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Due to the cost of veneer stone and its reputation for durability,
expectations for its performance are high. Failures in new construction in
stone are embarrassing for the project team and baffling to the customer.
Failures will happen because an unprecedented combination of factors has
come together to make sure that they do. First, with new materials
available and new tastes in architecture, the customer has options in
unproven materials or styles imported from outside the local climate and
building vernacular. Second, the reduced thickness of modern veneers
leaves less room for error. Third, after many decades of relative low
utilization, demand for this material has increased, both for new work and
restoration, but the reserve of skilled and experienced artisans and
designers has not kept up. To discover if the artisan's perspective could
contribute to more confident specification of this material, we consulted
with Robert Young, owner of
Gothic Stone Restoration of New Haven, CT and a journeyman stone mason
with New Haven Local #1 of the International Union of Bricklayers and
Allied Crafts. Building stone is a natural material, Mr. Young notes, with very
individual characteristics and many ways to fail. Every local climate is a
unique natural environment, placing different stresses upon the material.
Stone varies, for instance, in its ability to absorb heat and its rate of
expansion. Such variations affect how it interacts with mortars or
anchoring systems. If an attachment point is cut without full allowance
for the relative rates of expansion of the stone and the steel anchor,
stone breakage can result. Expanding stone can damage incorrectly
formulated mortars. Caulk can be made brittle by overheating masonry
units. Stone also varies in its capacity to absorb water and its ability
to survive cyles of freezing and thawing in a wet climate. An impermeable
mortar can damage a masonry unit by trapping freezing water within it.
Even profile details can affect its performance. Details in traditional
styles were developed to promote the runoff necessary in certain climates
to insure that no damage occurred to the stone. Local usages evolved to
match stone, mortar, anchorage and profiles for the most reliable
performance. Installations which deviate from these usages without
sufficient calculation risk failure. New materials without local
references or credentials are available from quarries in Asia, Africa and
South America. Beautiful and exciting as some of these materials are, the
primary role of an exterior veneer is as a durable part of the building
envelope and not as building ornament. Mr. Young suggests that the best template for developing durable
specifications is a historic building using similar materials and similar
details in a similar climate. The builders of historic facades relied upon
materials and designs whose performance in the local climate was well
understood. If there is any doubt about their choices, it should be
removed after decades or centuries to the weather. If any true flaw is to
be found, we can, with hindsight, make note and learn from it. The recent
dismantling of two historic veneers furnished him an inside look at the
anatomy of success and failure of historic facades. The 400 foot long
façade of the Red Cross building (see photo) was dismantled for no fault
of its own but rather to accommodate a major expansion of the building.
Before being reinstalled at its new location, however, its bearing and
anchorage details were extensively re-engineered to modern standards. The
seven story façade of the Sterling Library (see photo), on the other hand,
suffered from chronic leaking and saturation of the wall core, a condition
which damaged limestone trim and threatened an important library
collection. Mr. Young notes three suggestions for the uneventful and
painless specification of durable new stone facades and supports them with
observations from these notable historic stone veneer projects. The first suggestion is that by specifying a proven stone, risk and
issues can be more efficiently managed. In both of the cases cited, the
use of Indiana limestone provided value through the ready availability of
matching stone for replacements, repairs and for building additions. For
buildings with a long life cycle, this is essential. The choice proved its
value again in simplifying technical specification. There was at first
controversy over the mortar to be used in the Red Cross façade, but in the
end it was quite simply settled by following the long established
specifications furnished by the Indiana Limestone Institute. There are
several thousand historic quarries in North America from which veneer
stone is still more or less available. While some of these local materials
may not be well supported by industry groups, they have all produced an
inventory of existing historic structures which can be observed and
analyzed to establish the character of the material and its requirements
for success. Secondly, he suggests building big. Unit mass helps the veneer to
achieve stability, dissipate heat and to disperse moisture. The ten-inch
thick, one-ton units of the Red Cross façade demonstrated these advantages
by being fifty years to the weather with no damage. The thickness of the
material also allowed extensive restoration and retrofitting that today's
much thinner commercial veneers would not have tolerated. The project
required the units to accept the installation of dutchmen to repair damage
from the dismantling, new kerf cuts to accept a new anchorage system and
the lamination of "liner" posts to provide full bearing to wider units
high in the wall. Volume means durability. Third, pay close attention to the waterproofing details. Avoid details
in the masonry or associated flashings which will promote concentrations
of runoff likely to cause the leaking and saturations similar to those of
the Sterling Library wall. Mr. Young {link to TB member page} believes
that most of the damage to that façade was produced by the failure of
systems dispersing runoff from the roof, not in the masonry itself. Even
so, flashing and weeps were introduced into the masonry at key points to
better manage water flow. Coping stone widths, projection pitches and drip
profiles all help determine the rate at which the wall will shed water.
Reproduction of successful details can forestall chronic problems. More
assertive details may not be advisable in very porous stone. In the same
vein, he further advises continued observation of the completed façade,
for a period, to determine if the building manages to shed water as
intended. A timely adjustment to a flashing or masonry detail can
sometimes prevent premature and unnecessary damage to the stone façade.
Attention is the key to performance.
Gothic Stone
Restoration
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