piece of ceramic veneer.
- The mortar troweled on the back of the piece of ceramic veneer shall be screeded to a uniform thickness as measured from the face of the ceramic veneer.
- The other half of the mortar bed applied to the wall surface shall be screeded to a true and plumb level.
- The total thickness of the mortar coat shall average 3/4", but sufficient mortar shall be used to create a slight excess which will be forced out at the joints at the edge of the piece when it is tapped into place.
- When the piece is set against the wall it shall be driven home by repeated tapping with a rubber mallet to eliminate all voids in the setting bed.
- The combined thickness of the ceramic veneer and the mortar coat, when so installed, will average 1-3/4".
- Minimum thickness of mortar bed shall be not less than 1/2".
- Any piece of ceramic veneer tipped away from the wall for readjustment or improperly set for any reason shall immediately be removed, cleaned of all mortar, including the wall surface, and reset in accordance with the above procedure.
- Soffits: Each piece of ceramic veneer, in addition to the usual centers and wooden wedges, shall be supported by suitable bent vertical wooden shores exerting a constant upward pressure until the mortar coat is set for several days.
- Tooling and pointing:
- If pointing is not specified, all mortar joints shall be tooled as the setting progresses to form a concave joint.
- When pointing is specified, all mortar joints shall be raked to a depth of 1/2".
- Before pointing, joints shall be saturated with clean water, after which they shall be filled solidly with pointing mortar and tooled.
- All surplus mortar and stains shall be removed as the setting progresses and the surface shall be left clean.
- Wipe all excess mortar as the work progresses. Dry brush at the end of each day's work.
- After mortar is thoroughly set and cured, wash new masonry surfaces, walls, sills, overhangs, etc., of all loose mortar, and dirt with soap or washing powder detergent and rinse thoroughly with clean water. Abrasives or metal tools shall not be used.