Slate Roofing: Innovation in an Old Industry; Part II – Standards, Codes, Testing, and More

Part I of this article, published in the
January 2017 issue of RCI Interface, discussed
new materials and installation techniques
that have been introduced to the
slate industry over the past 10 to 20 years.
Those are far from the only recent changes,
however. Part II will explore new and
retired sources of slate, code and standard
updates, new test data, and new resources
that can assist with design and installation
of traditional slate roofs.
Sources
In the industry’s heyday—around the
turn of the 20th century—there were as
many as 219 quarries in operation in the
United States.1 Many of them have long
since closed their doors. The last of the
quarries producing Peach Bottom slate and
Pennsylvania Hard-Vein slate closed in the
1940s and ’50s, respectively. Attempts in
the 1990s to reopen quarries in Monson,
Maine, which originally closed in the 1950s,
were unsuccessful. Within the past ten
years, a quarry producing unfading purple
slate in Newfoundland, Canada, closed (in
fact, no slate is quarried in Newfoundland
anymore). Veins of clear, unfading purple
slate in Vermont have dried up in the last
several years, as well. Currently, all domestic
purple slate is weathering (meaning
a certain percentage will turn shades of
tan and brown over time) and bears green
markings. The Penrhyn quarry in northwest
Wales is, at the time of this writing, the only
known source in the world for clear, unfading
purple roofing slate.
Given the information above, one may
be tempted to think that the slate industry
is gradually grinding to a halt. But that’s
not true. Production and installation of
slate is starting to turn around, although
slate still constitutes less than 1% of the
multibillion-dollar steep-slope roofing market.
In 1998, North Country Black slate,
produced by the Glendyne quarry in St.
Marc du Lac Long, Quebec, Canada, first
became available in the United States. The
slate is an unfading black with an expected
service life of 100 years. Penrhyn purple
slate, mentioned above, was reintroduced
in the U.S. in 2009 after being unavailable
for many decades. The James River Slate
Company opened in Arvonia, Virginia, in
2013, producing an unfading black slate
with a micaceous sheen and an expected
service life of 175 years or more. In addition,
just this year, the New England Slate
Company opened a quarry in Vermont producing
“Eagle Purple” slate, a semi-weathering
slate characterized by a range of purple
shades with some green markings.
The increase in slate use in recent
years is undoubtedly due, in part, to an
increase in imported slates. In this regard,
the industry has come full circle. Before
domestic quarries became prolific and productive
in the mid- to late 19th century,
the majority of roofing slate in the United
States was imported from Europe. Slate
was frequently used as ballast in the cargo
holds of ships originating from Europe. The
slate was off-loaded at U.S. ports to make
room for more valuable cargo for the return
trip. Even after slate began being produced
in the United States, Europe remained the
largest source of slate in this country until
railways became extensive enough to facilitate
transportation of slate from quarries to
building sites. From that point on, domestic
sources dominated the slate industry in
the United States…until recently. Imported
slate from other countries, including China,
Spain, and Brazil, are becoming increasingly
available in the U.S., a situation that has
brought concerns about testing and quality
to the forefront of the industry.
Not all slate is created equal, and not
all slate is suitable for roofing. Laboratory
testing, as laid out in ASTM C406, Standard
Specification for Roofing Slate, evaluates
the rate of water absorption, breaking load,
and depth of softening of natural slate,
assigning it to one of three grades: S-1 (over
75-year expected service life), S-2 (40- to
75-year expected service life), and S-3 (20-
3 0 • I n t e r f a c e F e b r u a r y 2 0 1 7
Innovation in
an Old Industry
By Julie Palmer
Part II – Standards, Codes, Testing, and More
Slate
Roofing:
to 40-year expected
service life).
The International
Building Code
(IBC) mandates
that roofing slate
comply with
ASTM C406, and
industry standards
advise that
only S-1 grade
slate should be
used for roofing. ASTM C406 also specifies
allowable criteria for dimensional tolerances
and allowances for breakage of roofing slate.
Other countries have different standard
criteria and testing procedures for
judging the quality of slate. As a result,
slate produced in other countries may meet
their standards, but not comply with ASTM
C406. Reputable distributors of imported
slate undertake ASTM testing on a regular
basis in order to ensure compliance and
will produce the test data on request. Still,
it seems that a substantial quantity of
slates with knots (rounded projections on
the slate’s surface), or cramps (“steps” in
the cleaved surface of a slate; Figure 1), as
well as warped slates (Figure 2) are slipping
through the cracks. These problems can
prevent slates from laying flat on a roof
(Figure 3). Raised butt ends make the slate
more susceptible to breaking under load
and can impact the roof’s watertightness by
increasing the risk of wind-driven rain penetrating
below the raised slates and entering
the roof system through nail holes or bond
lines in the underlying slates.
Another discrepancy in testing and
quality control involves iron pyrites. A significant
quantity of the slate produced in
other countries contains iron pyrite inclusions,
some of which will rust over time
when exposed to the elements. ASTM C406
does not currently include a test for oxidation.
Therefore, imported slate can comply
with ASTM C406 and still develop rust
stains after installation (Figure 4).
Code and Standard Changes
Even ASTM C406 has changed. The
standard includes three tests: C120, Flexure
Testing of Slate; C121, Water Absorption of
Slate; and C217, Weather Resistance of
Slate. In 2005, the test method prescribed
by ASTM C120 was updated to use a
breaking load requirement rather than the
previous modulus of rupture. The test now
requires slates to achieve a minimum breaking
load of 575 pounds of force rather than
the 9000-psi modulus of rupture required
previously. The change was intended to
resolve a number of problems with the modulus
of rupture test, including that multiple
tests on one specimen could produce widely
varied results; and the mathematical formula
used to derive the modulus of rupture
placed slate thickness in the denominator,
thereby making thicker slates appear less
durable than thinner ones.
Another change in 2015 redefined the
thickness of “standard” slate as nominal ¼
in. to better reflect the sizes that domestic
quarries produce. Prior to 2015, the standard
had specified a nominal thickness of
3/16 to ¼ in., which was based on industry
standards dating back to the 1920s. Today,
quarries in the U.S. largely produce roofing
slate in the ¼- to 3/8-in.-thick range, with
very few, if any, producing true 3/16-in.-thick
slates to the previous ASTM criteria.
The IBC has changed in recent years,
as well. Prior to 2009, the IBC recognized
slate roofs as Class A fire-rated assemblies,
no matter what substrate the slate was
installed over. Per ASTM E108, a Class A
F e b r u a r y 2 0 1 7 I n t e r f a c e • 3 1
Figure 1 – A cramp on the
underside of this slate shingle
caused it to stick up and rock
from side to side when installed
on the roof.
Figure 2 – Warped slates like this
one do not lay flat on a roof and
should not be installed.
Figure 3 – Numerous slates with raised butt ends detract, aesthetically, from a building’s
appearance and may impact the longevity and watertightness of the roof system.
rating means the roof covering is “effective
against severe test exposure, affords a high
degree of fire protection to the roof deck,
does not slip from position, and does not
present a flying brand hazard.”2 The 2009
and 2012 IBC only accept slate installed
over noncombustible roof decks, such as
concrete, as Class A assemblies. In municipalities
that have adopted those versions
of the IBC, slate installed over combustible
roof decks, such as wood, must be backed
up with ASTM E108 or UL 790 test data
to document the fire rating the system can
achieve.
In 2010, the National Slate Association
(NSA), in partnership
with the National Roofing
Contractors Association
(NRCA), sponsored fire
testing of slate roofing
in compliance with
UL 790, Standard Test
Methods for Fire Tests of
Roof Coverings (Figures 5
and 6). The sample roof
assembly for the test was
comprised of a ½-in.-
thick plywood deck, a
single layer of #30 felt
underlayment, and
¼-in.-thick, S-1 grade
North American slate
shingles.
The assembly performed to Class A
standards.3 As a result of this testing, NRCA
was able to work with the International
Code Council to have the Class A rating
for slate installed over combustible roof
decks (with ASTM-approved underlayment)
reinstated in the 2015 version of the IBC.
In municipalities that are governed by the
2009 or 2012 IBC, local building code officials
have the authority, per Section 104.11
of the IBC, to accept slate installed over a
combustible roof deck as a Class A system,
if they determine that the test data indicate
that the proposed slate roof system is
equivalent to the Class A fire-rated systems
included in the code.
Testing
The ability of slate
to resist hail impact and
wind uplift has also been
questioned in recent
years, inspiring the NSA
to pursue additional testing.
In 2010, hail resistance testing of slate
roofing was conducted in compliance with
FM 4473, Specification Test Standard for
Impact Resistance Testing of Rigid Materials
by Impacting With Freezer Ice Balls. A slate
roof test deck, covered with S-1 grade North
American slate shingles, was subjected to
impact from ice balls fired from a compressed
air cannon. Three-eighths-inchthick
slate met Class 4 requirements (FM’s
highest hail resistance rating), showing
no signs of damage following the impact
of 2-in. ice balls traveling at 76 mph; and
¼-in.-thick slate met Class 3 requirements,
withstanding the impact of 1¾-in. ice balls
traveling at 69 mph.4
Prior to 2013, there was no wind uplift
test for slate. ASTM D3161, Standard Test
Method for Wind-Resistance of Asphalt
Shingles (Fan-Induced Method), applied only
to asphalt shingles. In 2013, the standard
was changed to apply to a much wider array
of steep-slope products. As such, the NSA
was able to sponsor wind uplift testing of
slate in 2015. Two test decks covered with
S-1 grade slate shingles from a variety of
North American quarries were subjected
to 110 mph winds, which is the highest
wind speed specified by the ASTM test. The
slate shingles exhibited no damage at the
completion of the two-hour test, thereby
meeting the Class F requirements. The wind
speed was then increased by 10 mph every
ten minutes, up to 150 mph, in compliance
with FM Approvals Class Number 4475.
Both panels, again, exhibited no damage.
Finally, the testing laboratory increased the
fan speed to 160 mph (the highest wind
speed their equipment could achieve), and
the slate still did not fail.5
One important thing to note about these
tests was that the slates were secured to the
plywood substrate
with smoothshank
copper
nails. Smoothshank
nails, as
opposed to ringshank
nails, run
the greatest risk
of pulling out of a
wood deck under
wind uplift pressure,
so these tests
show that even
in the worst-case
scenario, slate can
withstand 160-
mph winds.
3 2 • I n t e r f a c e F e b r u a r y 2 0 1 7
Figure 5 – Burning brand test, per UL 790, in
progress on a slate roof test deck. The “brand”
burned for about 18 minutes and reached
temperatures in excess of 2000°F.
Figure 6 – Slate roof test deck
following the burning brand test.
Figure 4 – Rainwater has washed particles of oxidizing iron
pyrite down the roof, leaving a very visible rust stain trail.
Resources
New resources to assist with proper
design, specification, and installation of
slate roofs have become available within the
last six years. Until recently, Slate Roofs,
published in 1926 by the original NSA,
was the main industry reference containing
technical information about installing
a slate roof and flashing common details.
Over the years, the 1926 book was republished
by slate quarries, the flashing details
showed up in other publications, and many
outline specifications referred to it. The
breadth of the 1926 book was somewhat
limited and, as can be imagined, some of the
information was a bit outdated by today’s
standards. For instance, at the time, there
was no industry-wide consensus as to what
type of nails were best for securing slate
shingles. Instead, the NSA stated their hope
“that research in this field may be undertaken
in the near future and definite results
furnished those interested.”6 Ninety years
later, practical experience has led to copper
nails being the most widely used in slate
roofing—both because their service life is
comparable to that of slate, and their relative
softness facilitates slate repairs.
In 2010, the NSA published Slate Roofs:
Design and Installation Manual, consisting
of nearly 300 pages of technical information,
from basic geology to instructions for
laying out a slate roof, descriptions of slate
roofing tools, recommendations for flashing
materials, and discussion and illustration
of proper soldering techniques. The book
also includes more than 100 pages of detail
drawings, ranging from basic underlayment
F e b r u a r y 2 0 1 7 I n t e r f a c e • 3 5
Figure 7 – Examples of detail drawings included in Slate Roofs: Design and Installation
Manual. Reprinted with permission from the National Slate Association.
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installation, to typical flashing details, to
appropriate repair techniques, and even
including some unusual details like installing
slate on a turret (Figure 7).
Even more recently, the NSA has
launched a mobile field guide containing
layer-by-layer animations of the most
common installation details, which can
be accessed on mobile devices.7 Building
owners, contractors, and designers can now
have slate roof details literally at their fingertips
anytime and anywhere. The NSA has
also introduced a double-sided illustrated
card designed to be affixed to slate pallets
by quarriers and distributors to give the
end-user basic slate installation and handling
instructions, as well as directing them
to other sources of details and information.
Now, more than ever, the information and
guidelines needed to properly install slate
roofs are readily at hand.
Conclusion
Sources of slate have always been, and
likely will always be, a frequent variable
within the industry. Some domestic quarries
close, and others open on a regular
basis. Today, quarries in foreign countries
are providing more slate to the U.S. than
they have since the early 19th century.
While the importation of slate is not a new
phenomenon, it is being imported from
more varied countries than ever before,
thereby bringing new issues and considerations
to the forefront.
Testing, standards, building codes, and
publications have adapted and must continue
to do so in order to keep up with changing
concerns and technology—even when
they focus on a roofing material or installation
that has been in use for centuries.
Many recent changes in the slate industry,
including code and standard changes, testing,
and new resources, are geared toward
supporting S-1 grade natural slate and traditional
slate roof installation, which have
proven, over and over again, an ability to
deliver service lives in excess of 75 years.
New materials and installation techniques
are constantly appearing and disappearing
from the market. Some may prove
to be beneficial to the slate roofing industry.
The best indicator of reliability, however,
will always be the test of time. Until new
materials and alternative installation methods
are able to demonstrate a long history
of successful, reliable performance, their
use should be approached with caution and
a thorough understanding of the potential
risks.
References
1. Jeffrey S. Levine. A History of the
United States Slate Industry. (Thesis,
Cornell University, 1988). p. 329.
2. ASTM Standard E108, 2011, Standard
Test Methods for Fire Tests of
Roof Coverings, ASTM International,
West Conshohocken, PA, 2003, DOI:
10.1520/E0108-11, www.astm.org.
3. More information, official test results,
and photographs of the testing
in action are available at: http://
slateassociation.org/slate-testing/,
http://levineco.net/wp-content/
uploads/2010/12/RidgewalkerNews-
V5N1-FireTesting1.pdf, and http://
docserver.nrca.net/technical/9562.
pdf. It was concluded that the felt
underlayment did play a role in the
Class A fire rating.
4. More information, official test
results, photographs, and a video
of the testing in progress are available
at: http://slateassociation.org/
slate-testing/.
5. More information is available at:
http://slateassociation.org/wp-content/
uploads/2016/07/white.
paper_wind.test_.pdf.
6. National Slate Association. Slate
Roofs (Poultney, VT: National Slate
Association, 1926). p. 25.
7. NSA’s mobile field guide can be accessed
at mobile.slateassociation.
org.
3 6 • I n t e r f a c e F e b r u a r y 2 0 1 7
Julie Palmer is a
roof consultant with
Levine & Company,
Ardmore, PA, and
has 14 years of
experience with
roof restoration
and rehabilitation
projects for existing
and historical buildings.
She served as
the National Slate
Association’s office
manager for eight years and produced drawings
for the Association’s Slate Roofs: Design
and Installation Manual and mobile field
guide. Palmer has a master’s in historic
preservation from Columbia University and a
master’s in architecture from the University of
Pennsylvania.
Julie Palmer
© Can Stock Photo / Niyazz
Judge Blocks
Overtime Rule
The U.S. Department of Labor’s (DOL’s) Overtime Rule, which was to have
taken effect on December 1, has been blocked by a federal judge in Texas,
who granted a preliminary injunction on November 22 in a lawsuit challenging
the DOL’s authority to raise the salary threshold. Judge Amos Mazzant of the
U.S. District Court for the Eastern District of Texas granted the injunction after
50 business groups and 21 states brought a lawsuit claiming that the DOL
exceeded its authority by raising the salary threshold too much and providing
automatic updates to the threshold without stakeholder input. On December
1, the DOL appealed the decision to the 5th Circuit Court.
The Fair Labor Standards Act’s (FLSA’s) salary threshold for exemption
from overtime pay would have been raised from $23,660 to $47,476, or $455
per week to $913 per week. The Overtime Rule was last updated in 2004.
Response briefs were due before January 17, 2017.