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Standards for Testing Roofing Slate
By Jonathan Hill, December 2003
It is common knowledge within the slate industry that identical samples prepared from the same source piece of slate will undoubtedly receive different ASTM C-406 values when tested by different labs. This posses a serious problem to all of those who use slate, specify slate, and, more importantly, manufacturer slate. A standard is only useful if it can consistently provide accurate information. When a standard becomes unpredictable, it can no longer serve as an industry's yardstick. The National Slate Association's Standards Committee has spent the last several months investigating the issues associated with the physical testing of roofing slate. The group has attempted to identify the underlying reasons why roofing slate test results can very so widely.
The NSA Standards Committee investigation has lead to a primary conclusion.
Precision takes practice. When describing the techniques required to accurately
complete the ASTM C-406 test, the analogy to cooking can be made. It takes
skill and technique to prepare a soufflé; the same is true to accurately
perform the test series for natural roofing slate. The fact that one can read
a recipe does not make that person a great chef. Many testing firms possess
the technicians and tools necessary to perform the test, but do not get the
opportunity to become proficient with it. A large firm may only test
slate several times a year. They may always use the same equipment, but seldom
the same technician. This is no fault of the lab, but rather an issue
of economy.
Most of ASTM's thousands of test methods can be replicated by performing a
series of objective procedures. But, just as too much salt can ruin a soufflé,
the ASTM C-406 test series can be flawed by point loading, or by inaccurate
orientation of a sample's grain. The sample can be shocked by over accelerating
the pressure asserted by the load cell, or the technician may have inadvertently
damaged the sample while preparing it with a segmented saw. None of these
issues are addressed in the ASTM C-406 recipe. Rather, their recognition is
at the discretion of the technician. It is this seemingly minor subjection
that causes most slates to be improperly tested. Without a wealth of slate
testing experience to draw upon, many technicians simply read the recipe
and test the slates based upon erroneous procedures.
The few technicians that have extensive experience can determine from the
fractured remnants created by the destructive Modulus of Rupture test, whether
the slate was on grain or whether it was off grain. The resulting fragments
of stone may add or detract from the validity of the test results. The experienced
technician will then make a conscious effort to include or disregard the sample
from the results. This decision is truly contrary to the ASTM recipe,
which reads six samples are tested, and six results are tabulated. The recipe
does not account for the specific idiosyncrasies that make slate roofing so
unique. The determination of a roofing slate's anticipated longevity is not
merely an objective practice but requires great skill and subjective reporting
based upon experience. To assume that anyone can read the recipe and create
this soufflé is incorrect.
From the lab technician's standpoint, the recipe is quite easily read, and
performed. Upon tabulating the results of the ASTM test, the technician merely
grades the slate according to its performance. To the technician the results
are indisputable. If the slate meets the current standard, it is a viable
product. If it fails, then it is undoubtedly inferior. This latter judgment
cannot be assumed to be true without a firm grasp of the history behind ASTM
C-406.
The history of ASTM C-406 is well documented. The test was conceived in the 1930's as a method to grade the various types of roofing slates produced in the United States. The ASTM C-406 test series consisted of three tests: ASTM C-120 Modulus of Rupture, ASTM C-121 Water Absorption, and ASTM C-217 Depth of Softening. Each test was designated to replicate the most common causes of slate failure.
The Depth of Softening –test is based on the fact that slates containing pyrite, calcite, and carbon undergo a chemical weathering which results in the conversion of the calcite particles to gypsum. The swelling action that results causes disintegration of the slate. The extent of this action on various slates in the test has been found to correlate with the durability of the materials in actual weathering” (ASTM C-406-58).
The Water Absorption test attempts to quantify the percentage of water a slate absorbs after being immersed for 48 hours. The correlation has been made that the greater the absorption percentage the more susceptible the slate is to freeze thaw cycles, which would directly reduce the expected service life of the slate.
The above-mentioned tests attempt to calculate the detrimental effects of erosion on slate. Like the more predictable tests associated with ASTM, they have remained relatively unchanged since their inception in the late 1940's. For the most part, they can be preformed from the ASTM recipe book by any technician on hand. They are often regarded as the two most dependable methods of determining a slate's potential longevity.
The third component of the ASTM C-406 series has proven to be more unpredictable. The Modulus of Rupture test attempts to simulate the effects of foot traffic and hail on natural roofing slate. The test tries to quantify the amount of load that can be placed upon a slate before it fails. The assumption is made that more impervious materials can withstand more force. Their increased durability would presumably result in increased longevity upon installation.
Unlike the absorption and depth of softening tests, the modulus of rupture test focused upon the manufacturing practices used by quarries to produce roofing slate. The absorption and softening levels associated with a slate are intrinsic mineral properties that cannot be manipulated by manufacturing technique. However, a quarry can modify the thickness of a slate, and the results of this modification can have great bearing on the reported durability of the slate.
Historically, quarries preferred to produce slates of 3/16” thickness. This thickness reduced the overall weight of the product, which resulted in a reduction in the cost of construction and transportation. Transportation was a major contributor to the rising cost of roofing slate during the 1930's and 1940's. With a very limited local infrastructure and dependence on the rail system, the slate quarries of the 1930's were forced to undertake methods of economy that would help sustain the industry's viability. The slate quarries were facing unprecedented competition from various new forms of lightweight roofing products. Thickness reduction by the quarry was the easiest method to reduce transportation costs. The benefits are very straightforward. The often-unrecognized advantage to the quarry was an increase in production yield. Quarries could greatly increase their yield from the stone by simply splitting the slate even thinner than 3/16”. This practice led some quarries to produce slate so thin that it made installation very difficult.
The ASTM Modulus of Rupture test was designed to help safeguard the roofing industry from slates that were too thin to use. As a result of its historical success, the 3/16” slate was used as the industry benchmark. Slates of this thickness were referred to as –slate tiles for ordinary service” by ASTM and the roofing industry referred to the same as –commercial standard.” The assumption was made that since 3/16” slates could withstand the test of time, all slates, regardless of thickness, would need to possess the same physical standards.
The theory had merit, and the United States Bureau of Commerce began to undertake the task of standardizing U.S. roofing slates. In an effort to perform this onerous task, Dr. William Kessler was commissioned. His work was to become monumental in establishing the ASTM standard for commercial grade roofing slate. Kessler would attempt to quantify the properties that were assumed to be the driving forces behind a slate's expected longevity.
Kessler spent the better part of a decade analyzing the various U.S. slates. By applying accepted physical formulas to slate, he was able to make objective physical comparisons between the different quarries. Utilizing scientific methods, he was able to establish levels of absorption and softening that were common in all slates with an expected 75-year service life. These values have withstood repeated scrutiny and can be replicated by –recipe” in any lab with a high degree of certainty.
His finding and analysis of the Modulus of Rupture were far more subjective. Kessler arbitrarily introduced the standard level of 9,000 psi for 3/16” –commercial standard” thickness slates. This number was determined after his experiments indicated that all slates of U.S. origin would meet or exceed this level. The standard was not to be applied to slates of greater thickness. The original standard specifically defined –thicknesses of slate tiles for ordinary service are 3/16 in.; thicknesses of tiles for promenade decks and extraordinary service are 1/4 and 3/8 in.” Kessler had no scientific basis that the 9,000 psi level would indeed lead to a 75-year serviceable life. The level was purely derived by his assumption and the actual requirement was probably substantially lower.
Kessler also included language within the test procedure that would allow the laboratory technician to disregard the results of tested samples based upon physical imperfections. He was very specific in stating that samples that performed at a level that was 25% lower than the rest of the group should be reviewed for physical flaws. He also suggested that a minimum of six samples be tested.
From the beginning, it was apparent to Kessler that the Modulus of Rupture standard would not be a simple objective –recipe.” The testing community accepted this fact, and Kessler's original test procedure remained relatively unmodified until the mid 1980's. At the time, numerous ASTM committees attempted to make the test less subjective by making the lab procedure and reporting process more stringent.
Upon comparison, it is quite evident that the original text of the ASTM Modulus of Rupture test is vastly different than that of the 2003 standard. Some of these modifications were merely rewordings of the original language. However, many of the changes have substantial implications to the validity of the test itself. It is impossible to make a subjective test objective simply by changing the test procedures when the comparative results of the test were arbitrary to begin with (9000 lbs.). The intentions of the committees were warranted and were made in good faith; however, their results can be considered disastrous to the roofing slate community.
At present, the term ASTM C-406 compliant is synomous with the term undefendable lawsuit. The ASTM standards on slate produce such unpredictable results that many quarries are discounting the whole test series and merely giving a warranty. This practice has limited the often-frivolous lawsuits associated with installing roofing slate, but it has left contractors, architects, and homeowners scrambling for information. Without the confidence created by testing standards many projects are foregoing the use of slate for more predictable products. ASTM C-406 has become the slate industries Achilles heel.
Currently, ASTM is addressing the issues of Modulus of Rupture. The association is attempting to revamp this often miscalculated test. Their work is diligent and thorough. Hopefully, this effort will create a more practical and replicatable testing method. The unfortunate side effect of their diligence is that ASTM is slow to change. It might take years for a new or modified test to be ratified.
In the meantime, NSA is attempting to devise a plan that would utilize the existing ASTM standards and concurrently provide accurate testing results. One direction under consideration is the creation of NSA approved testing firms. It is common knowledge that there are several very experienced technicians of the ASTM C-406 standard. These individuals and their firms could be recognized as NSA approved laboratories and provide the industry with reliable test results.
The work of NSA's Standards Committee is ongoing. We will provide you with continued updates. For more information please contact Mr. Pete Papay, Chair, NSA Standards Committee.