Historic Preservation - Technical Procedures
- Repairing Spalled Terra Cotta
- Procedure code:
- National Capitol Region Specifications - Agriculture Bldg
- Terra Cotta Unit Masonry
- Last Modified:
- Repairing Spalled Terra Cotta
- Last Modified:
REPAIRING SPALLED TERRA COTTA
A. This procedure includes guidance on repairing small
spalls in terra cotta using a cementitious patching
mortar. For alternative guidance in repairing spalls in
terra cotta using field-prepared mortar, see 04214-01-R.
B. See 01100-07-S for general project guidelines to be
reviewed along with this procedure. These guidelines
cover the following sections:
1. Safety Precautions
2. Historic Structures Precautions
4. Quality Assurance
5. Delivery, Storage and Handling
6. Project/Site Conditions
7. Sequencing and Scheduling
8. 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).
A. Samples: Color-match patch samples fabricated on pieces
of appropriate masonry from or on the building using the
specified repair mortar as required.
A. Jahn Restoration Mortars
B. Edison Coatings, Inc.
A. Terra Cotta Patch Mix: "Jahn M100 Terra Cotta Repair
Mortar" (Jahn Restoration Mortars, Cathedral Stone
Products, Inc.), "Custom System 45" (Edison Chemical
Systems, Inc.), or approved equal. Prepare samples for
approval. The RHPO will select the color most closely
matching the existing original terra cotta.
1. Mortar mix should be formulated to match color and
texture of existing masonry.
2. Mortar mix should not contain any synthetic
3. Mortar mix should be vapor permeable, frost and
salt resistant, shrink resistant, and be physically
compatible with the substrate, including, but not
limited to, porosity, tensile, and compressive
B. Water: Clean, free of oils, acids, alkalis and organic
A. Stiff, natural bristle brushes
E. Putty knife
A. For Jahn Restoration Mortar: Follow manufacturer's printed
B. For Custom System 45: Follow manufacturer's
1. Protect persons, motor vehicles, surrounding
surfaces of building whose masonry surfaces are
being restored, building site, and surrounding
buildings from injury resulting from masonry
2. Erect temporary protection covers over pedestrian
walkways and at points of entrance and exit for
persons and vehicles which must remain in operation
during course of masonry restoration work.
3. Contractor shall test those areaway drains, window
well drains, etc., which will be used to assure
that drains are functioning properly prior to
performing masonry restoration operations in those
areas. The Contractor shall report immediately to
the Construction Engineer the location of drains
which are found to be stopped up or blocked.
4. Prevent grout or mortar used in repointing and
repair work from staining face of surrounding
masonry and other surfaces. Remove immediately
grout and mortar in contact with exposed masonry
and other surfaces.
5. Protect sills, ledges, windows, and projections
from repair material droppings.
3.02 ERECTION, INSTALLATION, APPLICATION
NOTE: See manufacturer's product specifications for
additional requirements in making terra cotta patches.
A. Remove loose mortar and masonry at areas to be patched.
B. Cut back 1/2" to sound terra cotta with chisel and
hammer. Score surface to receive patch with chisel to
provide a mechanical key with patching material.
B. Thoroughly clean all loose terra cotta particles, mortar
and other dust and debris from surfaces to be repaired by
air and then with a soft brush and water.
C. Pre-moisten terra cotta with clean water and a stiff
natural bristle brush to prevent patching mortar from
drying out prematurely. Avoid over wetting masonry,
which inhibits adhesion. Prepare test patches as
required for approval by the Contracting Officer's
Representative to determine the correct degree of
D. Mix the cementitious patching material according to
1. The mixture should be compatible in texture and
porosity to the adjacent terra cotta.
2. Patching mortar should be shapable without using
molds and as it is being applied should hold its
shape right away.
3. Do not mix more material than can be used within 30
minutes, or as recommended by manufacturer.
E. Apply mortar mix with a trowel. Apply patching mortar in
layers no greater than 3". If a second layer is required,
allow the mortar to cure before applying the second
layer. If a cement skin forms, remove 1/16" of the
surface using a metal straight edge or scraper.
F. Build up layers so that patch is slightly higher than
adjacent surfaces. Do not feather edges.
G. Allow to harden for length of time as recommended by
manufacturer, then carefully cut back and tool to match
adjacent surfaces. Maintain mortar joints; do not
install patch across joints.
H. Keep patches damp for 72 hours after installations.
A. After mortar has fully hardened, thoroughly clean exposed
masonry surfaces of excess mortar and foreign matter
using stiff nylon or bristle brushes and clean water,
spray applied at low pressure.
B. Use of metal scrapers or brushes will not be permitted.
C. Use of acid or alkali cleaning agents will not be
END OF SECTION