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A Primer on Clay Tile Roof Inspection and Material Selection

May 15, 2002

By John Dashner, CSI
Evaluation Standards
A complete and thorough evaluation of the existing roofing
system is the first step in a successful re-roofing project. What
needs to be done to the roofing system and what will be the subsequent
impact on the over-all building? In order to answer these
basic questions, the design professional must first establish the
degree of preservation mandated by the owner and or governing
authority and determine the standard of treatment or care
required on this project. Is the building designated and protected
by landmark status and therefore subject to its state’s Interior
Department standards for historic properties?
Before beginning an extensive roof investigation program,
the standard of treatment or care required should be defined. Is
the goal preservation of the roof system? Is it restoration of the
roof system? Or is it the reconstruction of the roof system? The
Secretary of the Interior Standards are very specific and detailed
with respect to these standards of treatment. Excerpts follow:
Preservation Standard requires retention of the greatest
amount of historic fabric, along with the building’s historic
form, features, and detailing as they have evolved
over time.
Restoration Standard acknowledges the need to alter or
add to the historic building to meet continuing new uses
while retaining the building’s historic character.
Reconstruction Standards establish a limited framework
for re-creating a vanished to non-surviving building with
new materials, primarily for interpretive purposes.1
These standards create the framework within which the roof
designer will formulate a plan of action to address the scope of
work for the project.
Evaluation of the other building components that are impacted
by the roof covering is also necessary. To focus only upon the
roof covering and disregard its related parts is professionally irresponsible
and may adversely affect the performance of the overall
roof system. Other components or elements that should be
reviewed include, but are not limited to:
• Adjacent metal flashing
• Condition of the parapet walls
• Condition of the supporting deck
• Drainage issues
• Current code requirements
Understanding the interaction of all these components as
part of the overall roofing system will provide a clearer picture
of how this area of the building envelope functions. Developing
a thorough and complete evaluation protocol is necessary to be
certain that all parts of the building related to or affected by the
roofing system are examined and documented.
Evaluation Protocol
An interior condition survey is necessary to obtain an
overview of potentially deficient conditions within the roofing
system. The inspection of accessible interior spaces is carefully
documented and then superimposed over a plan of the exterior
roof to identify and locate the relative position of the leaks and
Clay roofing materials are commonly used on both
steep-sloped commercial and residential roofing projects.
However, if the project involves restoration of an “historic
building,” the needs and requirements are typically much
different. In this article we examine two key elements that
are necessary for the successful historic preservation or
restoration re-roofing project. These elements are the evaluation
of the current roofing system and establishment of a
standard for selection of clay tile roofing material.
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other deleterious conditions. Locations of stained ceiling tile,
falling plaster, and other damage to interior finishes should be
closely examined for possible leak locations.
Do not limit the inspection to just the point of interior damage.
Leaks on tile roofs have the ability and often the tendency
to travel beyond the source location. With these types of roofing
systems, water penetrating the building envelope will typically
follow the contour of the roof deck and rafters for several feet
before entering the interior and damaging the space. The detection
of interior damage is only a general indicator of where to
help focus attention on possible problem roof areas.
After recording the locations of leaks and potential problem
areas, the evaluation and inspection moves to the exterior building
envelope. It is important to note that the exterior building
survey is not just a visual survey but also a comprehensive,
hands-on look at the entire roof system and its related parts.
When actually on the field of the roof, one can observe the conditions
of the tile and its components. How is the existing installation
holding up? Are there missing or broken tiles in random
areas of the roof, or are the missing and broken tiles in critical
transition areas, such as hip, ridge, valley, or eave lines? Close,
visual observation and hands-on examination of the roof area
will help identify obvious leak and problem areas. However, in
many instances, actual samples of the roofing system will have to
be taken.
These probes or inspection openings into the roofing system
will help the roof designer gain the greatest amount of information
about the as-built construction and condition of the existing roof
system. The number and extent of probes or inspection openings
should be dictated by the condition of the roof, size of the area
to be re-roofed, and number of levels to be re-roofed.
It is also recommended that, at the time of the roof probe,
representative sample of the components of the roof system be
obtained for laboratory analysis. In many cases, the laboratory
analysis will allow a reasonable assessment to be made regarding
the expected service life remaining in the part and, by extrapolation,
the rest of the roof system. These results are helpful in
determining the scope of work required and the types of materials
needed to successfully complete the project.
Samples should be taken from several different locations to
obtain a reasonable cross section of the conditions that are representative
of the overall roof system. Water testing may be utilized
in areas of suspected leaks to duplicate conditions and
more definitively identify the source and cause(s) contributing to
water leakage. During the water test, closely observe the suspect
area. Is there clay tile missing or broken, resulting in underlayment
deterioration? Is there metal flashing that is severely worn
or weathered leading to underlayment damage and subsequent
water infiltrations? Masonry detailing that is suspect could also
lead to water infiltration. Recording detailed observations of the
existing roof system while water testing will provide a solid
foundation upon which to develop repair recommendations for
the next generation of roofing on the building.2
Material Selection
After determining that replacement of the existing clay tile is
needed, deciding the type required for the project is the next
hurdle. The American Society of Testing and Materials (ASTM)
has developed a material specification for clay roof tile that
establishes consensus standards for grading tiles having various
degrees of resistance to weathering. There are three grades of
clay roof tile established in ASTM C-1167. Grade 1 tile is resistant
to severe frost action. This quality of tile is appropriate for
use anywhere in the United States, including areas with cold,
harsh climate. Grade 2 provides resistance to moderate frost
action and is acceptable for use in more moderate temperate climate.
Grade 3 provides negligible resistance to frost action, and
is recommended only in very mild climates such as Southern
Florida and Southern California.
The weathering index is one of the most significant features
of this ASTM standard. It is established by the product of the
average numbers of freezing cycle days during which the air
temperature passes either above or below the freezing point and
the average annual winter rainfall (a measure of the precipitation
in inches, occurring between the first killing frost in the fall and
the last killing frost in the spring).
A clay tile’s moisture absorption rate is directly related to
how well it resists weathering. Under cyclic freeze/thaw conditions,
moisture within a tile unit expands and contracts. The
resulting internal pressure within the pores of the tile can lead to
deterioration. In areas of severe weathering, these repetitive
freeze/thaw cycles cause tiles with a high moisture absorption
rate to prematurely break apart. Once deterioration has been initiated,
the rate of product deterioration increases rapidly, leading
to material failure.3
Another important element of the ASTM standard is the
establishment of the minimum transverse breaking strength values
for different tile profiles. For Type 1 (high profile tiles with a
rise-to-width ratio greater than 1:4), tiles must have a minimum
allowable transverse breaking strength of 300 pounds. For Type
2 (low profile tiles with a rise-to-width ratio equal to or less than
1:4), the minimum allowable breaking strength is 225 pounds.
With Type 3 tiles (which account for all other types, including
flat), the minimum strength is also 225 pounds. Strength offers
protection against the various elements a roof is subjected to:
i.e., hail as well as damage during handling and installation.
The requirements of these ASTM standards set the minimum
level of material quality standard that a tile must meet. Because it
is a minimum level, this standard does not necessarily ensure that
first quality materials will be provided for the project. Specifiers
can and frequently do ask for additional performance requirements.
Note 2 of ASTM Standard C-1192 states that:
“…The best indication of clay tile durability is the service
record of experience with the specified product in the
environment of its intended use.”4
Roof designers should require a list of reference projects of
similar size and scale to the project at hand for which the manufacturer
has supplied material.
Go out and look at the referred projects. See how they have
fared under actual field conditions. Be sure to look at projects
that have several years of weathering behind them and not just
recent installations. The roof designer should specify or require a
higher level of quality or upgraded standards whenever he or she
feels that the greater requirement will benefit the project. This
would include manufacturers’ warranties for long-term product
performance. Some manufacturers warrant the performance of
the material up to 75 years.
In checking the validity of the warranty, see how long the
manufacturer has been making roof tile material. What is its
track record in the climate zone of the project? If the manufacturer
is offering a long-term product performance warranty, ask
to see a reference list that would include projects that have performed
for extended periods of time in similar types of climates.
Do not accept a project reference list from a manufacturer who
is primarily located in the southern climate if your project is in
the north. Demand to see projects of similar size, scope, and
location as your project. Finally, as always, read the fine print on
the warranty to make sure it includes the required items prior to
inclusion of the warranty in specifications. ■
When selecting materials for replacement components on an
historic building, one must consider the material that was on the
structure originally, the required regulations to maintain the
building’s historic designation, and material performance standards
and characteristics. Establishing an evaluation protocol and
adhering to at least a minimum standard of quality will help
ensure proper stewardship of the historic property.
1. K.D. Weeks and A. Grimes, “Secretary of the Interior’s
Standard for the Treatment of Historic Properties with
Guidelines for Preserving Rehabilitating, Restoring, and
Reconstructing Historic Buildings.” Roofing Handbook for
Historic Buildings, Deborah Slaton, Charles Fisher III, Eds.
Historic Preservation Education Foundation and the
National Park Service, Washington D.C., 1999.
2. M. LeBonte and K. Cash. “Evaluation of Clay Tile and
Terra Cotta Roofing Systems.” Roofing Handbook for Historic
Buildings, Deborah Slaton, Charles Fisher III, Eds. Historic
Preservation Education Foundation and the National Park
Service, Washington D.C., 1999.
3. Ed Ryser and Dr. Mike Noone. “Performance
Characteristics of Clay Roof Tile,” Western Roofing,
January/February 1993, pp. 80-81.
4. American Society for Testing and Materials. C1167-96
Standard Specification for Clay Tiles. West
Conchohocken, Pennsylvania, ASTM, 1996
John Dashner is president of A & D Building Products,
Ltd., a manufacturer’s representative firm for roofing and
waterproofing products in the Midwest. He is past president
of the Northern Illinois Chapter of the Construction
Specification Institute, and a member of the Northern Illinois
AIA chapter. He has participated in numerous historic preservation
projects in the Midwest.
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16 • Interface January 2002