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Roofing Slate standards are Changing

May 15, 2003

May 2003 Interface • 21
At the Annual Meeting of the National Slate Association
(NSA) on February 13, 2003, in New Orleans, two important
things about ASTM roofing slate standards were
agreed upon: The ASTM C 406 “Standard Specifications for
Roofing Slate” and the standard test methods need improvement.
The standards as they currently exist can be difficult to interpret
by quarries and testing labs and are of limited use to designers,
consultants, contractors and owners. What’s wrong with them?
• The ASTM C 406 stated service lives of S1 for more than
75 years, S2 for 40 to 75 years, and S3 for 20 to 40 years,
are unrealistically low. Also, there is a poor correlation
between the standards and the quality of a slate. There are
slates that have performed well for over 150 years; yet,
despite their proven reputation for durability, they are
unable to comply with the ASTM standards.
• The Modulus of Rupture test method, ASTM C 120, which
is currently undergoing revision, has placed undue emphasis
on the thinness of the slate, making it difficult if not
impossible for thicker slate to qualify. ASTM is exploring
the use of a breaking load test to accommodate stone that
cannot be practically split to 3/16 to 1/4 inch thickness.
• The Absorption test, ASTM C 121, considers initial water
absorption, but research shows there is a poor correlation
between this factor and life expectancy and that the rate of
increase in water absorption during weathering is more
• The way a small test sample is cut is known to affect test
results. The saw used and the edge preparation are critical
in order to avoid poor results due to micro fracture damage.
• Non-adherence to ASTM test methods and the uniqueness
of testing slate result in different labs showing different
results on similar stone. This encourages lab shopping.
Identifying preferred testing labs may help level the playing
• There is also a normal range of variance inherent in any
natural material. Some natural material standards, such
as for wood shingles, have a stated allowance for this variation.
A standard is of value only to the extent to which it can assess
the quality and durability of a slate objectively. Quarries will not
stop producing slate with a known and proven reputation for
quality that is non-conforming with the standard. Instead, it is in
a company’s interest to ignore and avoid the standard. Contractors,
designers, consultants, and owners also don’t want to use
the standard. If they do, they end up relying on a standard that
has poor correlation to performance and quality and they may get
themselves in trouble. Noncompliance with ASTM has been used
by some unscrupulous or uninformed owners or owner’s representatives
to disrupt construction and avoid payment for good
material and for construction services. An effort should be made
to articulate the problems with ASTM C 406 as a warning to
users, to encourage change, and to disarm those who would use
non-conformance ruthlessly. An interim alternative to using only
the ASTM Standards for material quality assurance would be to
specify particular reputable quarries and stone types or to require
that the slate has a satisfactory long-term performance record
that is acceptable to the designer/specifier. Knowledgeable and
reputable quarrymen and primary distributors are available to
help with this process.
Of course, inadequate standards may be better than no standards.
This is especially true at a time when roofing slate is
becoming part of a world economy, and not all worldwide producers
are familiar with traditional U.S. and Canadian expectations
for quality and service life that are based on our specific roofing
methods and climates.
The limitations of existing standards are a problem in North
America as well as Europe. In Europe, the lack of correlation
between national standards and proven performance quality is a
major concern. An important effort is being put into a new
European standard, European Committee for Standardisation
(CEN) 1999, “Slate and Stone Products for Discontinuous Roofing
and Cladding, European Standard prEN12326, Part 2 Methods of
Test (draft).”
22 • Interface May 2003
The NSA Standards Committee is developing a strategy for
working with ASTM and improving the existing standard. All four
members of the ASTM Technical Committee C 18 representing the
roofing slate industry are NSA members. Improvements in ASTM
C 120, the modulus of rupture test method, are in process to
eliminate the bias against thicker slate. The terminology “weathering,
semi-weathering, and unfading” also needs attention. The
development of the new European Standard is being watched
closely for its possible application in North America and its foreshadowing
of an eventual international standard identifying several
grades of acceptable roofing slate.
The National Slate Association’s Standards Committee
includes: Pete Papay, Penn Big Bed Slate Company, Inc.;
Jonathan Hill, Greenstone Slate; Judith Selwyn, Preservation
Technology Associates, Inc.; John Conlon; Chris Bean, Evergreen
Slate Company, LLC; David Large, North Country Slate; and
Chuck Smid, The New England Slate Company. Input on roofing
slate standards is appreciated from all who are interested.
Contact NSA at 1-866-256-2111. 
Matt Millen is a principle with Millen
Roofing Company, Milwaukee, WI, a
110-year-old firm specializing in slate
and tile roofing. He may be reached
at He serves
as the President of the National Slate
Association. NSA was first established
in 1922 and was reorganized
in 2002. It includes contractors,
quarries, distributors, designers, consultants,
owners, and all others concerned
with promoting excellence in
slate roofing practices. Visit
An amended version of section 18-13-303, Urban Heat
Islands, of the Chicago Energy Conservation Code was
accepted on January 16, 2003, and will go into effect on
April 1, 2003. Counterproposals and information provided by
the Metal Construction Association (MCA) helped to amend
the previous code proposal that would have inhibited the use
of metal roofing. The amendment now pertains only to lowslope
roofing. Steep slope roofing was deliberately omitted for
now. A phased-in approach and emittance criteria were eliminated.
The initial minimum reflectance criteria were also
lowered. The specific reference to “metal” roofing was eliminated.
The ENERGY STAR® performance standards were established
as the criteria as of January 1, 2009. Metal roofing
can comply with the new language in the amended code.
The applicable specific language is as follows:
Section 18-13-303.2.1 Roofing materials used in
roofs with slopes of 0:12 to 2:12 shall meet the following
1. Roofs installed prior to and including 12/31/08
shall have a minimum solar reflectance, both
initial and weathered, of 0.25.
2. Roofs installed after 12/31/08 shall utilize roofing
products that meet or exceed the minimum
criteria to quality for an ENERGY STAR® label as
designated by the U.S. EPA ENERGY STAR® program.
SPRI, the organization representing
sheet membrane and component suppliers
to the commercial industry, has
issued an updated version of its wind
design standard. In accordance with
guidelines set by the American National
Standards Institute (ANSI), SPRI recanvassed
its revised wind design standard,
entitled ANSI/SPRI RP-4 2002,
“Wind Design Standard for Ballasted
Single-Ply Roofing Systems.” The original
standard was issued in 1985.
Subsequently, the standard was incorporated
in the BOCA (Building Officials
and Code Administrators International
Inc.) and SBCCI (Southern Building
Code Congress International, Inc.)
codes is and now included in the
International Code Council’s (ICC)
building code.
This updated standard incorporates
minor adjustments to its wind tables
based on revised data from the
American Society of Civil Engineers
(ASCE) 7-98 guidelines, SPRI Technical
Director Dave Roodvoets explained.
The full text of all approved
ANSI/SPRI standards, along with an
array of other handy resources, are
available on CD-ROM in SPRI’s comprehensive
reference manual, Flexible
Membrane Roofing: A Professional’s
Guide to Specifications. Single copies of
the standard itself are also available
from SPRI. Phone 781-444-0242 for
more information.
SPRI Publishes