A decade of change

Slate roofing is on the cusp of a revival

by Matt Millen

During the past 10 years, slate roofing has experienced a major resurgence in North America. Quarry operations have been enlarged and modernized; new slate shingle mills have been built and expanded; operations that turn raw stone into roofing shingles have become semiautomated with modern equipment; pneumatic or automated splitters are used; and carbide cutting wheels do final trimming. In addition, there is a new generation of managers at quarries, shingle mills and primary distributors, and the volume of slate work for roofing contractors has grown. Because of these changes in the industry, the recently reorganized National Slate Association (NSA) is working toward improvements in slate roofing installation.

The modern market

Based on an NSA survey of 12 leading Eastern quarriers and direct national distributors, the amount of roofing slate produced and used in North America has doubled during the past 10 years. During this time, a major export market in black roofing slate has developed from North America to Europe. A significant amount of roofing slate also is imported by North America—the main sources have changed from Spain and Brazil to China.

Photo courtesy of Greenstone Slate Co. Inc., Poultney, Vt.

The amount of roofing slate produced and used in North America has doubled during the past 10 years.

But the market wasn't always this way. During the 1920s, the principal market for slate roofing shifted from general construction to high-quality, well-designed, durable steep-slope roof systems. Manufactured roofing materials, such as asbestos shingles, asphalt shingles and tile, competed with slate. Also, the proper installation of a slate roof system required more skill than factory-produced materials. Currently, the principal markets for slate roof systems are expensive residences and historical-restoration projects.

Slate roof systems continue to provide quality and a distinctive appearance from long-lasting, low-maintenance materials. Slate roof designs often are the systems of choice of architects and builders for new homes in the East Coast, Mid-Atlantic, Midwest and West Coast regions. Many projects feature thick slates and/or graduated slates to provide unique and truly distinctive looks. Coupled with the use of copper for flashings, gutters, accessories, bays, dormers and ornaments, high-end residential construction continues the tradition of fine residential designs with slate roof systems from North American quarries.

According to NSA, throughout North America, there are more than 200 contractors who specialize in slate roofing. Companies range from smaller firms that focus on repair and maintenance of slate roof systems to those that specialize in steep-slope roof system restoration and reroofing. Larger multispecialty firms that focus on projects requiring high levels of specialized skills sometimes have slate and tile divisions.

Many craftsmen who supervise and install slate roof systems are second- and third-generation slate roofing workers or attended European roofing trade schools. Many have backgrounds in skilled traditional roofing methods or are part of a new generation being trained according to current slate roofing methods.

NRCA has taken the lead in updating slate roof system methodology and design issues. Each of the past two editions of The NRCA Roofing and Waterproofing Manual substantially has served as a catalyst to improve slate roof system installations. Advances in construction safety, general roofing methods, materials and professionalism in managing roofing construction have permeated the slate segment of the roofing industry through the auspices of NRCA.


NSA, which was founded in 1922 and published Slate Roofs in 1926, reorganized in 2002 during a meeting in Sarasota Springs, N.Y. The meeting brought a wide range of quarriers, consultants and contractors together for the first time in decades. Their discussions included improving material and construction standards and establishing a forum to bring together those knowledgeable and interested in slate roofing issues.

Although the slate industry is expanding and modernizing, there are many issues still to be confronted to further slate roofing growth, enhance its image and raise the quality of installed roof systems. Some issues have challenged the slate roofing industry for some time. Primary among them are inferior imports, poor flashing details, fierce competition among producers, competition from substitute materials, dubious material grading standards and incorrect installation methods.

NSA currently is developing ways to achieve more consistent results when using ASTM International test methods in ASTM C406-00, "Standard Specification for Roofing Slate." In addition, the terminology for slate colors and weathering characteristics is being discussed to achieve a consensus about needed revisions. A program for explaining the slate roofing experience of contractors and tradespeople also is being planned, as are new educational materials for designers, consultants, contractors and owners.

Current issues

The slate industry's experience with testing slate using the test methods in ASTM C406 has been problematic. For example, identical samples prepared from the same slate pieces will receive different test values when tested by different labs, according to a recent NSA report. Consistent test results are critical for those who use, specify or manufacture slate if the physical standard is to be used as the measure for durability and contract compliance. Also, ASTM test methods, as well as test methods used by other slate-producing countries, do not permit roofing professionals to adequately ascertain the quality and durability of slate and are, therefore, of limited use to architects, consultants, owners and contractors.

The three ASTM test methods incorporated in ASTM C406 (depth of softening, water absorption and modules of rupture) were conceived during the 1930s to replicate the most common sources of roofing slate failure.

Photo courtesy of Preservation Technology Associates Inc., Boston.

During the 1920s, the principal market for slate roofing shifted from general construction to high-quality, durable steep-slope roof systems. Pictured here is Memorial Hall at Harvard University, Cambridge, Mass.

ASTM C217, "Standard Test Method for Weather Resistance of Slate," has a depth of softening test. The test, a chemical-weathering test, converts calcite particles, if present, to gypsum. The swelling action that results causes slate disintegration and correlates to the durability of slate in actual weathering. ASTM C121, "Standard Test Method for Water Absorption of Slate," quantifies water absorption during 48 hours, which correlates to susceptibility to freeze-thaw cycles. Results from these two tests are relatively consistent and have remained largely unchanged since the late 1940s. Recent research in Europe suggests revisions in the water-absorption test may improve its ability to indicate slate longevity.

A major problem occurs in the third component of ASTM C406—ASTM C120, "Standard Test Methods of Flexure Testing of Slate (Modulus of Rupture, Modulus of Elasticity)." The test attempts to simulate the effects of foot traffic and hail and quantify the load amount that can be placed on slate before it fails.

The test was developed by William Kessler for the U.S. Bureau of Commerce. Unlike the other two tests, Kessler had no scientific basis for using 9,000 pounds per square inch (psi) (62055 kPa) for 3/16-inch- (4.8-mm-) thick "commercial standard" slate. Actual in situ experience shows that slate with test levels below 9,000 psi (62055 kPa) is durable for more than 100 years, and there is no test provision for slate thicker than 3/16 of an inch (4.8 mm). In addition, the test can be affected by point loading or inaccurate orientation of a sample's grain. The sample can be shocked by overaccelerating the pressure associated by the load or the lab technician inadvertently may damage the sample while preparing it with a segmented saw. These issues are not addressed by ASTM C120 and result in technician subjectivity and inconsistent test results.

The nomenclature of slate contained in ASTM C406 and the information section also require attention. The terms "unfading," "weathering," "semi-weathering," "mottled," "variegated" and "gray/black," for example, can be confusing, especially when inconsistently applied by different quarries or quarry regions. Refinement of slate grading and nomenclature standards will provide terms the industry can rely on when specifying and selecting slate shingles.

Currently, ASTM is addressing these issues, and a change may occur in a few years. Meanwhile, NSA recommends interim guidelines for testing consistency be recognized and several commercial labs with most of the slate testing expertise be recognized as such. This short-term approach could provide the slate industry with more consistent, reliable test results.

Certification and training

Another issue the slate roofing industry must address is that designers, manufacturers and building owners strongly have stated their need to identify capable and experienced slate roofing contractors to instill confidence that slate roof systems will be installed correctly. Problems with workmanship on recent new slate projects exist because of inexperienced crews or poor estimating.

Problem roof systems threaten the reputation and centuries of positive experience with slate roof systems. For slate roofing, learning curves can range from about three years for simple roof systems to more than five years to 10 years for developing the skills necessary for complex installation methods and designs.

A group of NSA-member contractors is developing criteria for identifying types of slate roofing experience and expertise for contractors. Peer review is the next step. The initial registry may lead to certification and training if other segments of the roofing and construction industries continue in that direction. These programs will allow design professionals and building owners to identify and select qualified contractors dedicated to excellence in slate roofing practices. Several organizations have expressed interest in developing new educational materials for the design and construction of slate roof systems. These include NRCA, Roof Consultants Institute, and National Training Center for Store and Masonry Trades.

Design and changes

Design guidelines also will help improve the quality of installed slate roof systems. Slate roof systems' performances and durabilities largely depend on proper detailing at flashings, gutters, downspouts and use of durable underlayments. This information needs to be available to those who design and install slate roof systems. Currently, detailed and specific craft techniques are shared orally and are not included in recent technical manuals. NSA expects to publish technical bulletins and revise Slate Roofs.

In addition, several new building materials and wall-cladding systems have entered the marketplace requiring updates to past practices and new construction details.

Slate roofing has changed in some ways since industry-sanctioned standard methods were published in Slate Roofs. The market shift from general construction to good quality, well-designed roof systems and historical-preservation projects have shifted the focus to roof systems with service lives of more than 100 years and the use of the best materials. In addition, a lower-cost secondary market has developed to compete in the price range with some wood roofing products. These other slate roof systems typically have shorter service lives, use less-skilled installers, have easy access to imported slate, and are designed for warm or arid climates.

There are additional changes that occurred: Plywood has replaced wood boards as the typical roof deck; fireproof decks, insulated decks and venting within a roof system and deck are more common; and copper nails are standard with copper alloys, stainless-steel screws, clips, adhesives and wire ties used where wind, seismic and other design criteria warrant.

Photo courtesy of Preservation Technology Associates.

The slate roof system over the dining hall in Harvard University's Memorial Hall has been in service since the 1870s.

A major improvement has been made in underlayment systems as a result of the development of new products based on inorganic fiber mats, engineered or modified asphalts, and rubber.

There also has been a change in design philosophy related to underlayments. Slate roof systems used to be designed so all water was shed from shingle to shingle and, when necessary, to flashings. Climates, roof slope, shingle size, nail location, head laps and side margins carefully were considered to prevent water migration beyond shingles and flashings. Roof slope, shingles and flashings had to handle water in high-wind conditions, under wet snow and ice, and other conditions that enhanced capillary action and surface tension that moves water sideways and upslope under shingles. Now, underlayment systems can handle some water migration and ice dams and last for the long-term when multiple layers of the best materials are used. Slate roof systems now can be installed on lower slopes, and the size of shingles and side margins can be relaxed with appropriate detailing.

Another major change is in flashing methods and materials. Old roof systems have taught those in the slate industry to use the best stone and better flashings. Heavier gauge copper and even more durable metals commonly are used. Flashings now are integrated with underlayment systems to form watertight zones, and flashing heights and depths are increased where possible. The new construction sealants are used with proper cleaning and primers at the interface with other building components. Good methods for tying a slate roof system into other roof systems and new wall systems also have been developed.

A new beginning

Slate roofing may be a small and time-honored part of the roofing industry, but there is something about roofing with stone that is irresistible—its directness from the earth, pure functionality, durability and beauty. Slate roofing has been with us before written history, yet it changes and grows within the roofing and construction industries.

Matt Millen is a principal with Millen Roofing Co., Milwaukee, Wis., and NSA president. Danny Cornwell, president of CC & L Roofing, Portland, Ore.; Jonathan Hill, president of Greenstone Slate Co., Poultney, Vt.; and Jeffrey Levine, a principal of 1:1:6 Technologies, Media, Pa., contributed to this article.

The history of slate

by Judith E. Selwyn

The market appeal and market share for slate roofing always have been dynamic. The height of its popularity was the 1890s when it shared the bulk of the roofing market with wood shingles, metal and early built-up roof systems. Although slate represents only about 3 percent of all roofing in North America (according to NRCA's 2002-03 annual market survey), its use dates to the early part of the 18th century.

Traditional architectural forms in the eastern, mid-Atlantic states and some southern states were based on English and European styles that employed steep-slope roof systems well-suited to those regions' climates. Eighteenth-century Georgian-style buildings, based on forms from England, were popular from Virginia to the Canadian border. These buildings often were roofed in slate brought over as ballast in trade ships.

Welsh quarrymen emigrated to Pennsylvania and began slate quarries in York and Northampton counties. Some settlements began producing slate during the 1700s. Irish immigrants who moved to escape the famine were quarrying slate before 1850. In the third quarter of the 19th century, Welsh immigrants began to explore the colorful slate beds found in Rutland County, Vt., and Washington County, N.Y. Slate beds in Virgina and Maine also were developed by Welsh quarrymen during the 19th century.

The progression of popular architectural styles in the 19th century made full use of slate's design color and potential. The austere yet monumental Greek-revival style that was popular from about 1820-40 used slate to cover steep-slope, simple gable roofs. The French-inspired Second Empire, or mansard style, was popular for city and country buildings and featured a double-pitched roof with slate as the favored roofing material. Sometimes, the slate was limited to roofs with more visible, lower, steeper slopes. The potential for slate in design was more fully explored on mansard roofs; fancy cuts, including scallops and diamonds, and color patterns often were used to provide ornamental roofs.

The later Victorian styles, such as Queen Anne and Stick, with their whimsical decoration executed on wood and brick buildings, extensively used the variations of slate for color and design. Slate remained popular in residential and small commercial construction as the Shingle, Romanesque, Arts and Crafts, and Colonial Revival designs progressed in popularity from the 1880s through World War I.

The Gothic style was popularized for use on churches in the 19th century. Prominent architects provided designs for large congregations and country parishes, as well as "country" houses. Whether built of stone or board and batten wood siding, slate continued to be the pre-eminent roofing material for these designs. Institutional buildings, including city halls, libraries, schools and universities, chose masonry construction with durable slate roof systems.

For more than a quarter of a century, historical-preservation advocates have worked to restore and preserve historical structures and codify the methods and criteria for restoration and preservation. The Secretary of the Interior's Standards for the Treatment of Historic Properties and Preservation Brief 29, "The Repair, Replacement and Maintenance of Historic Slate Roofs," by Jeffrey Levine (available through the National Park Service or state historical commissions) set a standard for retaining historical buildings to the greatest extent possible and replacing materials in kind when original materials had deteriorated beyond repair.

Federal tax credits and state-funded historical-preservation grants administered by state historical commissions and national programs allow recognized historical properties listed on the National Register of Historic Places or National Historic Landmarks or located in designated local historical districts to be eligible for funding to assist in repairing historical slate roof systems or providing replacement slate roofing materials. Generally, this funding is not available to residential properties, but owners and managers of churches and educational, nonprofit and civic buildings can take advantage of these programs.

Many older slate roof systems can be restored through selective repairs and replacement of flashing and rainwater management systems, including gutters and rain leaders. Most often, maintenance repairs will outlast the useful life of lower quality steep-slope roof systems.

The historical-preservation, high-end builder and architectural communities have become educated about the potential and availability of North American slate, and many roofing contracting companies and experienced slate mechanics actively are involved in training a new generation to continue the best techniques, design and installation of slate materials.

Judith E. Selwyn is a principal with Boston-based Preservation Technology Associates Inc.

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