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

Slate ROOfIng: Innovation in an Old Industry Part II – Standards, Codes, Testing, and More By Julie Palmer

Part I of this article, published in the tan and brown over time) and bears green increase in imported slates. In this regard, January 2017 issue of RCI Interface, dis markings. The Penrhyn quarry in northwest the industry has come full circle. Before cussed new materials and installation tech Wales is, at the time of this writing, the only domestic quarries became prolific and pro niques that have been introduced to the known source in the world for clear, unfad ductive in the mid- to late 19th century, slate industry over the past 10 to 20 years. ing purple roofing slate. the majority of roofing slate in the United Those are far from the only recent chang Given the information above, one may States was imported from Europe. Slate es, however. Part II will explore new and be tempted to think that the slate industry was frequently used as ballast in the cargo retired sources of slate, code and standard is gradually grinding to a halt. But that’s holds of ships originating from Europe. The updates, new test data, and new resources not true. Production and installation of slate was off-loaded at U.S. ports to make that can assist with design and installation slate is starting to turn around, although room for more valuable cargo for the return of traditional slate roofs. slate still constitutes less than 1% of the trip. Even after slate began being produced multibillion-dollar steep-slope roofing mar in the United States, Europe remained the SouRCeS ket. In 1998, North Country Black slate, largest source of slate in this country until In the industry’s heyday—around the produced by the Glendyne quarry in St. railways became extensive enough to facili turn of the 20th century—there were as Marc du Lac Long, Quebec, Canada, first tate transportation of slate from quarries to many as 219 quarries in operation in the became available in the United States. The building sites. From that point on, domestic United States.1 Many of them have long slate is an unfading black with an expected sources dominated the slate industry in since closed their doors. The last of the service life of 100 years. Penrhyn purple the United States…until recently. Imported quarries producing Peach Bottom slate and slate, mentioned above, was reintroduced slate from other countries, including China, Pennsylvania Hard-Vein slate closed in the in the U.S. in 2009 after being unavailable Spain, and Brazil, are becoming increasing 1940s and ’50s, respectively. Attempts in for many decades. The James River Slate ly available in the U.S., a situation that has the 1990s to reopen quarries in Monson, Company opened in Arvonia, Virginia, in brought concerns about testing and quality Maine, which originally closed in the 1950s, 2013, producing an unfading black slate to the forefront of the industry. were unsuccessful. Within the past ten with a micaceous sheen and an expected Not all slate is created equal, and not years, a quarry producing unfading purple service life of 175 years or more. In addi all slate is suitable for roofing. Laboratory slate in Newfoundland, Canada, closed (in tion, just this year, the New England Slate testing, as laid out in ASTM C406, Standard fact, no slate is quarried in Newfoundland Company opened a quarry in Vermont pro Specification for Roofing Slate, evaluates anymore). Veins of clear, unfading purple ducing “Eagle Purple” slate, a semi-weather the rate of water absorption, breaking load, slate in Vermont have dried up in the last ing slate characterized by a range of purple and depth of softening of natural slate, several years, as well. Currently, all domes shades with some green markings. assigning it to one of three grades: S-1 (over tic purple slate is weathering (meaning The increase in slate use in recent 75-year expected service life), S-2 (40- to a certain percentage will turn shades of years is undoubtedly due, in part, to an 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 to 40-year expect Figure 1 – A cramp on the ed service life). underside of this slate shingle The International caused it to stick up and rock Building Code from side to side when installed (IBC) mandates on the roof. that roofing slate comply with Figure 2 – Warped slates like this ASTM C406, and one do not lay flat on a roof and industry stan should not be installed. dards advise that only S-1 grade slate should be used for roofing. ASTM C406 also specifies ing load of 575 pounds of force rather than allowable criteria for dimensional tolerances the 9000-psi modulus of rupture required and allowances for breakage of roofing slate. previously. The change was intended to Other countries have different stan resolve a number of problems with the mod dard criteria and testing procedures for ulus of rupture test, including that multiple judging the quality of slate. As a result, tests on one specimen could produce widely slate produced in other countries may meet varied results; and the mathematical for their standards, but not comply with ASTM mula used to derive the modulus of rupture C406. Reputable distributors of imported placed slate thickness in the denominator, slate undertake ASTM testing on a regular thereby making thicker slates appear less basis in order to ensure compliance and durable than thinner ones. will produce the test data on request. Still, Another change in 2015 redefined the it seems that a substantial quantity of thickness of “standard” slate as nominal ¼ slates with knots (rounded projections on in. to better reflect the sizes that domestic the slate’s surface), or cramps (“steps” in quarries produce. Prior to 2015, the stan the cleaved surface of a slate; Figure 1), as dard had specified a nominal thickness of well as warped slates (Figure 2) are slipping 3/16 to ¼ in., which was based on industry through the cracks. These problems can standards dating back to the 1920s. Today, prevent slates from laying flat on a roof quarries in the U.S. largely produce roofing (Figure 3). Raised butt ends make the slate slate in the ¼- to 3/8-in.-thick range, with more susceptible to breaking under load very few, if any, producing true 3/16-in.-thick and can impact the roof’s watertightness by slates to the previous ASTM criteria. increasing the risk of wind-driven rain pen The IBC has changed in recent years, etrating below the raised slates and entering as well. Prior to 2009, the IBC recognized the roof system through nail holes or bond slate roofs as Class A fire-rated assemblies, lines in the underlying slates. no matter what substrate the slate was Another discrepancy in testing and installed over. Per ASTM E108, a Class A quality control involves iron pyrites. A sig nificant quantity of the slate produced in other countries contains iron pyrite inclu sions, some of which will rust over time when exposed to the elements. ASTM C406 does not currently include a test for oxida tion. 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 Figure 3 – Numerous slates with raised butt ends detract, aesthetically, from a building’s requires slates to achieve a minimum break appearance and may impact the longevity and watertightness of the roof system.

f e b r u a r y 2 0 1 7 I n t e r f a c e • 3 1 (NSA), in partnership ing. In 2010, hail resistance testing of slate with the National Roofing roofing was conducted in compliance with Contractors Association FM 4473, Specification Test Standard for (NRCA), sponsored fire Impact Resistance Testing of Rigid Materials testing of slate roof by Impacting With Freezer Ice Balls. A slate ing in compliance with roof test deck, covered with S-1 grade North UL 790, Standard Test American slate shingles, was subjected to Methods for Fire Tests of impact from ice balls fired from a com Roof Coverings (Figures 5 pressed air cannon. Three-eighths-inch- and 6). The sample roof thick slate met Class 4 requirements (FM’s assembly for the test was highest hail resistance rating), showing comprised of a ½-in.- no signs of damage following the impact thick plywood deck, a of 2-in. ice balls traveling at 76 mph; and single layer of #30 felt ¼-in.-thick slate met Class 3 requirements, underlayment, and withstanding the impact of 1¾-in. ice balls Figure 4 – Rainwater has washed particles of oxidizing iron ¼-in.-thick, S-1 grade traveling at 69 mph.4 pyrite down the roof, leaving a very visible rust stain trail. North American slate Prior to 2013, there was no wind uplift shingles. test for slate. ASTM D3161, Standard Test rating means the roof covering is “effective The assembly performed to Class A Method for Wind-Resistance of Asphalt against severe test exposure, affords a high standards.3 As a result of this testing, NRCA Shingles (Fan-Induced Method), applied only degree of fire protection to the roof deck, was able to work with the International to asphalt shingles. In 2013, the standard does not slip from position, and does not Code Council to have the Class A rating was changed to apply to a much wider array present a flying brand hazard.”2 The 2009 for slate installed over combustible roof of steep-slope products. As such, the NSA and 2012 IBC only accept slate installed decks (with ASTM-approved underlayment) was able to sponsor wind uplift testing of over noncombustible roof decks, such as reinstated in the 2015 version of the IBC. slate in 2015. Two test decks covered with concrete, as Class A assemblies. In munic In municipalities that are governed by the S-1 grade slate shingles from a variety of ipalities that have adopted those versions 2009 or 2012 IBC, local building code offi North American quarries were subjected of the IBC, slate installed over combustible cials have the authority, per Section 104.11 to 110 mph winds, which is the highest roof decks, such as wood, must be backed of the IBC, to accept slate installed over a wind speed specified by the ASTM test. The up with ASTM E108 or UL 790 test data combustible roof deck as a Class A system, slate shingles exhibited no damage at the to document the fire rating the system can if they determine that the test data indi completion of the two-hour test, thereby achieve. cate that the proposed slate roof system is meeting the Class F requirements. The wind In 2010, the National Slate Association equivalent to the Class A fire-rated systems speed was then increased by 10 mph every included in the code. ten minutes, up to 150 mph, in compliance with FM Approvals Class Number 4475. teSting Both panels, again, exhibited no damage. The ability of slate Finally, the testing laboratory increased the to resist hail impact and fan speed to 160 mph (the highest wind wind uplift has also been speed their equipment could achieve), and questioned in recent the slate still did not fail.5 years, inspiring the NSA One important thing to note about these to pursue additional test tests was that the slates were secured to the plywood substrate with smooth- shank copper nails. Smooth- shank nails, as opposed to ring- shank nails, run the greatest risk Figure 5 – Burning brand test, per UL 790, in of pulling out of a progress on a slate roof test deck. The “brand” wood deck under burned for about 18 minutes and reached wind uplift pres temperatures in excess of 2000°F. sure, so these tests show that even in the worst-case scenario, slate can Figure 6 – Slate roof test deck withstand 160- following the burning brand test. 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 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, RhinoBond – The World’s Leading was the main industry reference contain ing technical information about installing a slate roof and flashing common details. Induction Based Roofing System Over the years, the 1926 book was repub lished 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 • 1,000’s of Projects. shingles. Instead, the NSA stated their hope “that research in this field may be under taken in the near future and definite results Figure 7 – Examples of detail drawings included in Slate Roofs: Design and Installation furnished those interested.”6 Ninety years Manual. Reprinted with permission from the National Slate Association. • 1.5 Billion Sq. Ft. installed. later, practical experience has led to copper In 2010, the NSA published Slate Roofs: roofing tools, recommendations for flashing nails being the most widely used in slate Design and Installation Manual, consisting materials, and discussion and illustration roofing—both because their service life is of nearly 300 pages of technical informa- of proper soldering techniques. The book comparable to that of slate, and their rela tion, from basic geology to instructions for also includes more than 100 pages of detail • 5,000+ Tools. tive softness facilitates slate repairs. laying out a slate roof, descriptions of slate drawings, ranging from basic underlayment

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f e b r u a r y 2 0 1 7 I n t e r f a c e • 3 5 installation, to typical flashing details, to they focus on a roofing material or instal pdf. It was concluded that the felt appropriate repair techniques, and even lation that has been in use for centuries. underlayment did play a role in the including some unusual details like install Many recent changes in the slate industry, Class A fire rating. ing slate on a turret (Figure 7). including code and standard changes, test 4. More information, official test Even more recently, the NSA has ing, and new resources, are geared toward results, photographs, and a video launched a mobile field guide contain supporting S-1 grade natural slate and tra of the testing in progress are avail ing layer-by-layer animations of the most ditional slate roof installation, which have able at: http://slateassociation.org/ common installation details, which can proven, over and over again, an ability to slate-testing/. be accessed on mobile devices.7 Building deliver service lives in excess of 75 years. 5. More information is available at: owners, contractors, and designers can now New materials and installation tech http://slateassociation.org/wp-con have slate roof details literally at their fin niques are constantly appearing and disap tent/uploads/2016/07/white. gertips anytime and anywhere. The NSA has pearing from the market. Some may prove paper_wind.test_.pdf. also introduced a double-sided illustrated to be beneficial to the slate roofing industry. 6. National Slate Association. Slate card designed to be affixed to slate pallets The best indicator of reliability, however, Roofs (Poultney, VT: National Slate by quarriers and distributors to give the will always be the test of time. Until new Association, 1926). p. 25. end-user basic slate installation and han materials and alternative installation meth 7. NSA’s mobile field guide can be ac- dling instructions, as well as directing them ods are able to demonstrate a long history cessed at mobile.slateassociation. to other sources of details and information. of successful, reliable performance, their org. Now, more than ever, the information and use should be approached with caution and guidelines needed to properly install slate a thorough understanding of the potential Julie Palmer is a roofs are readily at hand. risks. roof consultant with Levine & Company, ConCluSion RefeRenCeS Ardmore, PA, and Sources of slate have always been, and 1. Jeffrey S. Levine. A History of the has 14 years of likely will always be, a frequent variable United States Slate Industry. (Thesis, experience with within the industry. Some domestic quar Cornell University, 1988). p. 329. roof restoration ries close, and others open on a regular 2. ASTM Standard E108, 2011, Stan and rehabilitation basis. Today, quarries in foreign countries dard Test Methods for Fire Tests of projects for existing are providing more slate to the U.S. than Roof Coverings, ASTM International, and historical build they have since the early 19th century. West Conshohocken, PA, 2003, DOI: ings. She served as While the importation of slate is not a new 10.1520/E0108-11, www.astm.org. Julie Palmer the National Slate phenomenon, it is being imported from 3. More information, official test results, Association’s office more varied countries than ever before, and photographs of the testing manager for eight years and produced draw thereby bringing new issues and consider in action are available at: http:// ings for the Association’s Slate Roofs: Design ations to the forefront. slateassociation.org/slate-testing/, and Installation Manual and mobile field Testing, standards, building codes, and http://levineco.net/wp-content/ guide. Palmer has a master’s in historic publications have adapted and must contin uploads/2010/12/RidgewalkerNews- preservation from Columbia University and a ue to do so in order to keep up with chang V5N1-FireTesting1.pdf, and http:// master’s in architecture from the University of ing concerns and technology—even when docserver.nrca.net/technical/9562. Pennsylvania.

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